Ciphertext generating apparatus, cryptographic communication system, and group parameter generating apparatus

[0001] The present invention relates to a cryptographic communication system employing an ID-based cryptographic communication system.

[0002] The public key cryptosystem realizes cryptographic communication decryptable only by a transmission party by transmitting data encrypted with a transmission party's public key so that it can be decrypted by the transmission party with a secret key paired with the public key.

[0003] Conventionally, in order to guarantee that a public key belongs to a transmission party, verification is performed on a public key certificate issued by a public key certification authority.

[0004] Moreover, in order to guarantee the correspondence between a public key and its owner even if there is no infrastructure like the public key certification authority, there has been proposed the ID-based cryptographic communication system using an identification name (ID), such as a transmission party's name, a name, and an equipment number, as the public key.

[0005] For guaranteeing the security of a public key cryptosystem, proving is performed by letting the security of a public key cryptosystem reduce to the difficulty of solving a mathematical problem.

[0006] That is, assuming that there is an attacker who can stochastically break the cipher, when an algorithm exists that can solve a mathematical problem by utilizing the attacker, it can be said that such cryptographic system is reduced to the mathematical problem.

[0007] In the proving, what is important is whether the reduced mathematical problem is good or bad, the reduction rate is good or bad, and the model is good or bad.

[0008] The goodness or badness of the reduced mathematical problem indicates the difficulty of solving the problem. It can be said that the public key cryptosystem reducible to a problem being difficult to solve has high security by that much.

[0009] The goodness or badness of the reduction rate indicates a relation between the resources (time, memory, etc.) exploited by the attacker in order to break a cipher and the resources exploited in order to solve a mathematical problem by utilizing the attacker. If there is not so much difference between the resources required for breaking the cipher and the resources required for solving the mathematical problem, it can be said that the reduction rate is good. In this case, if it is possible to break the cipher, it means it is possible to solve the mathematical problem. Contrapositively, if it is difficult to solve the mathematical problem, to break the cipher is as difficult as the solving. On the other hand, when the reduction rate is bad, that is, when the resources required for solving the mathematical problem are very large in comparison with the resources required for breaking the cipher, even if it is difficult to solve the mathematical problem, to break the cipher is not necessarily as difficult as the solving.

[0010] The goodness or badness of the model indicates whether the model being a premise of the proving is practical or not. For example, a model without using a random oracle is better than a model assuming a random oracle.

• [0011] [Patent Literature 1] International Publication No. 2005-050908
• [0012] [Non-patent Literature 1] Ryuichi SAKAI, Kiyoshi OHGISHI, and Masao KASAHARA, “Cryptosystems based on Pairing over Elliptic Curve” Symposium on Cryptography and Information Security (SCIS 2001), 2001
• [0013] [Non-patent Literature 2] Dan Boneh, and Matt Franklin, “Identity-Based Encryption from the Weil Pairing”, Crypto 2001, LNCS 2139, pp. 213-229, 2001
• [0014] [Non-patent Literature 3] Xavier Boyen, “The BB1 Identity-Based Cryptosystem: A Standard for Encryption and Key Encapsulation”, Submissions for IEEE P1363.3, 2006 (http://grouper.ieee.org/groups/1363/IBC/submissions/index.html)
• [0015] [Non-patent Literature 4] Craig Gentry, “Practical Identity-Based Encryption Without Random Oracles”, Eurocrypt 2006, LNCS 4004, pp. 445-464, 2006
• [0016] [Non-patent Literature 5] Jung Hee Cheon, “Security Analysis of the Strong Diffie-Hellman Problem”, Eurocrypt 2006, pp. 1-13, 2006
• [0017] [Non-patent Literature 6] Mihir Bellare, Alexandra Boldyreva, and Silvio Micali, “Public-key Encryption in a Multi-User Setting: Security Proofs and Improvements”, Eurocrypt 2000, LNCS1807, 2000 (http://www-cse.ucsd.edu/users/mihir/crypto-research-papers.html)
• [0018] [Non-patent Literature 7] Mihir Bellare, Alexandra Boldyreva, and Jessica Staddon, “Multi-Recipient Encryption Schemes: Security Notions and Randomness Re-Use”, PKC 2003, LNCS 2567, 2003 (http://www-cse.used.edu/users.mihir/crypto-research-papers.html)
• [0019] [Non-patent Literature 8] Ronald Cramer, and Victor Shoup, “Design and Analysis of Practical Public-Key Encryption Schemes Secure against Adaptive Chosen Ciphertext Attack”, SIAM. J. Comput, vol. 33, 2003
• [0020] [Non-patent Literature 9] Dan Boneh, and Xavier Boyen, “Efficient Selective-ID Secure Identity Based Encryption Without Random Oracles”, Eurocrypt 2004, LNCS 3027, pp. 223-238, 2004 (http://crypto.stanford.edu/˜dabo/)
• [0021] [Non-patent Literature 10] Brent Waters, “Efficient Identity-Based Encryption Without Random Oracles”, Eurocrypt 2005 (http://www.csl.sri.com/users/bwaters/publications/publications.html)
• [0022] [Non-patent Literature 11] David Naccache, “Secure and Practical Identity-Based Encryption” (http://eprint.iacr.org/2005/369)
• [0023] [Non-patent Literature 12] Sanjit Chatterjee, and Palash Sarkar, “Trading Time for Space: Towards an Efficient IBE Scheme with Short(er) Public Parameters in the Standard Model”, ICISC 2005, LNCS 3935, pp. 424-440, 2006
• [0024] [Non-patent Literature 13] N. P. Smart, “Efficient Key Encapsulation to Multiple Parties”, SCN 2004, LNCS 3352, pp. 208-219, 2005
• [0025] [Non-patent Literature 14] M. Barbosa, and P. Farshim, “Efficient Identity-Based Key Encapsulation to Multiple Parties”, Cryptography and Coding, 10th IMA Int. Cof. 2005, LNCS 3796, Springer Verlog, pp. 428-441, 2005
• [0026] [Non-patent Literature 15] Joonsang Baek, Reihaneh Safavi-Naini, and Willy Susilo, “Efficient Multi-receiver Identity-Based Encryption and Its Application to Broadcast Encryption”, PKC 2005, LNCS 3386, pp. 380-397, 2005
• [0027] [Non-patent Literature 16] Sanjit Chatterjee, and Palash Sarkar, “Generalization of the Selective-ID Security Model for HIBE Protocols”, PKC 2006, 2006
• [0028] [Non-patent Literature 17] Sanjit Chatterjee, and Palash Sarkar, “Multi-receiver Identity-Based Key Encapsulation with ShortenedCiphertext”, Indocrypt2006, LNCS 4329, pp. 394-408, 2006
• [0029] [Non-patent Literature 18] Xavier Boyen, Qixiang Mei, and Brent Waters, “Direct Chosen Ciphertext Security from Identity-Based Techniques” ACM-CC 2005, pp. 320-329, 2005

[0030] As a method for utilizing cryptography, ciphertexts generated by encrypting the same plaintext are transmitted to multiple different recipients.

[0031] Since it takes time to perform encryption processing in the public key cryptosystem, usually, a session key is encrypted and transmitted to a recipient, and data to be transmitted is encrypted with the transmitted session key, based on the common key cryptosystem that can quickly perform encryption processing.

[0032] In the public key cryptosystem, a plurality of ciphertexts are generated by encrypting a plaintext (session key) with a public key of each recipient, and the generated plurality of ciphertexts are united in one to be transmitted by e-mail, data broadcasting, etc.

[0033] In the conventional public key cryptosystem, the security in the case of transmitting to such multiple recipients has been discussed. However, in the ID-based cryptosystem, the security in the case of such multiple recipients has not been sufficiently discussed yet.

[0034] Moreover, in the case of a large number of recipients, since it takes much time even to perform encryption processing for the session keys, the efficiency of the processing needs to be enhanced.

[0035] The present invention has been developed, for example, to solve the problem as described above and aims to provide an ID-based cryptosystem capable of performing encryption processing with a small amount of resources and performing high-speed processing in the multiple-recipient environment where the same plaintext is encrypted to be transmitted to multiple recipients.

[0036] A ciphertext generating apparatus according to the present invention, which generates a ciphertext C to notify n recipient (n being an integer greater than or equal to 1) of a plaintext M, comprises:

[0037] a storage device for storing information; a processing device for processing information; an encryption parameter storage unit; a recipient identification input unit; a plaintext input unit; a ciphertext body generating unit; a hash value calculation unit; a ciphertext verification text generating unit; and a ciphertext combining unit,

[0038] wherein the encryption parameter storage unit stores a public encryption parameter by using the storage device,

[0039] the recipient identification input unit inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) for identifying the n recipient respectively, by using the processing device,

[0040] the plaintext input unit inputs the plaintext M by using the processing device,

[0041] the ciphertext body generating unit generates n ciphertext body CB_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n recipient identification information ID_i input by the recipient identification input unit, and the plaintext M input by the plaintext input unit, by using the processing device,

[0042] the hash value calculation unit generates a combined ciphertext by combining the n ciphertext body CB_i generated by the ciphertext body generating unit, by using the processing device, and calculates a hash value H based on the combined ciphertext generated, by using the processing device,

[0043] the ciphertext verification text generating unit generates a ciphertext verification text CC, based on the public encryption parameter stored by the encryption parameter storage unit and the hash value H calculated by the hash value calculation unit, by using the processing device, and

[0044] the ciphertext combining unit makes one ciphertext C by combining the n ciphertext body CB_i generated by the ciphertext body generating unit and the ciphertext verification text CC generated by the ciphertext verification text generating unit, by using the processing device.

[0045] The ciphertext generating apparatus according to the present invention further comprises a random number generating unit,

[0046] wherein the random number generating unit randomly generates n integer s_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0047] the ciphertext body generating unit includes a first ciphertext generating unit, a second ciphertext generating unit, and a third ciphertext generating unit,

[0048] the first ciphertext generating unit generates n first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the plaintext M input by the plaintext input unit, and the n integer s_i generated by the random number generating unit, by using the processing device,

[0049] the second ciphertext generating unit generates n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n recipient identification information ID_i input by the recipient identification input unit, and the n integer s_i generated by the random number generating unit, by using the processing device,

[0050] the third ciphertext generating unit generates n third ciphertext C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit and then integer s_i generated by the random number generating unit, by using the processing device,

[0051] the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, and the n third ciphertext C3_i generated by the third ciphertext generating unit are treated as the n ciphertext body CB_i corresponding to the n recipient,

[0052] the hash value calculation unit makes one combined ciphertext by combining the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, and the n third ciphertext C3_i generated by the third ciphertext generating unit, by using the processing device, and calculates one hash value H based on the one combined ciphertext by using the processing device,

[0053] the ciphertext verification text generating unit generates n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one hash value H calculated by the hash value calculation unit, by using the processing device, and

[0054] the ciphertext combining unit makes the one ciphertext C by combining the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, the n third ciphertext C3_i generated by the third ciphertext generating unit, and the n ciphertext verification text CC_i generated by the ciphertext verification text generating unit, by using the processing device.

[0055] The ciphertext generating apparatus according to the present invention further has the following features:

[0056] the encryption parameter storage unit stores information, as public encryption parameters, indicating a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, a key generation function KDF for calculating a bit sequence of predetermined length from the element of the multiplicative group GT, a hash function HF for calculating a natural number being less than the natural number r from a bit sequence of arbitrary length, an element g of the multiplicative group G1, an element g1 of the multiplicative group G1, an element g2 of the multiplicative group G2, an element h1 of the multiplicative group G2, an element h2 of the multiplicative group G2, and an element h3 of the multiplicative group G2, by using the storage device,

[0057] the recipient identification input unit inputs n integer ID_i as n recipient identification information, by using the processing device,

[0058] the random number generating unit randomly generates the n integer s_i, being greater than or equal to 1 and less than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0059] the first ciphertext generating unit calculates n key bit sequence k_i=KDF (e(g, h1)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the element g of the multiplicative group G1, the element h1 of the multiplicative group G2, the pairing e, and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit, and the n integer s_i generated by the random number generating unit, by using the processing device, and generates the n first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient by respectively encrypting the plaintext M input by the plaintext input unit, with the n key bit sequence k_i calculated, by using the processing device,

[0060] the second ciphertext generating unit calculates n element u_i=(g1·g^−ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipient, based on the element g of the multiplicative group G1 and the element g1 of the multiplicative group G1 indicated by the public encryption parameters stored by the encryption parameter storage unit, the n integer ID_i input by the recipient identification input unit, and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated n element u_i of the multiplicative group G1, as the n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0061] the third ciphertext generating unit calculates n element v_i=e(g, g2)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipient, based on the element g of the multiplicative group G1, the element g2 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated element v_i of the multiplicative group GT, as the n third ciphertext C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0062] the hash value calculation unit calculates one natural number β₀ to be treated as the hash value H, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device, and

[0063] the ciphertext verification text generating unit calculates n element y_i=e(g, h2)^s_i·e(g, h3)^(s_i·β₀) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipient, based on the element g of the multiplicative group G1, the element h2 of the multiplicative group G2, the element h3 of the multiplicative group G2, and the pairing e indicated by the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one natural number β₀ calculated by the hash value calculation unit, and treats each bit sequence, indicating each calculated n element y_i of the multiplicative group GT, as the n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device.

[0064] The ciphertext generating apparatus according to the present invention further has the following features:

[0065] the encryption parameter storage unit stores information indicating, as public encryption parameters, a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, a key generation function KDF for calculating a bit sequence of predetermined length from an element of the multiplicative group GT, two functions V1 and V2 for calculating an element of the multiplicative group GT from a bit sequence of predetermined length, a hash function HF for calculating a bit sequence of predetermined length from a bit sequence of arbitrary length, an element g of the multiplicative group G1, an element g1 of the multiplicative group G1, and an element g2 of the multiplicative group G2, by using the storage device,

[0066] the recipient identification input unit inputs n bit sequence ID_i of predetermined length, as n recipient identification information, by using the processing device,

[0067] the random number generating unit randomly generates the n integer s_i, being greater than or equal to landless than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0068] the first ciphertext generating unit calculates n key bit sequence k_i=KDF (e(g1, g2)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the element g1 of the multiplicative group G1, the element g2 of the multiplicative group G2, the pairing e and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit, and the n integer s_i generated by the random number generating unit, by using the processing device, and generates the n first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient by respectively encrypting the plaintext M input by the plaintext input unit, with the n key bit sequence k_i calculated, by using the processing device,

[0069] the second ciphertext generating unit calculates n element u_i=V1 (ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G2 corresponding to the n recipient, based on the function V1 indicated by the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the n bit sequence ID_i of predetermined length input by the recipient identification input unit, and treats each bit sequence, indicating each calculated n element u_i of the multiplicative group G2, as the n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0070] the third ciphertext generating unit calculates n element v_i=g^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipient, based on the element g of the multiplicative group G1 indicated by the public encryption parameter stored by the encryption parameter storage unit and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated n element v_i of the multiplicative group G1, as the n third ciphertext C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0071] the hash value calculation unit calculates one bit sequence w₀ of predetermined length to be treated as the hash value H, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device, and

[0072] the ciphertext verification text generating unit calculates n element y_i=V2(w₀)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G2 corresponding to the n recipient, based on the function V2 indicated by the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one bit sequence w₀ of predetermined length calculated by the hash value calculation unit, and treats each bit sequence, indicating each calculated n element y_i of the multiplicative group G2, as the n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal ton) corresponding to then recipient, by using the processing unit.

[0073] The ciphertext generating apparatus according to the present invention further has the following features:

[0074] the encryption parameter storage unit stores information indicating m1 element h_ξ (ξ being an integer greater than or equal to 1 and less than or equal to m1) of the multiplicative group G2 and two elements h1′ and h2′ of the multiplicative group G2, as information indicating the two functions V1 and V2, by using the storage device,

[0075] the second ciphertext generating unit separates each of the n bit sequence ID_i of predetermined length input by the recipient identification input unit into m1 bit sequence ν_iξ (ξ being an integer greater than or equal to 1 and less than or equal to m1) of predetermined length, treats separated m1 bit sequence ν_iξ of predetermined length as m1 integer, by using the processing device, and calculates an element V1(ID_i)=h1′·Π(h_ξ^ν_iξ) (ξ being an integer greater than or equal to 1 and less than or equal to m1) of the multiplicative group G2, which is a value of the function V1, based on the m1 element h of the multiplicative group G2 and the element h1′ of the multiplicative group G2 indicated by the public encryption parameters stored by the encryption parameter storage unit, and the m1 integer y, separated, by using the processing device, and

[0076] the ciphertext verification text generating unit separates a bit sequence w of predetermined length calculated by the hash value calculation unit into m1 bit sequence ν_ξ (ξ being an integer greater than or equal to 1 and less than or equal to m1) of predetermined length, treats separated m1 bit sequence ν_ξ of predetermined length as m1 integer, by using the processing device, and calculates an element V2(w)=h2′·Π(h_ξ^ν_ξ) (ξ being an integer greater than or equal to 1 and less than or equal to m1) of the multiplicative group G2, which is a value of the function V2, based on the m1 element h_ξ (ξ being an integer greater than or equal to 1 and less than or equal to m1) of the multiplicative group G2 and the element h2′ of the multiplicative group G2 indicated by the public encryption parameters stored by the encryption parameter storage unit and the m1 integer ν_ξ separated, by using the processing device.

[0077] The ciphertext generating apparatus according to the present invention further has a feature that the ciphertext body generating unit generates n ciphertext body CB_i including n recipient identification information ID_i, which has been input by the recipient identification input unit, by using the processing device.

[0078] The ciphertext generating apparatus according to the present invention further comprises a ciphertext notification unit,

[0079] wherein the ciphertext notification unit notifies the n recipient identified by the n recipient identification information ID_i input by the recipient identification input unit, of one ciphertext C combined by the ciphertext combining unit, by using the processing device.

[0080] The ciphertext generating apparatus according to the present invention further comprises a session key generating unit,

[0081] wherein the session key generating unit randomly generates a bit sequence of predetermined length, as a session key K, by using the processing device, and

[0082] the plaintext input unit inputs the session key K generated by the session key generating unit, as the plaintext M, by using the processing device.

[0083] The ciphertext generating apparatus according to the present invention further comprises a data input unit; a data encryption unit; and a ciphertext notification unit,

[0084] wherein the data input unit inputs one plaintext data to be transmitted to the n recipient, by using the processing device,

[0085] the data encryption unit generates one encrypted data by encrypting the one plaintext data input by the data input unit, with the session key K generated by the session key generating unit, by using the processing device, and

[0086] the ciphertext notification unit notifies the n recipient identified by the n recipient identification information ID_i input by the recipient identification input unit of the one ciphertext C combined by the ciphertext combining unit and the one encrypted data encrypted by the data encryption unit, by using the processing device.

[0087] A cryptographic communication system according to the present invention, which notifies n recipient (n being an integer greater than or equal to 1), each having a corresponding ciphertext receiving apparatus, of a plaintext M through one ciphertext C, the system comprises:

[0088] an encryption parameter generating apparatus; a ciphertext generating apparatus; and a plurality of ciphertext receiving apparatuses

[0089] wherein the encryption parameter generating apparatus includes a storage device for storing information, a processing device for processing information, a secret information generating unit, a secret information storage unit, a public parameter generating unit, a public parameter publishing unit, an identification information input unit, a secret key generating unit, and a secret key notification unit,

[0090] the secret information generating unit randomly generates secret information by using the processing device,

[0091] the secret information storage unit stores the secret information generated by the secret information generating unit, by using the storage device,

[0092] the public parameter generating unit generates a public encryption parameter, based on the secret information generated by the secret information generating unit, by using the processing device,

[0093] the public parameter publishing unit publishes the public encryption parameter generated by the public parameter generating unit, by using the processing device,

[0094] the identification information input unit inputs recipient identification information ID for identifying one recipient corresponding to one ciphertext receiving apparatus among the plurality of ciphertext receiving apparatuses, by using the processing device,

[0095] the secret key generating unit generates a secret key d_ID corresponding to the one recipient, based on the secret information stored by the secret information storage unit, the public encryption parameter generated by the public parameter generating unit, and the recipient identification information ID input by the identification information input unit, by using the processing device,

[0096] the secret key notification unit secretly notifies the one ciphertext receiving apparatus corresponding to the one recipient of the secret key d_ID generated by the secret key generating unit, by using the processing device,

[0097] wherein the ciphertext generating apparatus includes a storage device for storing information, a processing device for processing information, an encryption parameter storage unit, a recipient identification input unit, a plaintext input unit, a ciphertext body generating unit, a hash value calculation unit, a ciphertext verification text generating unit, a ciphertext combining unit, and a ciphertext notification unit,

[0098] the encryption parameter storage unit stores the public encryption parameter published by the encryption parameter generating apparatus, by using the storage device,

[0099] the recipient identification input unit inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) for respectively identifying the n recipient, by using the processing device,

[0100] the plaintext input unit inputs the plaintext M by using the processing device,

[0101] the ciphertext body generating unit generates n ciphertext body CB_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n recipient identification information ID_i input by the recipient identification input unit, and the plaintext M input by the plaintext input unit, by using the processing device,

[0102] the hash value calculation unit generates a combined ciphertext by combining the n ciphertext body CB_i generated by the ciphertext body generating unit, by using the processing device, and calculates a hash value H based on the combined ciphertext generated, by using the processing device,

[0103] the ciphertext verification text generating unit generates a ciphertext verification text CC, based on the public encryption parameter stored by the encryption parameter storage unit and the hash value H calculated by the hash value calculation unit, by using the processing device,

[0104] the ciphertext combining unit makes one ciphertext C by combining the n ciphertext body CB_i generated by the ciphertext body generating unit and the ciphertext verification text CC generated by the ciphertext verification text generating unit, by using the processing device,

[0105] the ciphertext notification unit notifies n ciphertext receiving apparatus corresponding to the n recipient of the one ciphertext C combined by the ciphertext combining unit, by using the processing device,

[0106] wherein each of the plurality of ciphertext receiving apparatuses includes a storage device for storing information, a processing device for processing information, a public parameter storage unit, a secret key storage unit, a ciphertext receiving unit, a ciphertext decomposition unit, a verification hash value calculation unit, a ciphertext verification unit, and a ciphertext decryption unit,

[0107] the public parameter storage unit stores the public encryption parameter published by the encryption parameter generating apparatus, by using the storage device,

[0108] the secret key storage unit stores the secret key d_ID notified by the encryption parameter generating apparatus, by using the storage device,

[0109] the ciphertext receiving unit receives the one ciphertext C notified by the ciphertext generating apparatus, by using the processing device,

[0110] the ciphertext decomposition unit acquires the n ciphertext body CB_i and the ciphertext verification text CC, based on the one ciphertext C received by the ciphertext receiving unit, by using the processing device,

[0111] the verification hash value calculation unit calculates a verification hash value H′, based on a combined verification ciphertext made by combining the n ciphertext body CB_i acquired by the ciphertext decomposition unit, by using the processing device,

[0112] the ciphertext verification unit judges whether the one ciphertext C received by the ciphertext receiving unit has consistency or not, based on the public encryption parameter stored by the public parameter storage unit, one corresponding ciphertext body CB_ID corresponding to a recipient corresponding to a ciphertext receiving apparatus among the n ciphertext body CB_i acquired by the ciphertext decomposition unit, the ciphertext verification text CC, and the verification hash value H′ calculated by the verification hash value calculation unit, by using the processing device, and

[0113] when the ciphertext verification unit judges that the one ciphertext C received by the ciphertext receiving unit has consistency, the ciphertext decryption unit restores the plaintext M, based on the public encryption parameter stored by the public parameter storage unit, the secret key d_ID, stored by the secret key storage unit, and the one corresponding ciphertext body CB_ID corresponding to the recipient corresponding to the ciphertext receiving apparatus among the n ciphertext body CB_i acquired by the ciphertext decomposition unit, by using the processing device.

[0114] The cryptographic communication system according to the present invention further has the following features:

[0115] the ciphertext generating apparatus further includes a random number generating unit,

[0116] the random number generating unit randomly generates n integer s_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0117] the ciphertext body generating unit includes a first ciphertext generating unit, a second ciphertext generating unit, and a third ciphertext generating unit,

[0118] the first ciphertext generating unit generates n first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the plaintext M input by the plaintext input unit, and the n integer s_i generated by the random number generating unit, by using the processing device,

[0119] the second ciphertext generating unit generates n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n recipient identification information ID_i input by the recipient identification input unit, and the n integer s_i generated by the random number generating unit, by using the processing device,

[0120] the third ciphertext generating unit generates n third ciphertext C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, and the n integer s_i generated by the random number generating unit, by using the processing device,

[0121] the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, and the n third ciphertext C3_i generated by the third ciphertext generating unit are treated as the n ciphertext body CB_i corresponding to the n recipient,

[0122] the hash value calculation unit makes one combined ciphertext by combining the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, and the n third ciphertext C3_i generated by the third ciphertext generating unit, by using the processing device, and calculates one hash value H, based on the one combined ciphertext, by using the processing device,

[0123] the ciphertext verification text generating unit generates n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one hash value H calculated by the hash value calculation unit, by using the processing device,

[0124] the ciphertext combining unit makes the one ciphertext C by combining the n first ciphertext C1_i generated by the first ciphertext generating unit, the n second ciphertext C2_i generated by the second ciphertext generating unit, the n third ciphertext C3_i generated by the third ciphertext generating unit, and the n ciphertext verification text CC_i generated by the ciphertext verification text generating unit, by using the processing device,

[0125] the ciphertext decomposition unit acquires the n first ciphertext C1_i, the n second ciphertext C2_i, the n third ciphertext C3_i, and the n ciphertext verification text CC_i, based on the one ciphertext C received by the ciphertext receiving unit, by using the processing device, and acquires one corresponding first ciphertext C1_ID, one corresponding second ciphertext C2_ID, one corresponding third ciphertext C3_ID, and one corresponding ciphertext verification text CC_ID which are corresponding to the recipient corresponding to the ciphertext receiving apparatus, among the n first ciphertext C1_i, the n second ciphertext C2_i, the n third ciphertext C3_i, and the n ciphertext verification text CC_i which have been acquired, by using the processing device,

[0126] the verification hash value calculation unit calculates the verification hash value H′, based on the combined verification ciphertext made by combining the n first ciphertext C1_i, the n second ciphertext C2_i, and the n third ciphertext C3_i acquired by the ciphertext decomposition unit, by using the processing device,

[0127] the ciphertext verification unit judges whether the one ciphertext C received by the ciphertext receiving unit has consistency or not, based on the public encryption parameter stored by the public parameter storage unit, at least either one of the one corresponding second ciphertext C2_ID and the one corresponding third ciphertext C3_ID, and the one corresponding ciphertext verification text CC_ID which have been acquired by the ciphertext decomposition unit, and the verification hash value H′ calculated by the verification hash value calculation unit, by using the processing device, and

[0128] when the ciphertext verification unit judges that the one ciphertext C received by the ciphertext receiving unit has consistency, the ciphertext decryption unit restores the plaintext M, based on the public encryption parameter stored by the public parameter storage unit, the secret key d_ID stored by the secret key storage unit, and the one corresponding first ciphertext C1_ID, the one corresponding second ciphertext C2_ID and the one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, by using the processing device.

[0129] The cryptographic communication system according to the present invention further has the following features:

[0130] the encryption parameter generating apparatus further includes a group parameter storage unit,

[0131] the group parameter storage unit stores information, as group parameters, indicating a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, and a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, by using the storage device,

[0132] the secret information generating unit randomly generates an integer α being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the group parameter stored by the group parameter storage unit, by using the processing device,

[0133] the secret information storage unit stores information indicating the integer α generated by the secret information generating unit, as secret information, by using the storage device,

[0134] the public parameter generating unit determines a key generation function KDF for calculating a bit sequence of predetermined length from an element of the multiplicative group GT, and a hash function HF for calculating a natural number being less than the natural number r from a bit sequence of arbitrary length, by using the processing device, randomly selects an element g of the multiplicative group G1, an element g2 of the multiplicative group G2, an element h1 of the multiplicative group G2, an element h2 of the multiplicative group G2, and an element h3 of the multiplicative group G2, based on the multiplicative group G1 and the multiplicative group G2 indicated by the group parameter stored by the group parameter storage unit, by using the processing device, calculates an element g1=g^α of the multiplicative group G1, based on the integer α indicated by the secret information generated by the secret information generating unit and a selected element g of the multiplicative group G1, by using the processing device, and treats information indicating the group parameter stored by the group parameter storage unit, determined key generation function KDF and hash function HF, selected element g of the multiplicative group G1, element g2 of the multiplicative group G2, element h1 of the multiplicative group G2, element h2 of the multiplicative group G2, and element h3 of the multiplicative group G2, and a calculated element g1 of the multiplicative group G1, as public encryption parameters,

[0135] the identification information input unit inputs an integer ID, as recipient identification information, by using the processing device,

[0136] the secret key generating unit randomly generates an integer r_ID,1 being greater than or equal to 1 and less than the natural number r, an integer r_ID,2 being greater than or equal to 1 and less than the natural number r, and an integer r_ID,3 being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the public encryption parameter generated by the public parameter generating unit, by using the processing device, calculates an element h_ID,1=(h1·g2^(−r_ID,1))^(1/(α−ID)) of the multiplicative group G2, an element h_ID,2=(h2·g2^(−r_ID,2))^(1/(α−ID)) of the multiplicative group G2, and an element h_ID,3=(h3·g2^(−r_ID,3))^(1/(α−ID)) of the multiplicative group G2, based on the element g2 of the multiplicative group G2, the element h1 of the multiplicative group G2, the element h2 of the multiplicative group G2, and the element h3 of the multiplicative group G2 indicated by the public encryption parameters generated by the public parameter generating unit, the integer α stored by the secret information storage unit, the integer ID input by the identification information input unit, and generated integer r_ID,1, integer r_ID,2 and integer r_ID,3, by using the processing device, and treats information indicating the generated integer r_ID,1, integer r_ID,2 and integer r_ID,3 and calculated element h_ID,1 of the multiplicative group G2, element h_ID,2 of the multiplicative group G2, and element h_ID,3 of the multiplicative group G2, as the secret key d_ID,

[0137] the recipient identification input unit inputs n integer ID_i, as n recipient identification information, by using the processing device,

[0138] the random number generating unit randomly generates the n integer s_i, being greater than or equal to 1 and less than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0139] the first ciphertext generating unit calculates n key bit sequence k_i=KDF(e(g,h1)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the element g of the multiplicative group G1, the element h1 of the multiplicative group G2, the pairing e, and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit, and the n integer s_i generated by the random number generating unit, by using the processing device, and generates then first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient by respectively encrypting the plaintext M input by the plaintext input unit, with the n key bit sequence k_i calculated, by using the processing device,

[0140] the second ciphertext generating unit calculates n element u_i=(g1·g^−ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipient, based on the element g of the multiplicative group G1 and the element g1 of the multiplicative group G1 indicated by the public encryption parameters stored by the encryption parameter storage unit, the n integer ID_i input by the recipient identification input unit, and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated n element u_i of the multiplicative group G1, as the n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0141] the third ciphertext generating unit calculates n element v_i=e(g, g2)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipient, based on the element g of the multiplicative group G1, the element g2 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated n element v_i of the multiplicative group GT, as the n third ciphertext C3 corresponding to the n recipient, by using the processing device,

[0142] the hash value calculation unit calculates one natural number β₀ to be treated as the hash value H, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0143] the ciphertext verification text generating unit calculates n element y_i=e(g, h2)^s_i·e(g, h3)^(s_i·β₀) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipient, based on the element g of the multiplicative group G1, the element h2 of the multiplicative group G2, the element h3 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one natural number β₀ calculated by the hash value calculation unit, and treats each bit sequence, indicating each calculated element y_i of the multiplicative group GT, as the n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0144] the verification hash value calculation unit calculates a natural number β′ to be treated as the verification hash value H′, based on the hash function HF indicated by the public encryption parameter stored by the public parameter storage unit, by using the processing device,

[0145] the ciphertext verification unit, calculates an element y′=e (u_ID, h_ID,2·h_ID,3^/β′)·v_ID^(r_ID,2+r_ID,3·β′) of the multiplicative group GT, based on the pairing e indicated by the public encryption parameter stored by the public parameter storage unit, the integer r_ID,2 the integer r_ID,3, the element h_ID,2 of the multiplicative group G2, and the element h_ID,3 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit, an element u_ID of the multiplicative group G1 indicated by the one corresponding second ciphertext C2_ID and an element v_ID of the multiplicative group GT indicated by the one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, and the natural number β′ calculated by the verification hash value calculation unit, by using the processing device, compares a calculated element y′ of the multiplicative group GT with an element y_ID of the multiplicative group GT indicated by the corresponding ciphertext verification text CC_ID acquired by the ciphertext decomposition unit, by using the processing device, and when the element y′ of the multiplicative group GT accords with the element y_ID of the multiplicative group GT, judges that the one ciphertext C received by the ciphertext receiving unit has consistency, and

[0146] when the ciphertext verification unit judges that the one ciphertext C received by the ciphertext receiving unit has consistency, the ciphertext decryption unit calculates a key bit sequence k′=KDF(e(u_ID,h_ID,1)·v_ID^r_ID,1), based on the pairing e and the key generation function KDF indicated by the public encryption parameters stored by the public parameter storage unit, the integer r_ID,1 and the element h_ID,1 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit, the element u_ID of the multiplicative group G1 indicated by the one corresponding second ciphertext C2_ID, and the element v_ID of the multiplicative group G2 indicated by the corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, by using the processing device, and restores the plaintext M by decrypting the corresponding first ciphertext C1_ID acquired by the ciphertext decomposition unit, with the key bit sequence k′ calculated, by using the processing device.

[0147] The cryptographic communication system according to the present invention further comprises a group parameter generating apparatus,

[0148] wherein the group parameter generating apparatus includes a processing device for processing information, a group order candidate generating unit, a group order security judging unit, and a parameter generating unit,

[0149] the group order candidate generating unit generates a prime number r as a group order candidate by using the processing device, the group order security judging unit calculates a factor of (r−1) being a difference between the prime number r and 1, and a factor of (r+1) being a sum of the prime number r and 1, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and judges whether the prime number r generated by the group order candidate generating unit is a secure group order or not, based on a calculated factor of (r−1) and a calculated factor of (r+1), by using the processing device,

[0150] when the group order security judging unit judges that the prime number r generated by the group order candidate generating unit is a secure group order, the parameter generating unit calculates a multiplicative group whose group order is the prime number r, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and generates a group parameter including information indicating the multiplicative group calculated, by using the processing device, and

[0151] the group parameter storage unit stores the group parameter generated by the group parameter generating apparatus, by using the storage device.

[0152] The cryptographic communication system according to the present invention further has the following features:

[0153] the group order candidate generating unit generates the prime number r by adding 1 to a multiple of a predetermined integer k, as the group order candidate, by using the processing device,

[0154] the group order security judging unit includes a remainder calculation unit, a judgment value calculation unit, and a prime factor judging unit,

[0155] the remainder calculation unit calculates a remainder by dividing the prime number r by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device,

[0156] when the remainder calculated by the remainder calculation unit is 1, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,4), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM(k,4) of an integer k and 4, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/2, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 2, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and when the remainder calculated by the remainder calculation unit is 3, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,2), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,2) of the integer k and 2, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/4, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and

[0157] the prime factor judging unit judges whether the integer r1 has a prime factor less than a predetermined integer B1 or not, based on the integer r1 calculated by the judgment value calculation unit, by using the processing device, and judges whether the integer r2 has a prime factor less than a predetermined integer B2 or not, based on the integer r2 calculated by the judgment value calculation unit, by using the processing device, and when it is judged that the integer r1 has a prime factor less than the predetermined integer B1 and when it is judged that the integer r2 has a prime factor less than the predetermined integer B2, the prime factor judging unit judges that the prime number r generated by the group order candidate generating unit is not a secure group order.

[0158] The cryptographic communication system according to the present invention further has the following features:

[0159] the group order candidate generating unit generates the prime number r by adding 1 to a multiple of a predetermined integer k, as a group order candidate, by using the processing device,

[0160] the group order security judging unit includes a remainder calculation unit, a judgment value calculation unit, and a prime factor judging unit,

[0161] the remainder calculation unit calculates a remainder by dividing the prime number r by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device,

[0162] when the remainder calculated by the remainder calculation unit is 1, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,4), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM(k,4) of an integer k and 4, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/2, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 2, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and when the remainder calculated by the remainder calculation unit is 3, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,2), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,2) of the integer k and 2, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/4, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and

[0163] the prime factor judging unit judges whether the integer r1 is a prime number or not, based on the integer r1 calculated by the judgment value calculation unit, by using the processing device, and judges whether the integer r2 is a prime number or not, based on the integer r2 calculated by the judgment value calculation unit, by using the processing device, and when it is judged that the integer r1 is not a prime number and when it is judged that the integer r2 is not a prime number, the prime factor judging unit judges that the prime number r generated by the group order candidate generating unit is not a secure group order.

[0164] The cryptographic communication system according to the present invention further has the following features:

[0165] the encryption parameter generating apparatus further includes a group parameter storage unit,

[0166] the group parameter storage unit stores information, as group parameters, indicating a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, and a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, by using the storage device,

[0167] the secret information generating unit randomly generates an integer α being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the group parameter stored by the group parameter storage unit, by using the processing device,

[0168] the public parameter generating unit determines a key generation function KDF for calculating a bit sequence of predetermined length from an element of the multiplicative group GT, two functions V1 and V2 for calculating an element of the multiplicative group GT from a bit sequence of predetermined length, and a hash function HF for calculating a bit sequence of predetermined length from a bit sequence of arbitrary length, by using the processing device, randomly selects an element g of the multiplicative group G1 and an element g2 of the multiplicative group G2, based on the multiplicative group G1 and the multiplicative group G2 indicated by the group parameters stored by the group parameter storage unit, by using the processing device, calculates an element g1=g^α of the multiplicative group G1, based on the integer α generated by the secret information generating unit and a selected element g of the multiplicative group G1, by using the processing device, and treats information indicating the group parameter stored by the group parameter storage unit, and determined key generation function KDF, two functions V1 and V2, and hash function HF, selected element g of the multiplicative group G1 and element g2 of the multiplicative group G2, and a calculated element g1 of the multiplicative group G1, as public encryption parameters, by using the processing device,

[0169] the secret information generating unit further calculates an element msk=g2^α of the multiplicative group G2, based on the integer α generated and the element g2 of the multiplicative group G2 selected by the public parameter generating unit, by using the processing device,

[0170] the secret information storage unit stores information indicating the element msk of the multiplicative group G2 calculated by the secret information generating unit, as secret information, by using the storage device,

[0171] the identification information input unit inputs a bit sequence ID of predetermined length, as recipient identification information, by using the processing device,

[0172] the secret key generating unit randomly generates an integer r_ID being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the public encryption parameter generated by the public parameter generating unit, by using the processing device, and calculates an element d0_ID=msk·V1 (ID)^r_ID of the multiplicative group G2 and an element d1_ID=g^ r_ID of the multiplicative group G1, based on the element g of the multiplicative group G1 and the function V1 indicated by the public encryption parameters generated by the public parameter generating unit, the element msk of the multiplicative group G2 indicated by the secret information stored by the secret information storage unit, and the integer r_ID generated, and treats information indicating calculated element d0_ID of the multiplicative group G2 and element d1_ID, of the multiplicative group G1, as the secret key d_ID, by using the processing device,

[0173] the recipient identification input unit inputs n bit sequence ID_i of predetermined length, as n recipient identification information, by using the processing device,

[0174] the random number generating unit randomly generates the n integer s_i, which is greater than or equal to 1 and less than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0175] the first ciphertext generating unit calculates n key bit sequence k_i=KDF(e(g1,g2)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, based on the element g1 of the multiplicative group G1, the element g2 of the multiplicative group G2, the pairing e and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit, and the n integer s_i generated by the random number generating unit, by using the processing device, and generates the n first ciphertext C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient by respectively encrypting the plaintext M input by the plaintext input unit, with the n key bit sequence k_i calculated, by using the processing device,

[0176] the second ciphertext generating unit calculates n element u_i=V1 (ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G2 corresponding to the n recipient, based on the function V1 indicated by the public encryption parameter stored by the encryption parameter storage unit, the n bit sequence ID_i of predetermined length input by the recipient identification input unit, and the n integer s_i (i being an integer greater than or equal to 1 and less than or equal to n) generated by the random number generating unit by using the processing device, and treats each bit sequence, indicating each calculated n element u_i of the multiplicative group G2, as the n second ciphertext C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient,

[0177] the third ciphertext generating unit calculates n element v_i=g^s_i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipient, based on the element g of the multiplicative group G1 indicated by the public encryption parameter stored by the encryption parameter storage unit and the n integer s_i generated by the random number generating unit, and treats each bit sequence, indicating each calculated n element v_i of the multiplicative group G1, as the n third ciphertext C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipient, by using the processing device,

[0178] the hash value calculation unit calculates one bit sequence w₀ of predetermined length to be treated as the hash value H, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit, by using the processing device,

[0179] the ciphertext verification text generating unit calculates n element y_i=V2(w₀)^ s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G2 corresponding to the n recipient, based on the function V2 indicated by the public encryption parameter stored by the encryption parameter storage unit, the n integer s_i generated by the random number generating unit, and the one bit sequence w₀ of predetermined length calculated by the hash value calculation unit, by using the processing device, and treats each bit sequence, indicating each calculated n element y_i of the multiplicative group G2, as the n ciphertext verification text CC_i corresponding to the n recipient,

[0180] the verification hash value calculation unit calculates a bit sequence w′ of predetermined length to be treated as the verification hash value H′, based on the hash function HF indicated by the public encryption parameter stored by the public parameter storage unit, by using the processing device,

[0181] the ciphertext verification unit calculates an element e(v_ID, V2(w′)) of the multiplicative group GT, based on the pairing e and the function V2 indicated by the public encryption parameters stored by the public parameter storage unit, an element v_ID of the multiplicative group G1 indicated by the corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, and the bit sequence w′ of predetermined length calculated by the verification hash value calculation unit, by using the processing device, calculates an element e(g,y_ID) of the multiplicative group GT, based on the element g of the multiplicative group G1 and the pairing e indicated by the public encryption parameters stored by the public parameter storage unit, and an element y_ID of the multiplicative group G2 indicated by the corresponding ciphertext verification text CC_ID acquired by the ciphertext decomposition unit, by using the processing device, and compares a calculated element e(v_ID,V2(w′)) of the multiplicative group GT with a calculated element e(g,y_ID) of the multiplicative group GT by using the processing device, and when the element e(v_ID,V2(w′)) of the multiplicative group GT accords with the element e(g,y_ID) of the multiplicative group GT, judges that the one ciphertext C received by the ciphertext receiving unit has consistency, and

[0182] when the ciphertext verification unit judges that the one ciphertext C received by the ciphertext receiving unit has consistency, the ciphertext decryption unit calculates a key bit sequence k′=KDF(e(y_ID,d0_ID)/e(d1_ID,u_ID)), based on the pairing e and the key generation function KDF indicated by the public encryption parameters stored by the public parameter storage unit, the element d0_ID of the multiplicative group G2 and the element d1_ID of the multiplicative group G1 indicated by the secret key d_ID stored by the secret key storage unit, the element u_ID of the multiplicative group G2 indicated by the one corresponding second ciphertext C2_ID and the element v_ID of the multiplicative group G1 indicated by the corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, by using the processing device, and restores the plaintext M by decrypting the corresponding first ciphertext C1_ID acquired by the ciphertext decomposition unit, with a calculated key bit sequence k′, by using the processing device.

[0183] The cryptographic communication system according to the present invention further has a feature that the ciphertext body generating unit generates the n ciphertext body CB_i including the n recipient identification information ID_i input by the recipient identification input unit, by using the processing device.

[0184] The cryptographic communication system according to the present invention further has the following features:

[0185] the ciphertext generating apparatus further includes a session key generating unit, a data input unit, a data encryption unit, and a ciphertext notification unit,

[0186] the session key generating unit randomly generates a bit sequence of predetermined length, as a session key K, by using the processing device,

[0187] the plaintext input unit inputs the session key K generated by the session key generating unit, as the plaintext M, by using the processing device,

[0188] the data input unit inputs one plaintext data to be transmitted to the n recipient, by using the processing device,

[0189] the data encryption unit generates one encrypted data by encrypting the one plaintext data input by the data input unit, with the session key K generated by the session key generating unit, by using the processing device,

[0190] the ciphertext notification unit notifies the n recipient identified by the n recipient identification information input by the recipient identification input unit of one ciphertext C combined by the ciphertext combining unit and the one encrypted data encrypted by the data encryption unit, by using the processing device,

[0191] each of the plurality of ciphertext receiving apparatuses further includes a data decryption unit,

[0192] the ciphertext receiving unit receives the one ciphertext C and the one encrypted data notified by the ciphertext generating apparatus, by using the processing device, and

[0193] the data decryption unit treats the plaintext M decrypted by the ciphertext decryption unit as a session key K′, and decrypts the one encrypted data received by the ciphertext receiving unit with the session key K′, by using the processing device.

[0194] A group parameter generating apparatus according to the present invention, for generating a parameter of a multiplicative group used for encryption processing in a cryptographic communication system that notifies n recipient (n being an integer greater than or equal to 1), each having a corresponding ciphertext receiving apparatus, of a plaintext M through one ciphertext C, the group parameter generating apparatus comprises:

[0195] a processing device for processing information; a group order candidate generating unit; a group order security judging unit; and a parameter generating unit,

[0196] wherein the group order candidate generating unit generates a prime number r as a group order candidate, by using the processing device,

[0197] the group order security judging unit calculates a factor of (r−1) being a difference between the prime number r and 1, and a factor of (r+1) being a sum of the prime number r and 1, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and judges whether the prime number r generated by the group order candidate generating unit is a secure group order or not, based on a calculated factor of (r−1) and a calculated factor of (r+1), by using the processing device,

[0198] when the group order security judging unit judges that the prime number r generated by the group order candidate generating unit is a secure group order, the parameter generating unit calculates a multiplicative group whose group order is the prime number r, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and generates a group parameter including information indicating the multiplicative group calculated, by using the processing device, and

[0199] the group parameter storage unit stores the group parameter generated by the group parameter generating apparatus, by using the storage device.

[0200] The group parameter generating apparatus according to the present invention further has the following features:

[0201] the group order candidate generating unit generates the prime number r by adding 1 to a multiple of a predetermined integer k, as the group order candidate, by using the processing device,

[0202] the group order security judging unit includes a remainder calculation unit, a judgment value calculation unit, and a prime factor judging unit,

[0203] the remainder calculation unit calculates a remainder by dividing the prime number r by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device,

[0204] when the remainder calculated by the remainder calculation unit is 1, the judgment value calculation unit calculates an integer r1=(r−1)/LCM (k,4), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,4) of an integer k and 4, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/2, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 2, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and when the remainder calculated by the remainder calculation unit is 3, the judgment value calculation unit calculates an integer r1=(r−1)/LCM (k,2), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,2) of the integer k and 2, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/4, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and

[0205] the prime factor judging unit judges whether the integer r1 has a prime factor less than a predetermined integer B1 or not, based on the integer r1 calculated by the judgment value calculation unit, by using the processing device, and judges whether the integer r2 has a prime factor less than a predetermined integer B2 or not, based on the integer r2 calculated by the judgment value calculation unit, by using the processing device, and when it is judged that the integer r1 has a prime factor less than the predetermined integer B1 and when it is judged that the integer r2 has a prime factor less than the predetermined integer B2, the prime factor judging unit judges that the prime number r generated by the group order candidate generating unit is not a secure group order.

[0206] The group parameter generating apparatus according to the present invention further has the following features:

[0207] the group order candidate generating unit generates the prime number r by adding 1 to a multiple of a predetermined integer k, as a group order candidate, by using the processing device,

[0208] the group order security judging unit includes a remainder calculation unit, a judgment value calculation unit, and a prime factor judging unit,

[0209] the remainder calculation unit calculates a remainder by dividing the prime number r by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device,

[0210] when the remainder calculated by the remainder calculation unit is 1, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,4), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,4) of an integer k and 4, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/2, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 2, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and when the remainder calculated by the remainder calculation unit is 3, the judgment value calculation unit calculates an integer r1=(r−1)/LCM(k,2), which is a quotient obtained by dividing the difference (r−1) between the prime number r and 1 by a least common multiple LCM (k,2) of the integer k and 2, based on the predetermined integer k and the prime number r generated by the group order candidate generating unit, by using the processing device, and calculates an integer r2=(r+1)/4, which is a quotient obtained by dividing the sum (r+1) of the prime number r and 1 by 4, based on the prime number r generated by the group order candidate generating unit, by using the processing device, and

[0211] the prime factor judging unit judges whether the integer r1 is a prime number or not, based on the integer r1 calculated by the judgment value calculation unit, by using the processing device, and judges whether the integer r2 is a prime number or not, based on the integer r2 calculated by the judgment value calculation unit, by using the processing device, and when it is judged that the integer r1 is not a prime number and when it is judged that the integer r2 is not a prime number, the prime factor judging unit judges that the prime number r generated by the group order candidate generating unit is not a secure group order.

[0212] In the ID-based cryptographic communication system using the ciphertext generating apparatus of the present Embodiment, since only one processing of calculating a hash value and only one processing based on the calculated hash value, which are parts of processing for generating one ciphertext C to notify a plurality of recipients of the same plaintext M, are needed, the processing efficiency increases, thereby enabling high speed encryption processing.

[0213] Referring to FIGS. 1 to 10, Embodiment 1 will now be described.

[0214] FIG. 1 shows a system configuration illustrating an example of the whole configuration of an ID-based cryptographic communication system 800 according to the present Embodiment. The ID-based cryptographic communication system 800 includes an encryption parameter generating apparatus 100, a ciphertext generating apparatus 200, and a plurality of ciphertext receiving apparatuses 301 to 303.

[0215] The ciphertext receiving apparatuses 301 to 303 respectively correspond to recipients, and correspond to recipient identification information 531 to 533 for identifying a corresponding recipient.

[0216] The recipient identification information 531 to 533 is information capable of uniquely identifying a recipient, such as an address, a name and an e-mail address of the recipient. The recipient identification information 531 to 533 is a part of information 500 published in the ID-based cryptographic communication system 800, and is information whose correspondence relation can be easily verified to be right or not.

[0217] The encryption parameter generating apparatus 100 generates a public encryption parameter 510 required for encryption processing in the ID-based cryptographic communication system 800. The encryption parameter generating apparatus 100 publishes the generated public encryption parameter 510, and the published public encryption parameter 510 becomes a part of the information 500 published in the ID-based cryptographic communication system 800.

[0218] Moreover, the encryption parameter generating apparatus 100 inputs the recipient identification information 531 to 533 of the ciphertext receiving apparatuses 301 to 303, and generates secret keys 601 to 603 of the ciphertext receiving apparatuses 301 to 303, based on the input recipient identification information 531 to 533. The encryption parameter generating apparatus 100 secretly notifies each of the ciphertext receiving apparatuses 301 to 303 of the generated secret keys 601 to 603 by a secure method that is unknowable to a third party. For example, the encryption parameter generating apparatus 100 notifies the ciphertext receiving apparatus 301 of the secret key 601 by recording each of the generated secret keys 601 to 603 in a storage medium, such as a flexible disk, directly sending the flexible disk, in which the secret key 601 is recorded, to the recipient himself corresponding to the ciphertext receiving apparatus 301 and the recipient's installing the secret key 601 in the ciphertext receiving apparatus 301 by himself. The ciphertext receiving apparatuses 302 and 303 are notified of the secret keys 602 and 603 in the same way.

[0219] The ciphertext generating apparatus 200 inputs data (plaintext data 710) to be notified to all or some of the plurality of ciphertext receiving apparatuses 301 to 303. The case of intending to notify the ciphertext receiving apparatuses 301 and 303 of the plaintext data 710 and not intending to notify the ciphertext receiving apparatus 302 of it will now be described as an example.

[0220] The ciphertext generating apparatus 200 notifies the ciphertext receiving apparatuses 301 and 303 of notification data 720.

[0221] The ciphertext receiving apparatuses 301 and 303 are notified of the notification data 720, for example, through an e-mail communication, data broadcasting, etc. Therefore, the notification data 720 is the same data for all the recipients. Although it is not intended to publish the notification data 720, the channel used by the ciphertext generating apparatus 200 for notifying the notification data 720 is in the environment, such as the Internet and data broadcasting, where the third party can easily eavesdrop the notification data 720. Thus, the notification data 720 includes encrypted data 722 generated by encrypting the plaintext data 710.

[0222] Since the plaintext data 710 may be huge data, such as moving image data, it is encrypted by using a common session key K, regardless of the notified party. It is because if the session key K is changed for each notified party, it becomes necessary to generate the encrypted data 722 of the number same as that of notified parties, thereby increasing the load of encryption processing of the ciphertext generating apparatus 200 and increasing the traffic of the channel for notification.

[0223] The notification data 720 further includes a key ciphertext C 721 generated by encrypting the session key K which is used for encrypting the plaintext data 710.

[0224] The ciphertext receiving apparatus 301 acquires the session key K by decrypting the key ciphertext C 721 and acquires the plaintext data 710 by decrypting the encrypted data 722 with the acquired session key K.

[0225] The ciphertext generating apparatus 200 inputs the public encryption parameter 510 published by the encryption parameter generating apparatus 100, and the published recipient identification information 531 and 533 of the ciphertext receiving apparatuses 301 and 303.

[0226] The ciphertext generating apparatus 200 performs encryption processing of the ID-based cryptosystem, based on the input information, to generate the session key K and the key ciphertext C 721.

[0227] The ciphertext generating apparatus 200 encrypts the plaintext data 710 with the generated session key K, to generate the encrypted data 722.

[0228] The ciphertext generating apparatus 200 notifies the ciphertext receiving apparatuses 301 and 303 of the notification data 720 including the generated key ciphertext C 721 and the encrypted data 722.

[0229] The key ciphertext C721 includes a portion corresponding to each notified party. The key ciphertext C721 may further include a portion common regardless of the notified party.

[0230] The ciphertext receiving apparatus 301 acquires the portion corresponding to the ciphertext receiving apparatus 301 (and the common portion) from the key ciphertext C 721, and performs decryption processing of the ID-based cryptosystem by using the secret key 601, to acquire the session key K. Similarly, the ciphertext receiving apparatus 303 acquires the session key K by using the secret key 601.

[0231] On the other hand, since the key ciphertext C 721 does not include a portion corresponding to the ciphertext receiving apparatus 302, even if the ciphertext receiving apparatus 302 eavesdrops the notification data 720, it is impossible for the ciphertext receiving apparatus 302 to perform decryption with the secret key 602, thereby not acquiring the session key K. Therefore, the ciphertext receiving apparatus 302 cannot acquire the plaintext data 710.

[0232] FIG. 2 shows an example of the appearance of the encryption parameter generating apparatus 100, the ciphertext generating apparatus 200, and the ciphertext receiving apparatuses 301 to 303 according to the present Embodiment.

[0233] The encryption parameter generating apparatus 100, the ciphertext generating apparatus 200, and the ciphertext receiving apparatuses 301 to 303 include hardware resources, such as a system unit 910, a display device 901 having a display screen of CRT (Cathode Ray Tube) or LCD (Liquid Crystal), a keyboard (K/B) 902, a mouse 903, an FDD (Flexible Disk Drive) 904, a compact disk drive (CDD) 905, a printer 906, and a scanner 907, which are connected through cables and signal lines.

[0234] The system unit 910, being a computer, is connected to a facsimile machine 932 and a telephone 931 through cables, and to the Internet 940 through a local area network (LAN) 942 and a gateway 941.

[0235] FIG. 3 shows an example of hardware resources of the encryption parameter generating apparatus 100, the ciphertext generating apparatus 200, and the ciphertext receiving apparatuses 301 to 303 according to the present Embodiment.

[0236] The encryption parameter generating apparatus 100, the ciphertext generating apparatus 200, and the ciphertext receiving apparatuses 301 to 303 include a CPU 911 (also called a Central Processing Unit, central processing apparatus, processing device, operational device, microprocessor, microcomputer, or processor) which executes a program. The CPU 911 is connected through a bus 912 to a ROM 913, a RAM 914, a communication device 915, the display device 901, the keyboard 902, the mouse 903, the FDD 904, the CDD 905, the printer 906, the scanner 907, and a magnetic disk drive 920, and controls these hardware devices. Instead of the magnetic disk drive 920, a storage device, such as an optical disk drive and a memory card read/write device may be used. The RAM 914 is an example of a volatile memory. Storage media, such as the ROM 913, FDD 904, CDD 905, and magnetic disk drive 920 are examples of a nonvolatile memory. These are examples of a storage device or a storage unit. The communication device 915, keyboard 902, scanner 907, FDD 904, etc. are examples of an input unit or an input device. The communication board 915, display device 901, printer 906, etc. are examples of an output unit or an output device.

[0237] The communication device 915 is connected to the facsimile machine 932, telephone 931, LAN 942, etc. The communication device 915 may be connected not only to the LAN 942 but also to a WAN (Wide Area Network), such as the Internet 940 and ISDN. When connected to the WAN, such as the Internet 940 or ISDN, the gateway 941 becomes dispensable.

[0238] In the magnetic disk drive 920, an operating system (OS) 921, a window system 922, a program group 923, and a file group 924 are stored. Programs of the program group 923 are executed by the CPU 911, the operating system 921, and the window system 922.

[0239] In the program group 923, a program for executing functions indicated by the term “unit” described in the Embodiments stated below is stored. The program is read and executed by the CPU 911. In the file group 924, information, data, signal values, variable values, and parameters indicated by the terms “judgment result of”, “calculation result of”, or “processing result of” described in the Embodiments stated below are stored as each item of the “file” or “database”. Such “file” and “database” are stored in a recording medium, such as a disk and memory. The information, data, signal values, variable values, and parameters stored in the storage medium, such as a disk and a memory, are read to a main memory or a cache memory by the CPU 911 through a read and write circuit, and used for operations of CPU, such as extraction, search, reference, comparison, operation, calculation, processing, output, printing, and display. During the operations of CPU, namely during extraction, search, reference, comparison, operation, calculation, processing, output, printing, and display, the information, data, signal values, variable values, and parameters are temporarily stored in the main memory, cache memory, and buffer memory.

[0240] Moreover, arrows in the flowchart described in Embodiments stated below mainly show inputting and outputting of data or signals. Data and signal values are recorded on recording media, such as a memory of the RAM 914, a flexible disk of the FDD 904, a compact disk of the CDD 905, a magnetic disk of the magnetic disk drive 920, an optical disk, a mini disk, and a DVD (Digital Versatile Disk). Moreover, data and signals are transmitted on line by the bus 912, a signal line, a cable, or other transmission medium.

[0241] What is described by the term “unit” in the Embodiments stated below may be a “circuit”, a “device”, or an “apparatus” and may be a “step”, a “procedure”, or “processing”. That is, what is described as a “unit” may be realized by the firmware stored in the ROM 913. Alternatively, they may be implemented only by software, only by hardware, such as an element, a device, a substrate, and a wiring, or by a combination of hardware and software, and alternatively a combination of hardware, software and firmware. Firmware and software are stored, as programs, in a recording medium, such as a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, and DVD. Such programs are read by the CPU 911, and executed by the CPU 911. That is, such programs operate a computer, as units described below, or they cause a computer to execute a procedure or a method of the “unit” described below.

[0242] FIG. 4 is a block diagram showing an example of the functional block configuration of the encryption parameter generating apparatus 100 according to the present Embodiment. The encryption parameter generating apparatus 100 includes a parameter setting unit 110 and a secret key setting unit 130. The encryption parameter generating apparatus 100 may be separated into a parameter setting device which includes the parameter setting unit 110, and a secret key setting device which includes the secret key setting unit 130. In that case, there may be a plurality of secret key setting devices for one parameter setting device.

[0243] The parameter setting unit 110 generates and publishes public encryption parameters used as a basis for encryption processing and decryption processing of an ID-based cryptosystem in the ID-based cryptographic communication system 800.

[0244] The parameter setting unit 110 includes a group parameter storage unit 111, a secret information generating unit 112, a public parameter generating unit 113, a public parameter publishing unit 114, a secret information storage unit 121, and a parameter storage unit 122.

[0245] The group parameter storage unit 111 stores a group parameter by using a storage device, such as the magnetic disk drive 920. The group parameter is the information indicating a parameter defining a group that performs a group operation in the ID-based cryptosystem.

[0246] For example, in the case of using a group formed by points on an elliptic curve, as a group for performing a group operation in the ID-based cryptosystem, the group parameter storage unit 111 stores a coefficient a and a coefficient b that define an elliptic curve Y²=X³+aX+b, etc. as a group parameter.

[0247] As an operation in the group composed of points on an elliptic curve, addition of the points on the elliptic curve is usually defined. The group indicated by group parameters stored by the group parameter storage unit 111 is not limited to a group formed by points on an elliptic curve, and it may be other groups. In this case, the group indicated by the group parameters stored by the group parameter storage unit 111 shall be a multiplicative group. That is, multiplication shall be defined as an operation between elements of a group indicated by group parameters stored by the group parameter storage unit 111, and the group shall be formed under the multiplication. Therefore, in the following explanation, the group referred to as a “multiplicative group” shall include a group formed by points on an elliptic curve, and an operation referred to as “multiplication” shall generally include an operation called an addition of points on an elliptic curve. When describing “multiplication” in equations, “·” is used.

[0248] In addition, multiplication in the multiplicative group indicated by group parameters stored by the group parameter storage unit 111 is to be calculable by a predetermined algorithm within a practical time. Therefore, it is possible to obtain a multiplication result by using a processing device, such as the CPU 911.

[0249] The secret information generating unit 112 inputs group parameters stored by the group parameter storage unit 111 by using a processing device, such as the CPU 911.

[0250] The secret information generating unit 112 randomly generates secret information by using a processing device, such as the CPU 911.

[0251] The secret information generating unit 112 outputs the generated secret information, by using a processing device, such as the CPU 911.

[0252] The secret information is information used as a basis for generating public encryption parameters published in the ID-based cryptosystem. Moreover, since the secret information is used as a basis for generating a secret key, it must not be known to the third party by any means.

[0253] The content indicated by the secret information differs depending upon a cryptographic system adopted as the ID-based cryptosystem. For example, the secret information indicates an integer less than the order r of the multiplicative group indicated by a group parameter and greater than or equal to 1.

[0254] The public parameter generating unit 113 inputs the group parameter stored by the group parameter storage unit 111, and the secret information output by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[0255] The public parameter generating unit 113 generates a public encryption parameter, based on the input group parameter and secret information, by using a processing device, such as the CPU 911.

[0256] The public parameter generating unit 113 outputs the generated public encryption parameter, by using a processing device, such as the CPU 911.

[0257] The content indicated by a public encryption parameter differs depending upon the type of a cryptographic system adopted as the ID-based cryptosystem. The public encryption parameter is information indicating an element of a multiplicative group indicated by a group parameter, for example.

[0258] Secret information is skillfully incorporated into public encryption parameters, so that it is possible for the ciphertext generating apparatus 200 and the ciphertext receiving apparatuses 301 to 303 to indirectly utilize the secret information by using the public encryption parameter, but it is impossible to inversely calculate the secret information from the public encryption parameter.

[0259] For example, the secret information indicates an integer α and the public encryption parameter includes information of an element g of the multiplicative group G and an element g^α of the multiplicative group G indicated by the group parameters. In addition, “^” denotes an exponential operation and “g^α” denotes α-th power of g, namely, multiplying α number of g each other. If the multiplicative group G is a group in which solving DH problem (Diffie-Hellmann Problem) is difficult, it is impossible to obtain a from g and g^α. For this reason, even if the public encryption parameter is published, the secret of the secret information can be secured.

[0260] The public parameter publishing unit 114 inputs the public encryption parameter output by the public parameter generating unit 113, by using a processing device, such as the CPU 911.

[0261] The public parameter publishing unit 114 publishes the public parameter which has been input, by using a processing device, such as the CPU 911. The “publishing” includes notifying the ciphertext generating apparatus 200 and the ciphertext receiving apparatuses 301 to 303, and it means that the ciphertext generating apparatus 200 and the ciphertext receiving apparatuses 301 to 303 can acquire a right public encryption parameter, and, if acquiring a falsified public encryption parameter, can easily discriminate whether the acquired one has been falsified or not. For example, the public parameter publishing unit 114 publishes a public encryption parameter by posting it on a homepage. Although it is not beyond the realm of possibility that the homepage is falsified by a third party's attack, such falsification can be discovered by the public encryption parameter generating unit 100′ s checking the homepage. Moreover, it is also preferable to discover a falsification by publishing the same public encryption parameters, by using a plurality of server devices, to judge whether both the ones are in accordance with each other or not.

[0262] The secret information storage unit 121 inputs the secret information output by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[0263] The secret information storage unit 121 stores the input secret information by using a storage device, such as the magnetic disk drive 920.

[0264] The parameter storage unit 122 inputs the public encryption parameter output by the public parameter generating unit 113, by using a processing device, such as the CPU 911. The parameter storage unit 122 stores the input public encryption parameter, by using a storage device, such as the magnetic disk drive 920.

[0265] The secret key setting unit 130 generates secret keys required for decryption processing of the ID-based cryptosystem in the ID-based cryptographic communication system 800, and notifies each of the ciphertext receiving apparatuses 301 to 303 of the generated secret key in secrecy.

[0266] The secret key setting unit 130 includes an identification information input unit 131, a secret key generating unit 132, and a secret key notification unit 133.

[0267] The identification information input unit 131 inputs recipient identification information, by using a processing device, such as the CPU 911. The recipient identification information input by the identification information input unit 131 is information for identifying the recipient corresponding to one of the ciphertext receiving apparatuses 301 to 303, and is a part of the published information 500. For example, the identification information input unit 131 inputs the recipient identification information 531 for identifying the recipient corresponding to the ciphertext receiving apparatus 301, from the published information 500.

[0268] The identification information input unit 131 verifies whether the input recipient identification information is the one for properly identifying the recipient corresponding to the ciphertext receiving apparatus or not, based on the published information 500, by using a processing device, such as the CPU 911.

[0269] Then, if satisfactory as a result of the verification, the identification information input unit 131 outputs the input recipient identification information, by using a processing device, such as the CPU 911.

[0270] The secret key generating unit 132 inputs the recipient identification information output by the identification information input unit 131, the secret information stored by the secret information storage unit 121, and the public encryption parameter stored by the parameter storage unit 122, by using a processing device, such as the CPU 911.

[0271] The secret key generating unit 132 generates a secret key, based on the input recipient identification information, secret information, and public encryption parameter, by using a processing device, such as the CPU 911.

[0272] The secret key generating unit 132 outputs the generated secret key, by using a processing device, such as the CPU 911.

[0273] The secret key notification unit 133 inputs the secret key output by the secret key generating unit 132, by using a processing device, such as the CPU 911.

[0274] The secret key notification unit 133 secretly notifies the ciphertext receiving apparatus belonging to the corresponding recipient of the input secret key, by using a processing device, such as the CPU 911. For example, the secret key notification unit 133 notifies the ciphertext receiving apparatus 301 corresponding to the recipient identified by the recipient identification information 531 of the secret key 601 which the secret key generating unit 132 generated based on the recipient identification information 531.

[0275] Thus, preparation for the cryptographic communication based on the ID-based cryptosystem is completed by publishing the public encryption parameter which the parameter setting unit 110 has generated, and by notifying each of the ciphertext receiving apparatuses 301 to 303 of the secret keys 601 to 603 which the secret key setting unit 130 has generated.

[0276] FIG. 5 is a block diagram showing an example of the functional block configuration of the ciphertext generating apparatus 200 according to the present Embodiment.

[0277] The ciphertext generating apparatus 200 includes a key encryption unit 290, a data input unit 251, a data encryption unit 252, and a ciphertext notification unit 261.

[0278] The key encryption unit 290 generates a session key K and further a key ciphertext C, by using a processing device, such as the CPU 911.

[0279] The key ciphertext C is generated by encrypting the session key K which the key encryption unit 290 generated. The key ciphertext C is an example of the ciphertext C. The key encryption unit 290 includes an encryption parameter storage unit 210, a session key generating unit 221, a random number generating unit 222, a plaintext input unit 231, a recipient identification input unit 232, a ciphertext body generating unit 240, a hash value calculation unit, 244 a ciphertext verification text generating unit 245, and a ciphertext combining unit 246.

[0280] The encryption parameter storage unit 210 stores a public encryption parameter published by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0281] The session key generating unit 221 randomly generates a session key K, by using a processing device, such as the CPU 911. The session key K is a key for encrypting the plaintext data 710, and, for example, is a bit sequence of predetermined length. The cryptographic system for encrypting the plaintext data 710 is a common key cryptosystem, such as AES (Advanced Encryption Standard) and MISTY®.

[0282] The session key generating unit 221 outputs the generated session key K, by using a processing device, such as the CPU 911.

[0283] The plaintext input unit 231 inputs the session key K output by the session key generating unit 221, as a plaintext M, by using a processing device, such as the CPU 911.

[0284] The plaintext input unit 231 outputs the input plaintext M, by using a processing device, such as the CPU 911.

[0285] The recipient identification input unit 232 inputs recipient identification information of the recipient corresponding to the ciphertext receiving apparatus to which the plaintext data 710 is to be transmitted, by using a processing device, such as the CPU 911. For example, in the case of intending to transmit the plaintext data 710 to the ciphertext receiving apparatuses 301 and 303, the recipient identification input unit 232 inputs the recipient identification information 531 of the recipient corresponding to the ciphertext receiving apparatus 301 and the recipient identification information 533 of the recipient corresponding to the ciphertext receiving apparatus 303.

[0286] The number of parties to whom the plaintext data 710 is to be transmitted is hereinafter denoted as n. Moreover, in order to distinguish a plurality of recipient identification information, it will be described, using a suffix, such as recipient identification information ID₁, recipient identification information ID₂, . . . , recipient identification information ID_n. To summarize n number of recipient identification information ID₁, ID₂, . . . , ID_n, it will also be described as “recipient identification information ID_i” (i being an integer greater than or equal to 1 and less than or equal to n). In the following description, the suffix “i” indicates what is corresponding to each of n recipients, in principle.

[0287] The recipient identification input unit 232 inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to the n recipients.

[0288] The recipient identification input unit 232 outputs the n recipient identification information ID_i which has been input, by using a processing device, such as the CPU 911.

[0289] The number n of recipients is not fixed beforehand (namely, not fixed when generating a public encryption parameter), and it is fixed when actually transmitting data.

[0290] The ID-based cryptographic communication system 800 is a cryptographic communication system which can transmit data to recipients of an arbitrary number. Depending upon the necessity at each time, it may be n=1, or may be n=100.

[0291] The ciphertext generating apparatus 200 gets to know the number n of the recipients, for example, when the recipient identification input unit 232 inputs recipient identification information. The ciphertext generating apparatus 200 generates one key ciphertext C (and encrypted data) for any n greater than or equal to 1.

[0292] Moreover, it is assumed that all of the plurality of recipient identification information input by the recipient identification input unit 232 differ from each other. Alternatively, it may be preferable that the recipient identification input unit 232 discriminates whether or not there is any overlapping in the plurality of recipient identification information that has been input, and collects overlapping recipient identification information into one, to exclude the overlapping.

[0293] The random number generating unit 222 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0294] The random number generating unit 222 randomly generates n random numbers, by using a processing device, such as the CPU 911. The random number generated by the random number generating unit 222 is an integer s less than the order r of a multiplicative group indicated by the input public encryption parameter and greater than or equal to 1, for example.

[0295] Moreover, the n random numbers generated by the random number generating unit 222 respectively correspond to the n recipients. That is, the random number generating unit 222 generates n random numbers each of which corresponds to each of the n recipients.

[0296] The random number generating unit 222 outputs the information indicating the n generated random numbers.

[0297] The ciphertext body generating unit 240 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the plaintext M output by the plaintext input unit 231, the n recipient identification information output by the recipient identification input unit 232, and the information indicating the n random numbers output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0298] The ciphertext body generating unit 240 generates n ciphertext bodies CB_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to the n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0299] The ciphertext body generating unit 240 outputs the generated n ciphertext bodies CB_i, by using a processing device, such as the CPU 911.

[0300] The ciphertext body is what is generated by encrypting the plaintext M (session key K) for each recipient, by using the each recipient's identification information ID_i, based on the ID-based cryptosystem. As to the n ciphertext bodies CB_i, each of them corresponds to one recipient.

[0301] In addition, the ciphertext body may be separated into several portions. In the separated portions of the ciphertext body, there may be a portion common to all the recipients. The entire n ciphertext bodies CB_i may be shortened by gathering the portions common to all the recipients among the separated portions of the ciphertext body into one.

[0302] The ciphertext body generating unit 240 includes a first ciphertext generating unit 241, a second ciphertext generating unit 242, and a third ciphertext generating unit 243.

[0303] A ciphertext body CB_i output by the ciphertext body generating unit 240 is composed of a first ciphertext C1_i output by the first ciphertext generating unit 241, a second ciphertext C2_i output by the second ciphertext generating unit 242, and a third ciphertext C3_i output by the third ciphertext generating unit 243.

[0304] The ciphertext body CB_i may be made by combining the first ciphertext C1_i, the second ciphertext C2_i, and the third ciphertext C3_i, and alternatively it may be made of a group of the first ciphertext C1_i, the second ciphertext C2_i, and the third ciphertext C3_i.

[0305] The ciphertext body CB_i is composed of the three ciphertexts in this case, however this is just an example and may have other compositions.

[0306] For example, the ciphertext body CB_i may include recipient identification information ID_i in addition to the three ciphertexts.

[0307] The “including” as used herein means to include as a form in which the identification information ID_i of the recipient corresponding to the ciphertext body CB_i can be easily discriminated, and can be verified when necessary, such as the case of the ciphertext body CB_i made by combining the recipient identification information ID_i, the first ciphertext C1_i, the second ciphertext C2_i, and the third ciphertext C3_i, for example.

[0308] The first ciphertext generating unit 241 inputs the plaintext M output by the plaintext input unit 231, the public encryption parameter stored by the encryption parameter storage unit 210, and the information indicating the n random numbers output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0309] The first ciphertext generating unit 241 generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0310] The first ciphertext generating unit 241 outputs the generated n first ciphertexts C1_i, by using a processing device, such as the CPU 911.

[0311] The second ciphertext generating unit 242 inputs the n recipient identification information ID_i output by the recipient identification input unit 232, the public encryption parameter stored by the encryption parameter storage unit 210, and the information indicating the n random numbers output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0312] The second ciphertext generating unit 242 generates n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0313] The second ciphertext generating unit 242 outputs the generated n second ciphertexts C2_i, by using a processing device, such as the CPU 911.

[0314] The third ciphertext generating unit 243 inputs the public encryption parameter stored by the encryption parameter storage unit 210 and the information indicating the n random numbers output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0315] The third ciphertext generating unit 243 generates n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0316] The third ciphertext generating unit 243 outputs the generated n third ciphertexts C3_i, by using a processing device, such as the CPU 911.

[0317] The hash value calculation unit 244 inputs the public encryption parameter stored by the encryption parameter storage unit 210 and the n ciphertext bodies CB_i (that is, the n first ciphertexts C1_i generated by the first ciphertext generating unit 241, the n second ciphertexts C2_i generated by the second ciphertext generating unit 242, and the n third ciphertexts C3_i generated by the third ciphertext generating unit 243) generated by the ciphertext body generating unit 240, by using a processing device, such as the CPU 911.

[0318] The hash value calculation unit 244 calculates one hash value H, based on the input n ciphertext bodies CB_i, by using a processing device, such as the CPU 911.

[0319] The hash value calculation unit 244 outputs the calculated one hash value H, by using a processing device, such as the CPU 911.

[0320] The hash value H calculated by the hash value calculation unit 244 varies depending upon the ID-based cryptosystem being used. For example, the hash value H is a bit sequence of predetermined length, defined by a public encryption parameter. Alternatively, the hash value H is information indicating an integer less than the order r of a multiplicative group indicated by a public encryption parameter and greater than or equal to 1.

[0321] Moreover, it is also preferable to define a hash function HF for calculating a hash value H, in the public encryption parameter. In that case, the hash value calculation unit 244 calculates a hash value H by the hash function HF indicated by the public encryption parameter.

[0322] The hash value calculation unit 244 calculates one hash value H, based on all the n ciphertext bodies CB_i. For example, the hash value calculation unit 244 generates one combined ciphertext by combining all of the n ciphertext bodies CB_i, and calculates a hash value H of the generated one combined ciphertext.

[0323] The ciphertext verification text generating unit 245 inputs the hash value H output by the hash value calculation unit 244, the public encryption parameter stored by the encryption parameter storage unit 210, and the information indicating the n random numbers output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0324] The ciphertext verification text generating unit 245 generates n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0325] The ciphertext verification text generating unit 245 outputs the generated ciphertext verification text CC_i, by using a processing device, such as the CPU 911.

[0326] Although the number of the ciphertext verification texts generated by the ciphertext verification text generating unit 245 is n in this example, it is also preferable to have a configuration in which one ciphertext verification text being common regardless of the recipient is generated.

[0327] The ciphertext combining units 246 inputs the n ciphertext bodies CB_i (that is, the n first ciphertexts C1_i output by the first ciphertext generating unit 241, the n second ciphertexts C2_i output by the second ciphertext generating unit 242, and the n third ciphertexts C3_i output by the third ciphertext generating unit 243) output by the ciphertext body generating unit 240 and the ciphertext verification text CC output by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0328] The ciphertext combining unit 246 generates one key ciphertext C, based on the input n first ciphertexts C1_i, n second ciphertexts C2_i, n third ciphertexts C3_i, and ciphertext verification text CC, by using a processing device, such as the CPU 911. For example, the ciphertext combining unit 246 makes one key ciphertext C by combining the input n first ciphertexts C1_i, n second ciphertexts C2_i, n third ciphertexts C3_i, and ciphertext verification text CC, by using a processing device, such as the CPU 911.

[0329] The one key ciphertext C combined by the ciphertext combining unit 246 is equally transmitted to all the ciphertext receiving apparatuses corresponding to the n recipients.

[0330] In the combined one key ciphertext C, which portion corresponds to which recipient shall be indicated by the order of combination defined previously, for example, and the ciphertext receiving apparatuses 301 to 303 shall easily extract the portions corresponding to themselves from the key ciphertext C.

[0331] The ciphertext combining unit 246 outputs the generated one key ciphertext C, by using a processing device, such as the CPU 911.

[0332] The data input unit 251 inputs the plaintext data 710, by using a processing device, such as the CPU 911.

[0333] The data input unit 251 outputs the input plaintext data 710, by using a processing device, such as the CPU 911.

[0334] The data encryption unit 252 inputs the plaintext data 710 output by the data input unit 251, and the session key K output by the session key generating unit 221, by using a processing device, such as the CPU 911.

[0335] The data encryption unit 252 encrypts the input plaintext data 710 with the input session key K to generate encrypted data, by using a processing device, such as the CPU 911.

[0336] The data encryption unit 252 outputs the generated encrypted data, by using a processing device, such as the CPU 911.

[0337] The ciphertext notification unit 261 inputs the one key ciphertext C output by the ciphertext combining unit 246, and the encrypted data output by the data encryption unit 252, by using a processing device, such as the CPU 911.

[0338] The ciphertext notification unit 261 generates the notification data 720 including the input key ciphertext C and encrypted data 722, by using a processing device, such as the CPU 911.

[0339] The ciphertext notification unit 261 notifies a plurality of ciphertext receiving apparatuses, which are notified parties, of the generated notification data 720, by using a processing device, such as the CPU 911.

[0340] The plurality of ciphertext receiving apparatuses are notified of the notification data 720 by way of e-mail communication specifying the plurality of recipients to be transmitted, for example. The notification data 720 is not different for each recipient, and is common to all the recipients.

[0341] FIG. 6 is a block diagram showing an example of the functional block configuration of the ciphertext receiving apparatus 301 according to the present Embodiment.

[0342] The ciphertext receiving apparatus 301 includes a ciphertext receiving unit 351, a key decryption unit 390, and a data decryption unit 352.

[0343] The block configuration of the ciphertext receiving apparatuses 302 and 303 is the same as that of the ciphertext receiving apparatus 301.

[0344] The ciphertext receiving unit 351 receives the notification data 720 notified by the ciphertext generating apparatus 200, by using a processing device, such as the CPU 911.

[0345] The ciphertext receiving unit 351 acquires the one key ciphertext C and encrypted data included in the received notification data 720, by using a processing device, such as the CPU 911.

[0346] The ciphertext receiving unit 351 outputs the acquired one key ciphertext C and encrypted data, by using a processing device, such as the CPU 911.

[0347] The key decryption unit 390 decrypts the key ciphertext C output by the ciphertext receiving unit 351, and acquires the session key for decrypting the encrypted data.

[0348] The key decryption unit 390 includes a public parameter storage unit 310, a secret key storage unit 323, the ciphertext receiving unit 351, a ciphertext decomposition unit 346, a verification hash value calculation unit 344, a ciphertext verification unit 345, a ciphertext decryption unit 341, and the data decryption unit 352.

[0349] The public parameter storage unit 310 stores the public encryption parameter published by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0350] The secret key storage unit 323 stores the secret key notified by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0351] The ciphertext decomposition unit 346 inputs the one key ciphertext C output by the ciphertext receiving unit 351, by using a processing device, such as the CPU 911. The ciphertext decomposition unit 346 decomposes the input one key ciphertext C, and acquires the n ciphertext bodies CB_i (that is, n first ciphertexts C1_i, n second ciphertexts C2_i, and n third ciphertexts C3_i) and the ciphertext verification text CC_i by using a processing device, such as the CPU 911.

[0352] Among the n ciphertext bodies CB_i corresponding to the n recipients, the second ciphertext corresponding to the recipient corresponding to the ciphertext receiving apparatus 301 is called a corresponding ciphertext body CB_ID. The same can be applied to the first ciphertext, the second ciphertext, the third ciphertext, and the ciphertext verification text.

[0353] The ciphertext decomposition unit 346 acquires the corresponding ciphertext body CB_i′, (that is, a corresponding first ciphertext C1_ID, a corresponding second ciphertext C2_ID and a corresponding third ciphertext C3_ID) from the n ciphertext bodies CB_i, by using a processing device, such as the CPU 911.

[0354] Similarly, the ciphertext decomposition unit 346 acquires a corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0355] When the ciphertext verification text is one ciphertext verification text common regardless of the recipient, the corresponding ciphertext verification text CC_ID acquired by the ciphertext decomposition unit 346 is the ciphertext verification text itself.

[0356] The ciphertext decomposition unit 346 outputs the acquired n ciphertext bodies CB_i, corresponding ciphertext body CB_ID, and corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0357] When information indicating which ciphertext body is a corresponding ciphertext body CB_ID corresponding to which ID is included in the n ciphertext bodies CB_i, the ciphertext decomposition unit 346 may not acquire the corresponding ciphertext body CB_ID.

[0358] The verification hash value calculation unit 344 inputs the n ciphertext bodies CB_i output by the ciphertext decomposition unit 346, and the public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0359] The verification hash value calculation unit 344 calculates a verification hash value H′ by the same method as calculating a hash value H by the hash value calculation unit 244 of the ciphertext generating apparatus 200, by using a processing device, such as the CPU 911.

[0360] The verification hash value calculation unit 344 outputs the calculated verification hash value H′, by using a processing device, such as the CPU 911.

[0361] The verification hash value calculation unit 344 calculates one verification hash value H′ by the same method as that of the hash value calculation unit 244, based on the input n ciphertext bodies CB_i.

[0362] For example, the verification hash value calculation unit 344 calculates a verification hash value H′, based on the combined verification ciphertext made by combining all the input n ciphertext bodies CB_i.

[0363] The ciphertext verification unit 345 inputs the corresponding ciphertext body CB_ID and the corresponding ciphertext verification text CC_ID output by the ciphertext decomposition unit 346, the verification hash value H′ output by the verification hash value calculation unit 344, the public encryption parameter stored by the public parameter storage unit 310, and the secret key stored by the secret key storage unit 323, by using a processing device, such as the CPU 911.

[0364] The ciphertext verification unit 345 generates verification information, based on the input information, by using a processing device, such as the CPU 911.

[0365] The ciphertext verification unit 345 judges whether the one key ciphertext C received by the ciphertext receiving unit 351 has consistency or not, based on the generated verification information and the input corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0366] For example, when the generated verification information accords with the input corresponding ciphertext verification text CC_ID, the ciphertext decomposition unit 346 judges that the one key ciphertext C received by the ciphertext receiving unit 351 has consistency.

[0367] Alternatively, it is also preferable that the ciphertext verification unit 345 generates another verification information, based on the input corresponding ciphertext verification text CC_ID (and the public encryption parameter and the secret key), and when the two generated verification information accord with each other, judges that the one key ciphertext C received by the ciphertext receiving unit 351 has consistency.

[0368] The ciphertext verification unit 345 outputs the information indicating the judgment result, by using a processing device, such as the CPU 911.

[0369] The ciphertext decryption unit 341 inputs the information indicating the judgment result output by the ciphertext decomposition unit 346, the corresponding ciphertext body CB_ID output by the ciphertext decomposition unit 346, the public encryption parameter stored by the public parameter storage unit 310, and the secret key stored by the secret key storage unit 323, by using a processing device, such as the CPU 911.

[0370] When the ciphertext verification unit 345 judges that the one key ciphertext C received by the ciphertext receiving unit 351 has consistency, based on the input information, the ciphertext decryption unit 341 restores the plaintext M, by using a processing device, such as the CPU 911.

[0371] The ciphertext decryption unit 341 outputs the restored plaintext M, by using a processing device, such as the CPU 911.

[0372] The data decryption unit 352 inputs the plaintext M output by the ciphertext decryption unit 341 and the encrypted data output by the ciphertext receiving unit 351, and treats the input plaintext M as a session key K′, by using a processing device, such as the CPU 911.

[0373] The data decryption unit 352 decrypts the input encrypted data with the input session key K′, to treat it as decrypted data, by using a processing device, such as the CPU 911.

[0374] The data decryption unit 352 outputs the decrypted data, by using a processing device, such as the CPU 911.

[0375] Operations will now be described.

[0376] FIG. 7 is a flowchart showing an example of the flow of parameter setting processing in which the encryption parameter generating apparatus 100 according to the present Embodiment sets public encryption parameters.

[0377] In the secret information generation step S11, the secret information generating unit 112 inputs the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0378] The secret information generating unit 112 randomly generates secret information, based on the input group parameter, by using a processing device, such as the CPU 911.

[0379] The secret information generating unit 112 outputs the generated secret information, by using a processing device, such as the CPU 911.

[0380] The secret information storage unit 121 inputs the secret information output by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[0381] The secret information storage unit 121 stores the input secret information, by using a storage device, such as the magnetic disk drive 920.

[0382] In the encryption parameter generation step S12, the public parameter generating unit 113 inputs the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0383] The public parameter generating unit 113 inputs the secret information output by the secret information generating unit 112 at the secret information generation step S11, by using a processing device, such as the CPU 911.

[0384] The public parameter generating unit 113 generates a public encryption parameter, based on the input group parameter and the input secret information, by using a processing device, such as the CPU 911.

[0385] The public parameter generating unit 113 outputs the generated public encryption parameter, by using a processing device, such as the CPU 911.

[0386] The parameter storage unit 122 inputs the public encryption parameter output by the public parameter generating unit 113, by using a processing device, such as the CPU 911.

[0387] The parameter storage unit 122 stores the input public encryption parameter, by using a storage device, such as the magnetic disk drive 920.

[0388] In the encryption parameter publishing step S13, the public parameter publishing unit 114 inputs the public encryption parameter output by the public parameter generating unit 113 at the encryption parameter generation step S12, by using a processing device, such as the CPU 911.

[0389] The public parameter publishing unit 114 publishes the input public encryption parameter, by using a processing device, such as the CPU 911.

[0390] The public encryption parameter published by the public parameter publishing unit 114 is acquired by the ciphertext generating apparatus 200 and the ciphertext receiving apparatuses 301 to 303, and stored by the encryption parameter storage unit 210 and the public parameter storage unit 310, by using a storage device, such as the magnetic disk drive 920.

[0391] The parameter setting processing may be executed only once before starting the operation of the ID-based cryptographic communication system 800. However, in order to maintain the security of the cryptographic communication, it is also preferable to perform the parameter setting processing periodically, so as to change the public encryption parameter, and in order to enhance the cryptographic strength, it is also preferable to execute the parameter setting processing with changing the multiplicative group to be used.

[0392] FIG. 8 is a flowchart showing an example of the flow of secret key setting processing in which the encryption parameter generating apparatus 100 according to the present Embodiment sets a secret key.

[0393] In the identification input step S21, the identification information input unit 131 inputs a recipient identification information ID, by using a processing device, such as the CPU 911.

[0394] The identification information input unit 131 outputs the input recipient identification information ID, by using a processing device, such as the CPU 911.

[0395] In the secret key generation step S22, the secret key generating unit 132 inputs the recipient identification information ID output by the identification information input unit 131 at S21, by using a processing device, such as the CPU 911.

[0396] The secret key generating unit 132 inputs the secret information stored by the secret information storage unit 121, by using a processing device, such as the CPU 911.

[0397] The secret key generating unit 132 inputs the public encryption parameter stored by the parameter storage unit 122, by using a processing device, such as the CPU 911.

[0398] The secret key generating unit 132 generates a secret key d_ID, based on the input secret information, the input public encryption parameter, and the input recipient identification information ID, by using a processing device, such as the CPU 911.

[0399] The secret key generating unit 132 outputs the generated secret key d_ID, by using a processing device, such as the CPU 911.

[0400] In the secret key notification step S23, the secret key notification unit 133 inputs the recipient identification information ID output by the identification information input unit 131 at S21, by using a processing device, such as the CPU 911. The secret key notification unit 133 inputs the secret key d_ID output by the secret key generating unit 132 at S22, by using a processing device, such as the CPU 911. The secret key notification unit 133 secretly notifies the ciphertext receiving apparatus corresponding to the recipient identified by the input recipient identification information ID of the input secret key d_ID, by using a processing device, such as the CPU 911. The secret key d_ID notified by the secret key notification unit 133 is acquired by the notified ciphertext receiving apparatus, and stored by the secret key storage unit 323 by using a storage device, such as the magnetic disk drive 920.

[0401] The secret key setting processing indicates to set a secret key corresponding to each ciphertext receiving apparatus in the ID-based cryptographic communication system 800. The encryption parameter generating apparatus 100 may generate a secret key beforehand, and notify all the ciphertext receiving apparatuses 301 to 303 in the ID-based cryptographic communication system 800 of the secret key, or may perform execution when required from the ciphertext receiving apparatuses 301 to 303. The secret key setting processing may be executed after notifying the notification data 720 by the ciphertext generating apparatus 200.

[0402] FIG. 9 is a flowchart showing an example of the flow of ciphertext generation processing in which the ciphertext generating apparatus 200 according to the present Embodiment generates a ciphertext.

[0403] In the plaintext data input step S31, the data input unit 251 inputs the plaintext data which is to be notified to n ciphertext receiving apparatuses, by using a processing device, such as the CPU 911.

[0404] The data input unit 251 outputs the input plaintext data, by using a processing device, such as the CPU 911.

[0405] In the recipient identification input step S32, the recipient identification input unit 232 inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) which respectively identify each of the n recipients corresponding to the n ciphertext receiving apparatuses that should be notified of the plaintext data input by the data input unit 251 at the plaintext data input step S31, by using a processing device, such as the CPU 911.

[0406] The recipient identification input unit 232 outputs the input n recipient identification information ID_i, by using a processing device, such as the CPU 911.

[0407] In the session key generation step S33, the session key generating unit 221 randomly generates a session key K, by using a processing device, such as the CPU 911.

[0408] The session key generating unit 221 outputs the generated session key K, by using a processing device, such as the CPU 911.

[0409] The plaintext input unit 231 inputs the session key K output by the session key generating unit 221, by using a processing device, such as the CPU 911.

[0410] The plaintext input unit 231 outputs the input session key K, as a plaintext M, by using a processing device, such as the CPU 911.

[0411] In the random number generation step S34, the random number generating unit 222 randomly generates n random numbers respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0412] The random number generating unit 222 outputs the information indicating the generated n random numbers, by using a processing device, such as the CPU 911.

[0413] In the first ciphertext generation step S35, the first ciphertext generating unit 241 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0414] The first ciphertext generating unit 241 inputs the plaintext M output by the plaintext input unit 231 at the session key generation step S33, by using a processing device, such as the CPU 911.

[0415] The first ciphertext generating unit 241 inputs the information indicating the n random numbers output by the random number generating unit 222 at the random number generation step S34, by using a processing device, such as the CPU 911.

[0416] The first ciphertext generating unit 241 generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0417] The first ciphertext generating unit 241 outputs the generated n first ciphertexts C1_i, by using a processing device, such as the CPU 911.

[0418] In the second ciphertext generation step S36, the second ciphertext generating unit 242 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0419] The second ciphertext generating unit 242 inputs the n recipient identification information ID_i output by the recipient identification input unit 232 at the recipient identification input step S32, by using a processing device, such as the CPU 911.

[0420] The second ciphertext generating unit 242 inputs the information indicating the n random numbers output by the random number generating unit 222 at the random number generation step S34, by using a processing device, such as the CPU 911.

[0421] The second ciphertext generating unit 242 generates n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to the n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0422] The second ciphertext generating unit 242 outputs the generated n second ciphertexts C2_i, by using a processing device, such as the CPU 911.

[0423] In the third ciphertext generation step S37, the third ciphertext generating unit 243 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0424] The third ciphertext generating unit 243 inputs the information indicating the n random numbers output by the random number generating unit 222 at the random number generation step S34, by using a processing device, such as the CPU 911.

[0425] The third ciphertext generating unit 243 generates n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) respectively corresponding to the n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0426] The third ciphertext generating unit 243 outputs the generated n third ciphertexts C3_i, by using a processing device, such as the CPU 911.

[0427] In the hash value calculation step S38, the hash value calculation unit 244 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0428] The hash value calculation unit 244 inputs the n first ciphertexts C1_i output by the first ciphertext generating unit 241 at the first ciphertext generation step S35, by using a processing device, such as the CPU 911.

[0429] The hash value calculation unit 244 inputs the n second ciphertexts C2_i output by the second ciphertext generating unit 242 at the second ciphertext generation step S36, by using a processing device, such as the CPU 911.

[0430] The hash value calculation unit 244 inputs the n third ciphertexts C3_i output by the third ciphertext generating unit 243 at the third ciphertext generation step S37, by using a processing device, such as the CPU 911.

[0431] The hash value calculation unit 244 calculates a hash value H, based on the input information, by using a processing device, such as the CPU 911.

[0432] The hash value calculation unit 244 outputs the calculated hash value H, by using a processing device, such as the CPU 911.

[0433] In the ciphertext verification text generation step S39, the ciphertext verification text generating unit 245 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0434] The ciphertext verification text generating unit 245 inputs the information indicating the n random numbers output by the random number generating unit 222 at the random number generation step S34, by using a processing device, such as the CPU 911.

[0435] The ciphertext verification text generating unit 245 inputs the hash value H output by the hash value calculation unit 244 at the hash value calculation step S38, by using a processing device, such as the CPU 911.

[0436] The ciphertext verification text generating unit 245 generates n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the input information, by using a processing device, such as the CPU 911.

[0437] The ciphertext verification text generating unit 245 outputs the generated n ciphertext verification texts CC_i, by using a processing device, such as the CPU 911.

[0438] In the ciphertext combination step S40, the ciphertext combining unit 246 inputs the n first ciphertexts C1_i output by the first ciphertext generating unit 241 at the first ciphertext generation step S35, by using a processing device, such as the CPU 911.

[0439] The ciphertext combining unit 246 inputs the n second ciphertexts C2_i output by the second ciphertext generating unit 242 at the second ciphertext generation step S36, by using a processing device, such as the CPU 911.

[0440] The ciphertext combining unit 246 inputs the n third ciphertexts C3_i output by the third ciphertext generating unit 243 at the third ciphertext generation step S37, by using a processing device, such as the CPU 911.

[0441] The ciphertext combining unit 246 inputs the n ciphertext verification texts CC_i output by the ciphertext verification text generating unit 245 at the ciphertext verification text generation step S39, by using a processing device, such as the CPU 911.

[0442] The ciphertext combining unit 246 makes one key ciphertext C by combining the input n first ciphertexts C1_i, the input n second ciphertexts C2_i, the input n third ciphertexts C3_i, and the input n ciphertext verification texts CC_i, by using a processing device, such as the CPU 911.

[0443] The ciphertext combining unit 246 outputs the combined one key ciphertext C, by using a processing device, such as the CPU 911.

[0444] In the data encryption step S41, the data encryption unit 252 inputs the plaintext data output by the data input unit 251 at the plaintext data input step S31, by using a processing device, such as the CPU 911.

[0445] The data encryption unit 252 inputs the session key K output by the session key generating unit 221 at the session key generation step S33, by using a processing device, such as the CPU 911.

[0446] The data encryption unit 252 generates encrypted data by encrypting the input plaintext data with the input session key K, by using a processing device, such as the CPU 911.

[0447] The data encryption unit 252 outputs the encrypted data, by using a processing device, such as the CPU 911.

[0448] In the ciphertext notification step S42, the ciphertext notification unit 261 inputs the n recipient identification information ID_i output by the recipient identification input unit 232 at the recipient identification input step S32, by using a processing device, such as the CPU 911.

[0449] The ciphertext notification unit 261 inputs the one key ciphertext C output by the ciphertext combining unit 246 at the ciphertext combination step S40, by using a processing device, such as the CPU 911.

[0450] The ciphertext notification unit 261 inputs the encrypted data output by the data encryption unit 252 at the data encryption step S41, by using a processing device, such as the CPU 911.

[0451] The ciphertext notification unit 261 generates data including the input one key ciphertext C and the input encrypted data, to be treated as notification data, by using a processing device, such as the CPU 911.

[0452] The ciphertext notification unit 261 notifies the ciphertext receiving apparatuses corresponding to the recipients identified by the input n recipient identification information ID_i of the generated notification data, by using a processing device, such as the CPU 911.

[0453] FIG. 10 is a flowchart showing an example of the flow of ciphertext reception processing in which the ciphertext receiving apparatuses 301 to 303 according to the present Embodiment receive ciphertexts.

[0454] In the notification data receiving step S51, the ciphertext receiving unit 351 receives the notification data notified by the ciphertext generating apparatus 200, by using a processing device, such as the CPU 911.

[0455] The ciphertext receiving unit 351 acquires the one key ciphertext C and the encrypted data included in the received notification data, by using a processing device, such as the CPU 911.

[0456] The ciphertext receiving unit 351 outputs the acquired one key ciphertext C and encrypted data, by using a processing device, such as the CPU 911.

[0457] In the ciphertext decomposing step S52, the ciphertext decomposition unit 346 inputs the one key ciphertext C output by the ciphertext receiving unit 351 at the notification data receiving step S51, by using a processing device, such as the CPU 911.

[0458] The ciphertext decomposition unit 346 acquires n first ciphertexts C1_i, n second ciphertexts C2_i, n third ciphertexts C3_i, and n ciphertext verification texts CC_i, based on the input one key ciphertext C, by using a processing device, such as the CPU 911.

[0459] The ciphertext decomposition unit 346 outputs the acquired n first ciphertexts C1_i, n second ciphertexts C2_i, n third ciphertexts C3_i, and n ciphertext verification texts CC_i, by using a processing device, such as the CPU 911.

[0460] Moreover, the ciphertext decomposition unit 346 acquires a corresponding first ciphertext C1_ID, a corresponding second ciphertext C2_ID, a corresponding third ciphertext C3_ID, and a corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0461] The ciphertext decomposition unit 346 outputs the acquired corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID, corresponding third ciphertext C3_ID, and corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0462] In the verification hash value calculation step S53, the verification hash value calculation unit 344 inputs the public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0463] The verification hash value calculation unit 344 inputs the n first ciphertexts C1_i, the n second ciphertexts C2_i, and the n third ciphertexts C3_i output by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, by using a processing device, such as the CPU 911.

[0464] The verification hash value calculation unit 344 calculates a verification hash value H′, based on the input information, by using a processing device, such as the CPU 911.

[0465] The verification hash value calculation unit 344 outputs the calculated verification hash value H′, by using a processing device, such as the CPU 911.

[0466] In the ciphertext verification step S54, the ciphertext verification unit 345 inputs the public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0467] The ciphertext verification unit 345 inputs the secret key stored by the secret key storage unit 323, by using a processing device, such as the CPU 911.

[0468] The ciphertext verification unit 345 inputs the corresponding second ciphertext C2_ID, the corresponding third ciphertext C3_ID, and the corresponding ciphertext verification text CC_ID output by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, by using a processing device, such as the CPU 911.

[0469] The ciphertext verification unit 345 inputs the verification hash value H′ output by the verification hash value calculation unit 344 at the verification hash value calculation step S53, by using a processing device, such as the CPU 911.

[0470] The ciphertext verification unit 345 generates verification information, based on the input information, by using a processing device, such as the CPU 911.

[0471] The ciphertext verification unit 345 judges whether the one key ciphertext C received by the ciphertext receiving unit 351 has consistency or not, based on the generated verification information and the input corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0472] When judged there is consistency, it goes to the ciphertext decrypting step S55.

[0473] When judged there is no consistency, the ciphertext reception processing is ended.

[0474] In the ciphertext decrypting step S55, the ciphertext decryption unit 341 inputs the public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0475] The ciphertext decryption unit 341 inputs the secret key stored by the secret key storage unit 323, by using a processing device, such as the CPU 911.

[0476] The ciphertext decryption unit 341 inputs the corresponding first ciphertext C1_ID, the corresponding second ciphertext C2_ID, and the corresponding third ciphertext C3_ID output by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, by using a processing device, such as the CPU 911.

[0477] The ciphertext decryption unit 341 restores the plaintext M, based on the input information, by using a processing device, such as the CPU 911.

[0478] The ciphertext decryption unit 341 outputs the restored plaintext M, by using a processing device, such as the CPU 911.

[0479] In the data decrypting step S56, the data decryption unit 352 inputs the encrypted data output by the ciphertext receiving unit 351 at the notification data receiving step S51, by using a processing device, such as the CPU 911.

[0480] The data decryption unit 352 inputs the plaintext M output by the ciphertext decryption unit 341 at the ciphertext decrypting step S55, as a session key K′, by using a processing device, such as the CPU 911.

[0481] The data decryption unit 352 decrypts the input encrypted data with the input session key K′, to be treated as decrypted data, by using a processing device, such as the CPU 911.

[0482] The data decryption unit 352 outputs the decrypted data, by using a processing device, such as the CPU 911.

[0483] As described above, only the recipient to whom the plaintext data should be notified can restore the session key K, and then can decrypt the encrypted data.

[0484] The security of the ID-based cryptographic communication system 800 according to the present Embodiment is discussed below.

[0485] First, as the security in the multiple user case in the ID-based cryptosystem, MU-IND-ID-CCA2 security is defined as follows: “MU-IND-ID-CCA2 security” as used herein means the security against an attacker capable of adaptively selecting a target ID in the multiple user (Multi-User) case.

[0486] The cryptographic system 11 is an ID-based cryptographic communication system that includes a key generation capability S for PKG (Private Key Generator), a secret key derivation capability X, an encryption capability C, and a decryption capability D.

[0487] The key generation capability S for PKG inputs initial information Γ, and generates a master key msk for PKG, and a common parameter params.

[0488] The key generation capability S for PKG is equivalent to the function of the parameter setting unit 110 of the public encryption parameter generating apparatus 100 in the ID-based cryptographic communication system 800. The initial information Γ is equivalent to a group parameter. The master key msk for PKG is equivalent to secret information. The common parameter params is equivalent to a public encryption parameter.

[0489] The secret key derivation capability X inputs a user's identification information ID and a master key msk for PKG, and outputs a secret key d_ID.

[0490] The secret key derivation capability X is equivalent to the function of the secret key setting unit 130 of the encryption parameter generating apparatus 100 in the ID-based cryptographic communication system 800. The user's identification information ID is equivalent to recipient identification information.

[0491] The encryption capability E generates a ciphertext C, based on a user's identification information ID, a common parameter params, and a message M. The ciphertext C is equivalent to a key ciphertext C. The encryption capability E is equivalent to the function of the ciphertext generating apparatus 200 of the ID-based cryptographic communication system 800. The message M is equivalent to the plaintext M (=session key K).

[0492] The decryption capability D inputs a secret key d_ID, a common parameter params, and a ciphertext C, and decrypts a message. The decryption capability D returns a decrypted message. When failing in decryption, not a message but a “rejection” is returned.

[0493] The decryption capability D is equivalent to the function of the ciphertext receiving apparatuses 301 to 303 of the ID-based cryptographic communication system 800.

[0494] An attacker against the cryptographic system Π configured as mentioned above is defined as A.

[0495] Moreover, n=n(k) shall be a polynomial with respect to k.

[0496] The attacker A can use an encryption oracle O_E.

[0497] The encryption oracle O_E previously selects either 0 or 1 at random to be treated as b.

[0498] The encryption oracle O_E inputs the user's identification information ID* and two messages M0* and M1*.

[0499] The encryption oracle O_E selects either one of the message M0* and the message M1*, based on b selected beforehand, generates a ciphertext C*, based on the input user's identification information ID* and common parameter params, and the selected message, by the encryption capability E, and replies it.

[0500] The attacker A can call the encryption oracle O_E with respect to a different user's identification information ID*. However, the number of user's identification information ID* which the attacker A can input into the encryption oracle O_E when calling the encryption oracle O_E shall be less than or equal to n.

[0501] Moreover, the attacker A can input different messages M0* and M1* for the same user's identification information ID* when calling the encryption oracle O_E.

[0502] Thus, the attacker A calls the encryption oracle O_E a plurality of times. The encryption oracle O_E always selects the same one (if selecting the former, the former is always selected, and if selecting the latter, the latter is always selected) of the input two messages.

[0503] Moreover, the attacker A can use a key derivation oracle O_X.

[0504] The key derivation oracle O_X inputs a user's identification information ID.

[0505] The key derivation oracle O_X generates a secret key d_ID, based on the master key msk for PKG which is not known to the attacker A and the input the user's identification information ID, by the key derivation capability X, and replies it.

[0506] Moreover, the attacker A can use a decryption oracle O_D.

[0507] The decryption oracle O_D inputs a user's identification information ID and a ciphertext C.

[0508] The decryption oracle O_D generates a secret key d_ID, based on the master key msk for PKG which is not known to the attacker A, and the input user's identification information ID_i by the key derivation capability X. The decryption oracle O_D decrypts a message, based on the generated secret key d_ID, the common parameter params, and the input ciphertext C, by the decryption capability E. The decryption oracle O_D replies the decrypted message (or “rejection”).

[0509] The attacker A tries the game of guessing b to be 0 or 1, by using these oracles. The attacker A cannot input the user's identification information ID which has been input into the key derivation oracle O_X, into the encryption oracle O_E. If the attacker A knows the secret key d_ID, since he can decrypt the ciphertext C* output by the encryption oracle, it is easy for him to guess which message has been encrypted, which means no cryptography has been broken.

[0510] The attacker A cannot even input, inversely to the above, the user's identification information ID* which has been input into the encryption oracle O_E, into the key derivation oracle O_X.

[0511] Moreover, the attacker A cannot even input a combination of the user's identification information ID* which has been input into the encryption oracle O_E, and the ciphertext C which has been output by the encryption oracle O_E in responsive to the user's identification information ID*, into the decryption oracle O_D. Also, in the case of decrypting the ciphertext C* output by the encryption oracle, by using the decryption oracle O_D, since it is easy to guess which message has been encrypted, which means no cryptography has been broken.

[0512] Except for the restriction mentioned above, the attacker A calls the oracles freely and guesses b to be 0 or 1. The attacker A outputs the guess result b′.

[0513] The number of times of the attacker A's calling the encryption oracle O_E per user's identification information ID is made to be q_e. That is, there is a possibility of the encryption oracle O_E being called n·q_e times at the maximum.

[0514] The number of times of the attacker A's calling the decryption oracle O_D is made to be q_d and calling the key derivation oracle O_X is made to be q_x.

[0515] When the attacker A cannot break the cryptography at all, the probability of b=b′ is ½. When the attacker A can always break the cryptography, the probability of b=b′ is 1. When the attacker A may be able to decipher the cryptography, the probability of b=b′ is between ½ and 1. Also, although the probability of b=b′ may become less than ½, in that case it can be said that the attacker A has broken the cryptography in the opposite meaning.

[0516] Then, the attacker A's advantage Adv is defined as follows: Adv^mu_n,Π,Γ(A)=/Pr[Game^mu_n,Π,Γ(A,0)=0]−Pr[Game^mu_n,Π,Γ(A,1)=0]/.

[0517] Pr[ . . . ] as used herein indicates probability of generation of the event.

[0518] Game^mu_n,Π,Γ(A,0) denotes a value of the guess result b′ guessed by the attacker A in the case of b=0. Therefore, Game^mu_n,Π,Γ(A,0)=0 indicates a event in which the attacker A succeeds in guessing when b=0. Game^mu_n,Π,Γ(A, 1) denotes a value of the guess result b′ guessed by the attacker A in the case of b=1. Therefore, Game^mu_n,Π,Γ(A,1)=0 indicates a event in which the attacker A fails in guessing when b=1.

[0519] That is, when the attacker A outputs a guess result b′=0, Adv^mu_n,Π,Γ(A) denotes the absolute value of the difference between the probability of succeeding in guessing and the probability of failing in guessing. In the case the attacker A cannot break the cryptography at all, Adv^mu_n,Π,Γ(A) is 0. In the case the attacker A can always break the cryptography, Adv^mu_n,Π,Γ(A) is 1. When the attacker A may be able to break the cryptography, Adv^mu_n,Π,Γ(A) is a value between 0 and 1.

[0520] n user security of the cryptographic system Π is defined as follows:
Adv^mu_n,Π,Γ(t,q_e,q_d,q_x)=max {Adv^mu_n,Π,Γ(A)}.

[0521] This indicates the advantage of the attacker having the highest advantage among the attackers A who provide an answer within the time t by calling the encryption oracle O_E q_e times per ID, the decryption oracle O_D q_d times, and the key derivation oracle O_X q_x times.

[0522] When Adv^mu_n,Π,Γ(t,q_e,q_d,q_x) is ignorable (negligible) for all polynomials n(k) with respect to k, the cryptographic system Π is said to have (t,q_e,q_d,q_x)−MU security.

[0523] What is conventionally defined as the security of the ID-based cryptosystem is a special case of the security herein defined, and is the MU security in the case of q_e=1 and n=1.

[0524] Next, as the security in the multiple recipient case in the ID-based cryptosystem, SMMR-IND-ID-CCA2 security is defined as follows: “SMMR-IND-ID-CCA2 security” as used herein means the security against an attacker capable of adaptively selecting a target ID_i in the case of transmitting a single message to multiple recipients (Multi-Recipient).

[0525] The cryptographic system is a multiple recipient ID-based cryptographic system in which a single message is multicasted. The cryptographic system Π′ includes a key generation capability S′ for PKG, a secret key derivation capability X′, an encryption capability E′, a ciphertext separation capability T′ and a decryption capability D′.

[0526] Since the key generation capability S′ for PKG, and the secret key derivation capability X′ are similar to the key generation capability S for PKG, and the secret key derivation capability X′ in the definition of the MU-IND-ID-CCA2 security, explanation is omitted.

[0527] The encryption capability E′ inputs multiple users' identification information ID=(ID₁, ID₂, . . . , ID_n), a common parameter params, and a message M, to generate a ciphertext C.

[0528] The ciphertext separation capability T′ inputs the ciphertext C, and separates it for each ID_i to generate a ciphertext C_ID.

[0529] The ciphertext separation capability T′ is equivalent to the function of the ciphertext decomposition unit 346 of the ciphertext receiving apparatus of the ID-based cryptographic communication system 800.

[0530] The decryption capability D′ inputs the secret key d_ID, the common parameter params, and the (separated) ciphertext C_ID for the ID, and returns the decrypted message (or “rejection”).

[0531] An attacker against the cryptographic system Π′ configured as mentioned above is defined as A.

[0532] The attacker A can use an encryption oracle O_E′. The encryption oracle O_E′ inputs each identification information ID*=(ID₁*, ID₂*, ID_n*) of n users and two messages M0* and M1*.

[0533] The encryption oracle O_E′ randomly selects either 0 or 1 to be treated as b.

[0534] The encryption oracle O_E′ selects either one of the input two messages M0* and M1*, based on the selected b, and generates a ciphertext C*, based on the input each identification information ID* of n users, and common parameter params, and the selected message, by the encryption capability E, and replies it.

[0535] Moreover, the attacker A can use a key derivation oracle O_X′. The key derivation oracle O_X′ inputs (one) user's identification information ID. The key derivation oracle O_X, generates a secret key d_ID, based on the master key msk for PKG which is not known to the attacker A, and the input user's identification information ID_i by the key derivation capability X′, and replies it.

[0536] Moreover, the attacker A can use a decryption oracle O_D′. The decryption oracle O_D′ inputs a user's identification information ID and a (separated) ciphertext C_ID for the ID. The decryption oracle O_D′ generates a secret key d_ID, based on the master key msk for PKG which is not known to the attacker A, and the input user's identification information ID, by the key derivation capability X′. The decryption oracle O_D′ decrypts the message, based on the generated secret key d_ID, the common parameter params, and the input ciphertext C_ID, by the decryption capability D′. The decryption oracle O_D′ replies the decrypted message (or “rejection”).

[0537] The attacker A tries the game of guessing b to be 0 or 1, by calling the encryption oracle O_E′ once.

[0538] The attacker A can call the key derivation oracle O_X′ and the decryption oracle O_D′ a plurality of times at both before and after calling the encryption oracle O_E′.

[0539] However, the attacker A cannot input the user's identification information included in each identification information ID* of n users, which has been input or is to be input into the encryption oracle O_E′, into the key derivation oracle O_X′.

[0540] Moreover, the attacker A cannot even input a combination of user's identification information included in each identification information ID* of n users, which has been input into the encryption oracle O_E′, and the ciphertext C_ID corresponding to the identification information among the ciphertexts C which the encryption oracle Op has output in responsive to the user's identification information, into the decryption oracle O_D′, either.

[0541] Except for the restriction mentioned above, the attacker A freely calls the key derivation oracle O_X′ and the decryption oracle O_D′, and guesses b to be 0 or 1. The attacker A outputs the guess result b′.

[0542] Based on the above game, the advantage Adv of the attacker is defined as follows similarly to the case of the MU-IND-ID-CCA2 security.
Adv^smmr_n,Π′,Γ(A)=/Pr[Game^smmr_n,Π′,Γ(A,0)=0]−Pr[Game^smmr_n,Π′,Γ(A,1)=0]/.
Moreover, n recipient security of the cryptographic system Π′ is similarly defined as follows:
Adv^smmr_n,Π′,Γ(t,q_d,q_x)=max{Adv^smmr_n,Π′,Γ(A)}.
This indicates the advantage of the attacker having the highest advantage among the attackers A who provide an answer within the time t by calling the decryption oracle O_D′ q_d times and the key derivation oracle O_X′ q_x times.

[0543] When Adv^smmr_n,Π,Γ(t,q_d,q_x) is negligible for all polynomials n(k) with respect to k, the cryptographic system Π′ is said to have (t,q_d,q_x)-SMMR security.

[0544] Based on the above definition, the SMMR-IND-ID-CCA2 security of the ID-based cryptographic communication system 800 according to the present Embodiment will be explained.

[0545] In the ID-based cryptographic communication system 800 of the present Embodiment, the hash value calculation unit 244 calculates one hash value H, based on n ciphertext bodies CB_i corresponding to n recipients. This method is called a “common hash value method.”

[0546] On the other hand, there may be a method in which the hash value calculation unit 244 calculates a hash value, based on each of n ciphertext bodies CB_i corresponding to n recipients, and calculates n hash values corresponding to the n recipients. This method will be called a “conjunctive method.” First, security of the conjunctive method is now described.

[0547] The encryption oracle O_E′in the definition of the SMMR-IND-ID-CCA2 security inputs each of identification information ID*=(ID₁*,ID₂*, . . . , ID_n*) of n users, to perform encryption processing.

[0548] On the other hand, the encryption oracle O_E in the definition of the MU-IND-ID-CCA2 security inputs an identification information ID* of (one) user, to perform encryption processing.

[0549] Therefore, performing encryption processing for each user's identification information by calling the encryption oracle O_E in the definition of the MU-IND-ID-CCA2 security n times is equivalent to the encryption oracle O_E′in the definition of the SMMR-IND-ID-CCA2 security.

[0550] Moreover, the key derivation oracle O_X and the decryption oracle O_D in the definition of the MU-IND-ID-CCA2 security are equivalent to the key derivation oracle O_X′ and the decryption oracle O_D′ in the definition of the SMMR-IND-ID-CCA2 security.

[0551] As mentioned above, the n recipient SMMR security of the cryptographic system Π′ based on the conjunctive method is equivalent to the n user MU security in the case of q_e=1 in the original cryptographic system Π. That is, Adv^smmr_n,Π′,Γ(t,q_d,q_x)=Adv^mu_n,Π,Γ(t,1,q_d,q_x).

[0552] The MU security for the general public key cryptosystem Π can be proved to be Adv^mu_n,Π,Γ(t,q_e,q_d,q_x)≦q_e·n·Adv^mu_1,Π,Γ(t′,1,q_d,q_x). This can also be applicable to the case of the ID-based cryptosystem.

[0553] Since Adv^mu_1,Π,Γ(t′,1,q_d,q_x) is the security in the case of n=1 and q_e=1 of the n user MU security, it is what is conventionally defined as the security of the ID-based cryptosystem.

[0554] Therefore, the SMMR security of the multiple recipient ID-based cryptosystem made by extending the ID-based cryptosystem, whose security has been proved in the conventional meaning, by the conjunctive method becomes as follows:

Adv^smmr_n,Π′,Γ(t,q_d,q_x)≦n·Adv^mu_1,Π,Γ(t′,1,q_d,q_x) Thereby, the security can be proved.

[0555] Thus, the SMMR security based on the conjunctive method can be directly led from the MU security of the original (single recipient) ID-based cryptosystem, but the SMMR security of the common hash value method needs to be concretely proved.

[0556] In the common hash value method, since it is enough for the hash value calculation unit 244 to calculate one hash value H, the amount of processing of calculating a hash value by the hash value calculation unit 244 can be reduced compared with the conjunctive method in which n hash values corresponding to n recipients are calculated. Moreover, as to the processing of generating a ciphertext verification text CC by the ciphertext verification text generating unit 245, since it is possible to make calculations from the beginning to the middle be common to perform the calculation once, in the common hash value method, the amount of processing of generating a ciphertext verification text CC by the ciphertext verification text generating unit 245 can also be reduced compared with the conjunctive method in which most of calculations need to be repeated n times. Thereby, since the processing speed of the ciphertext generation processing in the ciphertext generating apparatus 200 becomes fast, the ciphertext generation processing can be completed within a practical time even if the throughput of the processing device, such as the CPU 911, of the ciphertext generating apparatus 200 is low.

[0557] The ciphertext generating apparatus 200 according to the present Embodiment that generates a ciphertext C (key ciphertext C) to notify n recipients (n being an integer greater than or equal to 1) of a plaintext M has the following features:

[0558] The ciphertext generating apparatus 200 includes a storage device for storing information, such as the magnetic disk drive 920, a processing device for processing information, such as the CPU 911, the encryption parameter storage unit 210, the recipient identification input unit 232, the plaintext input unit 231, the ciphertext body generating unit 240, the hash value calculation unit 244, the ciphertext verification text generating unit 245, and the ciphertext combining unit 246.

[0559] The encryption parameter storage unit 210 stores a public encryption parameter by using a storage device, such as the magnetic disk drive 920.

[0560] The recipient identification input unit 232 inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) for identifying the n recipients respectively, by using a processing device, such as the CPU 911.

[0561] The plaintext input unit 231 inputs the plaintext M by using a processing device, such as the CPU 911.

[0562] The ciphertext body generating unit 240 generates n ciphertext bodies CB_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n recipient identification information ID_i input by the recipient identification input unit 232, and the plaintext M input by the plaintext input unit 231, by using a processing device, such as the CPU 911.

[0563] The hash value calculation unit 244 generates a combined ciphertext by combining the n ciphertext bodies CB_i generated by the ciphertext body generating unit 240, by using a processing device, such as the CPU 911, and calculates a hash value H based on the combined ciphertext generated, by using a processing device, such as the CPU 911.

[0564] The ciphertext verification text generating unit 245 generates a ciphertext verification text CC, based on the public encryption parameter stored by the encryption parameter storage unit 210 and the hash value H calculated by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0565] The ciphertext combining unit 246 makes one ciphertext C by combining the n ciphertext bodies CB_i generated by the ciphertext body generating unit 240 and the ciphertext verification text CC generated by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0566] The ciphertext generating apparatus 200 according to the present Embodiment has the following effect:

[0567] In the ID-based cryptographic communication system, since only once processing of calculating a hash value and only once processing based on the calculated hash value, which are parts of processing for generating one ciphertext C to notify a plurality of recipients of the same plaintext M, are required, the processing efficiency is increased, thereby enabling high speed encryption processing.

[0568] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[0569] The ciphertext generating apparatus 200 further includes the random number generating unit 222.

[0570] The random number generating unit 222 randomly generates n integers s_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to n recipients, by using a processing device, such as the CPU 911.

[0571] The ciphertext body generating unit 240 includes the first ciphertext generating unit 241, the second ciphertext generating unit 242, and the third ciphertext generating unit 243.

[0572] The first ciphertext generating unit 241 generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the plaintext M input by the plaintext input unit 231, and then integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0573] The second ciphertext generating unit 242 generates n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n recipient identification information ID_i input by the recipient identification input unit 232, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0574] The third ciphertext generating unit 243 generates n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210 and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0575] The hash value calculation unit 244 makes one combined ciphertext by combining the n first ciphertexts C1_i generated by the first ciphertext generating unit 241, the n second ciphertexts C2_i generated by the second ciphertext generating unit 242, and the n third ciphertexts C3_i generated by the third ciphertext generating unit 243, by using a processing device, such as the CPU 911, and calculates one hash value H₀ based on the one combined ciphertext by using a processing device, such as the CPU 911.

[0576] The ciphertext verification text generating unit 245 generates n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n integers s_i generated by the random number generating unit 222, and the one hash value H₀ calculated by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0577] The ciphertext combining unit 246 makes one ciphertext C (key ciphertext C) by combining the n first ciphertexts C1_i generated by the first ciphertext generating unit 241, the n second ciphertexts C2_i generated by the second ciphertext generating unit 242, the n third ciphertexts C3_i generated by the third ciphertext generating unit 243, and the n ciphertext verification texts CC_i generated by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0578] The ciphertext generating apparatus 200 according to the present Embodiment has the following effects:

[0579] The hash value calculation unit 244 calculates one hash value H₀ being common regardless of the recipient, thereby reducing the processing amount of the ciphertext generation processing in the ciphertext generating apparatus 200.

[0580] Moreover, in the ID-based cryptographic communication system whose recipient has been extended to multiple recipients from the ID-based cryptographic communication system whose security has been proved with respect to a single recipient, it is possible to prove the security similar to that of the single recipient case.

[0581] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[0582] The ciphertext body generating unit 240 generates n ciphertext bodies CB_i including n recipient identification information ID_i input by the recipient identification input unit 232, by using a processing device, such as the CPU 911.

[0583] The ciphertext generating apparatus 200 according to the present Embodiment has the following effect:

[0584] Since n ciphertext bodies CB_i used as the basis of the hash value calculated by the hash value calculation unit 244 include n recipient identification information, the security of the ID-based cryptographic communication system is increased.

[0585] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[0586] The ciphertext generating apparatus 200 further includes the ciphertext notification unit 261.

[0587] The ciphertext notification unit 261 notifies the n recipients identified by the n recipient identification information ID_i input by the recipient identification input unit 232, of the one ciphertext C combined by the ciphertext combining unit 246, by using a processing device, such as the CPU 911.

[0588] The ciphertext generating apparatus 200 according to the present Embodiment has the following effect:

[0589] Since the ciphertext notification unit 261 notifies n recipients of the same ciphertext C, the load applied to the traffic of the communication path for notifying the ciphertext is small.

[0590] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[0591] The ciphertext generating apparatus 200 further includes the session key generating unit 221.

[0592] The session key generating unit 221 randomly generates a bit sequence of predetermined length, as a session key K, by using a processing device, such as the CPU 911.

[0593] The plaintext input unit 231 inputs the session key K generated by the session key generating unit, as the plaintext M, by using a processing device, such as the CPU 911.

[0594] The ciphertext generating apparatus 200 according to the present Embodiment has the following effect:

[0595] Since the session key generating unit 221 generates a session key K, and then generates a ciphertext C for notifying a notified party of the generated session key, the security is higher compared with the case of inputting the session key K from the outside.

[0596] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[0597] The ciphertext generating apparatus 200 further includes the data input unit 251, the data encryption unit 252, and the ciphertext notification unit 261.

[0598] The data input unit 251 inputs one plaintext data to be transmitted to n recipients, by using a processing device, such as the CPU 911.

[0599] The data encryption unit 252 generates one encrypted data by encrypting one plaintext data input by the data input unit 251, with the session key K generated by the session key generating unit 221, by using a processing device, such as the CPU 911.

[0600] The ciphertext notification unit 261 notifies the n recipients identified by the n recipient identification information ID_i input by the recipient identification input unit 232 of one ciphertext C (key ciphertext C) combined by the ciphertext combining unit 246 and one encrypted data encrypted by the data encryption unit 252, by using a processing device, such as the CPU 911.

[0601] The ciphertext generating apparatus 200 according to the present Embodiment has the following effect:

[0602] Since the data encryption unit 252 encrypts plaintext data with one session key K being common regardless of the recipient, and the ciphertext notification unit 261 notifies the recipient of one ciphertext C being common regardless of the recipient and one encrypted data, the load applied to the traffic of the communication path is small.

[0603] The ID-based cryptographic communication system 800 (cryptographic communication system) according to the present Embodiment that notifies n recipients (n being an integer greater than or equal to 1), each having a corresponding ciphertext receiving apparatus, of a plaintext M (session key K) through one ciphertext C (key ciphertext C) has the following features:

[0604] The ID-based cryptographic communication system 800 includes the encryption parameter generating apparatus 100, the ciphertext generating apparatus 200, and a plurality of ciphertext receiving apparatuses 301 to 303.

[0605] The encryption parameter generating apparatus 100 includes a storage device for storing information, such as the magnetic disk drive 920, a processing device for processing information, such as the CPU 911, the secret information generating unit 112, the secret information storage unit 121, the public parameter generating unit 113, the public parameter publishing unit 114, the identification information input unit 131, the secret key generating unit 132, and the secret key notification unit 133.

[0606] The secret information generating unit 112 randomly generates secret information by using a processing device, such as the CPU 911.

[0607] The secret information storage unit 121 stores the secret information generated by the secret information generating unit 112, by using a storage device, such as the magnetic disk drive 920.

[0608] The public parameter generating unit 113 generates a public encryption parameter, based on the secret information generated by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[0609] The public parameter publishing unit 114 publishes the public encryption parameter generated by the public parameter generating unit 113, by using a processing device, such as the CPU 911.

[0610] The identification information input unit 131 inputs recipient identification information ID for identifying one recipient corresponding to one ciphertext receiving apparatus among a plurality of ciphertext receiving apparatuses 301 to 303, by using a processing device, such as the CPU 911.

[0611] The secret key generating unit 132 generates a secret key d_ID corresponding to the one recipient, based on the secret information stored by the secret information storage unit 121, the public encryption parameter generated by the public parameter generating unit 113, and the recipient identification information ID input by the identification information input unit 131, by using a processing device, such as the CPU 911.

[0612] The secret key notification unit 133 secretly notifies one ciphertext receiving apparatus corresponding to the one recipient of the secret key d_ID generated by the secret key generating unit 132, by using a processing device, such as the CPU 911.

[0613] The ciphertext generating apparatus 200 includes a storage device for storing information, such as the magnetic disk drive 920, a processing device for processing information, such as the CPU 911, the encryption parameter storage unit 210, the recipient identification input unit 232, the plaintext input unit 231, the random number generating unit 222 the ciphertext body generating unit 240, the hash value calculation unit 244, the ciphertext verification text generating unit 245, the ciphertext combining unit 246, and the ciphertext notification unit 261.

[0614] The encryption parameter storage unit 210 stores the public encryption parameter published by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0615] The recipient identification input unit 232 inputs n recipient identification information ID_i (i being an integer greater than or equal to 1 and less than or equal to n) for respectively identifying n recipients, by using a processing device, such as the CPU 911.

[0616] The plaintext input unit 231 inputs the plaintext M by using a processing device, such as the CPU 911.

[0617] The second ciphertext generating unit 242 generates n ciphertext bodies CB_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n recipient identification information ID_i input by the recipient identification input unit 232, and the plaintext M input by the plaintext input unit 231, by using a processing device, such as the CPU 911.

[0618] The hash value calculation unit 244 generates a combined ciphertext by combining the n ciphertext bodies CB_i generated by the ciphertext body generating unit 240, by using a processing device, such as the CPU 911, and calculates a hash value H based on the combined ciphertext generated, by using a processing device, such as the CPU 911.

[0619] The ciphertext verification text generating unit 245 generates a ciphertext verification text CC_i based on the public encryption parameter stored by the encryption parameter storage unit 210 and the hash value H calculated by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0620] The ciphertext combining unit 246 makes one ciphertext C (key ciphertext C) by combining the n ciphertext bodies CB_i generated by the ciphertext body generating unit 240 and the ciphertext verification text CC generated by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0621] The ciphertext notification unit 261 notifies n ciphertext receiving apparatuses corresponding to the n recipients of the one ciphertext C combined by the ciphertext combining unit 246, by using a processing device, such as the CPU 911.

[0622] Each of a plurality of ciphertext receiving apparatuses 301 to 303 includes a storage device for storing information, such as the magnetic disk drive 920, a processing device for processing information, such as the CPU 911, the public parameter storage unit 310, the secret key storage unit 323, the ciphertext receiving unit 351, the ciphertext decomposition unit 346, the verification hash value calculation unit 344, the ciphertext verification unit 345, and the ciphertext decryption unit 341.

[0623] The public parameter storage unit 310 stores the public encryption parameter published by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0624] The secret key storage unit 323 stores the secret key d_ID notified by the encryption parameter generating apparatus 100, by using a storage device, such as the magnetic disk drive 920.

[0625] The ciphertext receiving unit 351 receives the one ciphertext C (key ciphertext C) notified by the ciphertext generating apparatus 200, by using a processing device, such as the CPU 911.

[0626] The ciphertext decomposition unit 346 acquires the n ciphertext bodies C2_i and the ciphertext verification text CC_i based on the one ciphertext C received by the ciphertext receiving unit 351, by using a processing device, such as the CPU 911, and acquires one corresponding ciphertext body CB_ID, which is corresponding to the recipient corresponding to the ciphertext receiving apparatus, among the acquired n ciphertext bodies CB_i, by using a processing device, such as the CPU 911.

[0627] The verification hash value calculation unit 344 calculates a verification hash value H′, based on a combined verification ciphertext made by combining the n ciphertext bodies CB_i acquired by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0628] The ciphertext verification unit 345 judges whether the one ciphertext C received by the ciphertext receiving unit 351 has consistency or not, based on the public encryption parameter stored by the public parameter storage unit 310, the one corresponding ciphertext body CB_ID and the ciphertext verification text CC acquired by the ciphertext decomposition unit 346, and the verification hash value H′ calculated by the verification hash value calculation unit 344, by using a processing device, such as the CPU 911.

[0629] When the ciphertext verification unit 345 judges that the one ciphertext C received by the ciphertext receiving unit 351 has consistency, the ciphertext decryption unit 341 restores the plaintext M, based on the public encryption parameter stored by the public parameter storage unit 310, the secret key d_ID stored by the secret key storage unit 323, and the one corresponding ciphertext body CB_ID acquired by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0630] The ID-based cryptographic communication system 800 according to the present Embodiment has the following effect:

[0631] In the ID-based cryptographic communication system, a plurality of recipients are notified of the same plaintext M through one ciphertext C and other recipients cannot decipher the ciphertext C.

[0632] The ID-based cryptographic communication system 800 (cryptographic communication system) according to the present Embodiment has the following features:

[0633] The ciphertext generating apparatus 200 further includes the random number generating unit 222.

[0634] The random number generating unit 222 randomly generates n integers s_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to n recipients, by using a processing device, such as the CPU 911.

[0635] The ciphertext body generating unit 240 includes the first ciphertext generating unit 241, the second ciphertext generating unit 242, and the third ciphertext generating unit 243.

[0636] The first ciphertext generating unit 241 generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the plaintext M input by the plaintext input unit 231, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0637] The second ciphertext generating unit 242 generates n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n recipient identification information ID_i input by the recipient identification input unit 232, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0638] The third ciphertext generating unit 243 generates n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0639] The hash value calculation unit 244 makes one combined ciphertext by combining the n first ciphertexts C1_i generated by the first ciphertext generating unit 241, the n second ciphertexts C2_i generated by the second ciphertext generating unit 242, and the n third ciphertexts C3_i generated by the third ciphertext generating unit 243, by using a processing device, such as the CPU 911, and calculates one hash value H₀, based on the one combined ciphertext, by using a processing device, such as the CPU 911.

[0640] The ciphertext verification text generating unit 245 generates n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the public encryption parameter stored by the encryption parameter storage unit 210, the n integers s_i generated by the random number generating unit 222, and the one hash value H₀ calculated by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0641] The ciphertext combining unit 246 makes one ciphertext C (key ciphertext C) by combining the n first ciphertexts C1_i generated by the first ciphertext generating unit 241, the n second ciphertexts C2_i generated by the second ciphertext generating unit 242, the n third ciphertexts C3_i generated by the third ciphertext generating unit 243, and the n ciphertext verification texts CC_i generated by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0642] The ciphertext decomposition unit 346 acquires the n first ciphertexts C1_i, the n second ciphertexts C2_i, the n third ciphertexts C3_i, and the n ciphertext verification texts CC_i, based on the one ciphertext C received by the ciphertext receiving unit 351, by using a processing device, such as the CPU 911, and acquires one corresponding first ciphertext C1_ID, one corresponding second ciphertext C2_ID, one corresponding third ciphertext C3_ID, and one corresponding ciphertext verification text CC_ID, which are corresponding to the recipient corresponding to the ciphertext receiving apparatus, from the acquired n first ciphertexts C1_i, n second ciphertexts C2_i, n third ciphertexts C3_i, and n ciphertext verification texts CC_i, by using a processing device, such as the CPU 911.

[0643] The verification hash value calculation unit 344 calculates a verification hash value H′, based on the combined verification ciphertext made by combining the n first ciphertexts C1_i, the n second ciphertexts C2_i, and the n third ciphertexts C3_i acquired by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0644] The ciphertext verification unit 345 judges whether the one ciphertext C received by the ciphertext receiving unit 351 has consistency or not, based on the public encryption parameter stored by the public parameter storage unit 310, at least either one of the one corresponding second ciphertext C2_ID and the one corresponding third ciphertext C3_ID, and the one corresponding ciphertext verification text acquired by the ciphertext decomposition unit 346, and the verification hash value H′ calculated by the verification hash value calculation unit 344, by using a processing device, such as the CPU 911.

[0645] When the ciphertext verification unit 345 judges that the one ciphertext C received by the ciphertext receiving unit 351 has consistency, the ciphertext decryption unit 341 restores the plaintext M, based on the public encryption parameter stored by the public parameter storage unit 310, the secret key d_ID stored by the secret key storage unit 323, and the one corresponding first ciphertext C1_ID, the one corresponding second ciphertext C2_ID, and the one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0646] The ID-based cryptographic communication system 800 according to the present Embodiment has the following effect:

[0647] Since the hash value calculation unit 244 calculates one hash value H₀ being common regardless of the recipient, the processing amount of the ciphertext generation processing in the ciphertext generating apparatus 200 can be reduced.

[0648] Moreover, in the ID-based cryptographic communication system whose recipient has been extended to multiple recipients from the ID-based cryptographic communication system whose security has been proved with respect to a single recipient, it is possible to prove the security similar to that of the single recipient case.

[0649] The ID-based cryptographic communication system 800 (cryptographic communication system) according to the present Embodiment has the following features:

[0650] The ciphertext body generating unit 240 generates n ciphertext bodies CB_i including the n recipient identification information ID_i input by the recipient identification input unit 232, by using a processing device, such as the CPU 911.

[0651] The ID-based cryptographic communication system 800 according to the present Embodiment has the following effect:

[0652] The n ciphertext bodies CB_i used as the basis of the hash value calculated by the hash value calculation unit 244 includes n recipient identification information, thereby increasing the security of the ID-based cryptographic communication system.

[0653] The ID-based cryptographic communication system 800 (cryptographic communication system) according to the present Embodiment has the following features:

[0654] The ciphertext generating apparatus 200 further includes the session key generating unit 221, the data input unit 251, the data encryption unit 252, and the ciphertext notification unit 261.

[0655] The session key generating unit 221 randomly generates a bit sequence of predetermined length, as a session key K, by using a processing device, such as the CPU 911.

[0656] The plaintext input unit 231 inputs the session key K generated by the session key generating unit 221, as the plaintext M, by using a processing device, such as the CPU 911.

[0657] The data input unit 251 inputs one plaintext data to be transmitted to the n recipients, by using a processing device, such as the CPU 911.

[0658] The data encryption unit 252 generates one encrypted data by encrypting the one plaintext data input by the data input unit 251, with the session key K generated by the session key generating unit 221, by using a processing device, such as the CPU 911.

[0659] The ciphertext notification unit 261 notifies the n recipients identified by the n recipient identification information input by the recipient identification input unit 232 of the one ciphertext C (key ciphertext C) combined by the ciphertext combining unit 246 and the one encrypted data encrypted by the data encryption unit 252, by using a processing device, such as the CPU 911.

[0660] Each of the plurality of ciphertext receiving apparatuses 301 to 303 further includes the data decryption unit 352.

[0661] The ciphertext receiving unit 351 receives the one ciphertext C and the one encrypted data notified by the ciphertext generating apparatus 200, by using a processing device, such as the CPU 911.

[0662] The data decryption unit 352 treats the plaintext M decrypted by the ciphertext decryption unit 341, as a session key K′, and decrypts the one encrypted data received by the ciphertext receiving unit 351 with the session key K′, by using a processing device, such as the CPU 911.

[0663] The ID-based cryptographic communication system 800 according to the present Embodiment has the following effect:

[0664] Since the data encryption unit 252 encrypts the plaintext data with one session key K being common regardless of the recipient, and the ciphertext notification unit 261 notifies the recipient of one ciphertext C being common regardless of the recipient and one encrypted data, the load applied to the traffic of the communication path is small.

[0665] Although it has been explained that the number n of recipients is not fixed beforehand (namely, not fixed when generating a public encryption parameter), and is fixed when actually transmitting data in the present Embodiment, it is also preferable to fix the number n of recipients in advance.

[0666] For example, if the recipient has been determined to be one person, it may be fixed as n=1. Alternatively, it may be fixed as n=10.

[0667] Moreover, it is also preferable to have a configuration in which the range of the number n of recipients is determined in advance, and n is specified within the range when transmitting data. For example, n may be variable within the range of 1≦n<100. That is, the ID-based cryptographic communication system 800 may be a cryptographic communication system in which the number of recipients is fixed beforehand, or in which the number of recipients can be varied when transmitting data.

[0668] Referring to FIGS. 11 to 18, Embodiment 2 will now be explained.

[0669] Before explaining the ID-based cryptographic communication system 800 according to the present Embodiment, an ID-based cryptographic communication system, which is a comparative example, will be described first.

[0670] Since the whole configuration of the ID-based cryptographic communication system, which is a comparative example, according to the present Embodiment is the same as that described in the Embodiment 1, explanation is herein omitted.

[0671] First, the encryption parameter generating apparatus 100 will be described.

[0672] Since the block configuration of the encryption parameter generating apparatus 100 is the same as that described in Embodiment 1, a portion different from Embodiment 1 will be explained with reference to FIG. 4.

[0673] The group parameter storage unit 111 stores information indicating a natural number r, a multiplicative group G1, a multiplicative group G2, a multiplicative group GT and a pairing e, as group parameters, by using a storage device, such as the magnetic disk drive 920.

[0674] The natural number r is a large prime number in which solving DH problem is difficult.

[0675] The multiplicative group G1, the multiplicative group G2, and multiplicative group GT are multiplicative groups whose order is the natural number r.

[0676] The multiplicative group G1 and the multiplicative group G2 may be the same multiplicative groups or different multiplicative groups. When the multiplicative group G1 and the multiplicative group G2 are the same multiplicative groups, the group parameter storage unit 111 does not need to store the information indicating the multiplicative group G2.

[0677] Pairing e is a mapping which calculates an element of the multiplicative group GT, from the element of the multiplicative group G1 and the element of the multiplicative group G2. The element of the multiplicative group GT calculated from an element p of the multiplicative group G1 and an element q of the multiplicative group G2 will be hereafter described as a pairing value e(p,q).

[0678] The pairing e shall be a bilinear pairing. That is, the pairing e shall have a feature that a pairing value e(p^a,q^b) of the a-th power of the element p of the multiplicative group G1 (a being an integer greater than or equal to 0 and less than the natural number r) and the b-th power of the element q of the multiplicative group G2 (b being an integer greater than or equal to 0 and less than the natural number r) is equal to the c-th power of the pairing value e(p,q) of the element p of the multiplicative group G1 and the element q of the multiplicative group G2 (c being a remainder obtained by dividing a product of the integer a and the integer b by the natural number r), which is expressed by the equation “e(p^a,q^b)=e(p,q)^(a·b)”.

[0679] Tate Pairing, Weil Pairing, etc. are known as a bilinear pairing that can be calculated within a practical time by using a processing device, such as the CPU 911 of a computer.

[0680] The pairing e is not limited to be a specific pairing but may any pairing as long as being a bilinear pairing, and shall be calculated within a practical time by using a processing device, such as the CPU 911 of a computer.

[0681] The secret information generating unit 112 inputs a group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0682] The secret information generating unit 112 randomly generates an integer α being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the input group parameter, by using a processing device, such as the CPU 911.

[0683] The secret information generating unit 112 outputs information (secret information) indicating the generated integer α, by using a processing device, such as the CPU 911.

[0684] The public parameter generating unit 113 inputs the group parameter stored by the group parameter storage unit 111, and the information indicating the integer α output by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[0685] The public parameter generating unit 113 determines a key generation function KDF, based on the multiplicative group GT indicated by the input group parameter, by using a processing device, such as the CPU 911.

[0686] The key generation function KDF is a function for calculating a bit sequence of predetermined length from an element of the multiplicative group GT.

[0687] The public parameter generating unit 113 determines a hash function HF, based on the natural number r indicated by the input group parameter, by using a processing device, such as the CPU 911. The hash function HF is a function for calculating a natural number less than the natural number r from a bit sequence of arbitrary length. The hash function HF shall have a very low probability of calculating the same natural numbers from different bit sequences.

[0688] The public parameter generating unit 113 randomly selects an element g of the multiplicative group G1, based on the multiplicative group G1 indicated by the input group parameter, by using a processing device, such as the CPU 911.

[0689] The public parameter generating unit 113 randomly selects four elements g2, h1, h2, and h3 of the multiplicative group G2, based on the multiplicative group G2 indicated by the input group parameter, by using a processing device, such as the CPU 911.

[0690] The elements g2, h1, h2, and h3 of the multiplicative group G2 shall be selected as different elements each other.

[0691] Moreover, the element g of the multiplicative group G1 and the element g2 of the multiplicative group G2 are selected so that the element e(g,g2) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element g2 of the multiplicative group G2, may not be an identity of the multiplicative group GT.

[0692] When the multiplicative group G1 and the multiplicative group G2 are the same multiplicative groups, the element g of the multiplicative group G1 and the element g2 of the multiplicative group G2 may be the same elements.

[0693] The public parameter generating unit 113 calculates an element g1=g^α of the multiplicative group G1, based on the multiplicative group G1 indicated by the input group parameter, the selected element g of the multiplicative group G1, and the integer α indicated by the input information, by using a processing device, such as the CPU 911.

[0694] The public parameter generating unit 113 outputs information indicating the input group parameter, the determined key generation function KDF, the determined hash function HF, the selected element g of the multiplicative group G1, the calculated element g1 of the multiplicative group G1, and the selected four elements g2, h1, h2, and h3 of the multiplicative group G2, as public encryption parameters, by using a processing device, such as the CPU 911.

[0695] The identification information input unit 131 inputs a recipient identification information ID, by using a processing device, such as the CPU 911.

[0696] In the description below, the recipient identification information ID will be treated as an integer, by the encryption parameter generating apparatus 100.

[0697] For example, the encryption parameter generating apparatus 100 treats recipient identification information as an integer by treating a bit sequence of arbitrary length, such as an e-mail address, as a bit sequence indicating an integral value.

[0698] Alternatively, the encryption parameter generating apparatus 100 may input a bit sequence of arbitrary length, such as an e-mail address, into a hash function (that may be the same hash function as the hash function HF determined by the public parameter generating unit 113, or may be a different hash function) and obtain an integer calculated by the hash function, to treat it as recipient identification information.

[0699] That is, the identification information input unit 131 inputs an integer ID_i as recipient identification information, by using a processing device, such as the CPU 911.

[0700] The identification information input unit 131 outputs the input integer ID by using a processing device, such as the CPU 911.

[0701] The secret key generating unit 132 inputs the information indicating the integer α stored by the secret information storage unit 121, the public encryption parameter stored by the parameter storage unit 122, and the integer ID output by the identification information input unit 131, by using a processing device, such as the CPU 911.

[0702] The secret key generating unit 132 randomly generates three integers r_ID,1, r_ID,2, and r_ID,3, based on the natural number r indicated by the input public encryption parameter, by using a processing device, such as the CPU 911. Each of the three integers r_ID,1, r_ID,2, and r_ID,3 has been generated to be greater than or equal to 1 and less than the natural number r and to be different from each other.

[0703] The secret key generating unit 132 calculates an inverse number 1/(α−ID), which is an inverse of the difference (α−ID) between α and ID, based on the integer α indicated by the input information and the input integer ID, by using a processing device, such as the CPU 911. The inverse number as used herein is an inverse number in the finite field Z/rZ. That is, the inverse number of (α−ID) means an integer x (let the inverse number of (α−ID) be x) greater than or equal to 1 and less than r, where the remainder obtained by dividing the product (α−ID) x by the natural number r is 1. (The product (α−ID) x indicates (α−ID) multiplied by the inverse number x.) The inverse number of (α−ID) is described as 1/(α−ID).

[0704] Since the natural number r is a prime number, unless α−ID is 0 (or in multiples of the natural number r), there exists one inverse number of (α−ID), which can be calculated within a practical time by using a processing device, such as the CPU 911 of a computer.

[0705] Since the natural number r is a very large natural number, the probability that α−ID is 0 (or in multiples of the natural number r) is extremely low and may be ignored.

[0706] Based on the multiplicative group G2 and the element g2 of the multiplicative group G2 indicated by the input public encryption parameter, and the generated integer r_ID,1, the secret key generating unit 132 calculates an element g2^(−r_ID,1) of the multiplicative group G2, which is the (−r_ID,1) th power of the element g2 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0707] Based on the multiplicative group G2 and the element h1 of the multiplicative group G2 indicated by the input public encryption parameter, and the calculated element g2^(−r_ID,1) of the multiplicative group G2, the secret key generating unit 132 calculates an element h1·g2^(−r_ID,1) of the multiplicative group G2, which is a product of the element h1 of the multiplicative group G2 and the element g2^(−r_ID,1) of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0708] Based on the calculated element h1·g2^(−r_ID,1) of the multiplicative group G2 and the calculated integer 1/(α−ID), the secret key generating unit 132 calculates an element h_ID,1=(h1·g2^(−r_ID,1))^(1/(α−ID)) of the multiplicative group G2, which is the 1/(α−ID) th power of the element h1·g2^(−r_ID,1) of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0709] Similarly, the secret key generating unit 132 calculates an element h_ID,2=(h2·g2^(−r_ID,2))^(1/(α−ID)) of the multiplicative group G2 and an element h_ID,3=(h3·g2^(−r_ID,3))^(1/(α−ID)) of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0710] The secret key generating unit 132 outputs information indicating the calculated three integers r_ID,1, r_ID,2 and r_ID,3, and the calculated three elements h_ID,1, h_ID,2, and h_ID,3 of the multiplicative group G2, as a secret key d_ID, by using a processing device, such as the CPU 911.

[0711] Next, the ciphertext generating apparatus 200 will be described.

[0712] Since the configuration of the functional block of the ciphertext generating apparatus 200 is also the same as that explained in Embodiment 1, different points from Embodiment 1 will be explained with reference to FIG. 5.

[0713] The ciphertext generating apparatus 200 treats a recipient identification information ID_i as an integer, similarly to the encryption parameter generating apparatus 100.

[0714] The recipient identification input unit 232 inputs n integers ID_i (n being an integer greater than or equal to 1, i being an integer greater than or equal to 1 and less than or equal to n), as n recipient identification information which respectively identifies n recipients, by using a processing device, such as the CPU 911.

[0715] The recipient identification input unit 232 outputs the input n integers ID_i, by using a processing device, such as the CPU 911.

[0716] The random number generating unit 222 inputs the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0717] The random number generating unit 222 randomly generates n integers s_i greater than or equal to 1 and less than the natural number r (i being an integer greater than or equal to 1 and less than or equal to n) which respectively correspond to n recipients, based on the natural number r indicated by the input public encryption parameter, by using a processing device, such as the CPU 911.

[0718] The random number generating unit 222 outputs information indicating the generated n integers s_i, by using a processing device, such as the CPU 911.

[0719] The first ciphertext generating unit 241 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the information indicating the n integers s_i output by the random number generating unit 222, and the plaintext M output by the plaintext input unit 231, by using a processing device, such as the CPU 911.

[0720] The first ciphertext generating unit 241 calculates an element e(g, h1) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element h1 of the multiplicative group G2, based on the pairing e, the element g of the multiplicative group G1, and the element h1 of the multiplicative group G2 indicated by the input public encryption parameter, by using a processing device, such as the CPU 911.

[0721] Since the element e(g, h1) of the multiplicative group GT is constant regardless of the integer s_i and the plaintext M, it may be calculated beforehand and stored by using a storage device, such as the magnetic disk drive 920.

[0722] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated element e(g, h1) of the multiplicative group GT, and the n integers s_i indicated by the input information, the first ciphertext generating unit 241 calculates n elements e(g,h1)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT, which is the s_i-th power of the element e(g, h1) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0723] Based on the key generation function KDF indicated by the input public encryption parameter, and the calculated n elements e(g,h1)^s_i of the multiplicative group GT, the first ciphertext generating unit 241 calculates n key bit sequences k_i=KDF (e(g, h1)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n), which is a bit sequence of predetermined length obtained by converting the element e(g, h1)^s_i of the multiplicative group GT by the key generation function KDF, by using a processing device, such as the CPU 911.

[0724] Based on the input plaintext M and the calculated n key bit sequences k_i, the first ciphertext generating unit 241 generates n bit sequences by encrypting the plaintext M respectively with the n key bit sequences k_i, to be treated as n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n), by using a processing device, such as the CPU 911.

[0725] When the bit length of the plaintext M and the bit length of the key bit sequence k are equal, the first ciphertext generating unit 241 calculates an exclusive OR for each bit between the plaintext M and the key bit sequence k_i, to be treated as a first ciphertext C1_i, for example.

[0726] The method for encrypting the plaintext M is not limited to the method using the exclusive OR operation, and it may be other cryptographic system as long as the cryptographic system can restore the plaintext M from the first ciphertext C1_i when there is the same bit sequence as the key bit sequence k_i.

[0727] The first ciphertext generating unit 241 outputs the generated n first ciphertexts C1_i, by using a processing device, such as the CPU 911.

[0728] The second ciphertext generating unit 242 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the information indicating then integers s_i output by the random number generating unit 222, and the n integers ID_i output by the recipient identification input unit 232, by using a processing device, such as the CPU 911.

[0729] The second ciphertext generating unit 242 calculates n elements g^(−ID_i) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1, which is the (−ID_i)th power of the element g of the multiplicative group G1, based on the multiplicative group G1 indicated by the input public encryption parameter, the element g of the multiplicative group G1, and the input n integers ID_i, by using a processing device, such as the CPU 911.

[0730] Based on the multiplicative group G1 and the element g1 of the multiplicative group G1 indicated by the input public encryption parameter, and the calculated n elements g^(−ID_i) of the multiplicative group G1, the second ciphertext generating unit 242 calculates elements g1·g^(−ID_i) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1, which is a product of the element g1 of the multiplicative group G1 and the element g^ (−ID_i) of the multiplicative group G1, by using a processing device, such as the CPU 911.

[0731] Based on the multiplicative group G1 indicated by the input public encryption parameter, the n integers s_i indicated by the input information, and the calculated n elements g1·g^(−ID_i) of the multiplicative group G1, the second ciphertext generating unit 242 calculates n elements u_i=(g1·g^(−ID_i))^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1, which is the s_i-th power of the element g1·g^(−ID_i) of the multiplicative group G1, by using a processing device, such as the CPU 911.

[0732] The second ciphertext generating unit 242 outputs bit sequences respectively indicating the calculated n elements u_i of the multiplicative group G1, as n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n), by using a processing device, such as the CPU 911.

[0733] The third ciphertext generating unit 243 inputs the public encryption parameter stored by the encryption parameter storage unit 210, and the information indicating the n integers s_i output by the random number generating unit 222, by using a processing device, such as the CPU 911.

[0734] Based on a pairing e, the element g of the multiplicative group G1, and the element g2 of the multiplicative group G2 indicated by the input public encryption parameter, the third ciphertext generating unit 243 calculates an element e(g,g2) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element g2 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0735] Since the element e(g,g2) of the multiplicative group GT is constant regardless of the integer s_i, it may be calculated beforehand and stored by using a storage device, such as the magnetic disk drive 920.

[0736] Based on the multiplicative group GT indicated by the input public encryption parameter, the n integers s_i indicated by the input information, and the calculated element e(g, g2) of the multiplicative group GT, the third ciphertext generating unit 243 calculates n elements e(g,g2)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT, which is the s_i-th power of the element e(g,g2) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0737] The third ciphertext generating unit 243 outputs bit sequences respectively indicating the calculated n elements e(g,g2)^s_i of the multiplicative group GT, as n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n), by using a processing device, such as the CPU 911.

[0738] The hash value calculation unit 244 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the n first ciphertexts C1_i output by the first ciphertext generating unit 241, the n second ciphertexts C2_i output by the second ciphertext generating unit 242, and the n third ciphertexts C3_i output by the third ciphertext generating unit 243, by using a processing device, such as the CPU 911.

[0739] The hash value calculation unit 244 generates n combined ciphertexts by combining each of the input n first ciphertexts C1_i, n second ciphertexts C2_i, and n third ciphertexts C3_i, by using a processing device, such as the CPU 911.

[0740] Based on the hash function HF indicated by the public encryption parameter and the generated n combined ciphertexts, the hash value calculation unit 244 calculates n natural numbers β_i by converting the n combined ciphertexts respectively by the hash function HF, as n hash values H_i, by using a processing device, such as the CPU 911.

[0741] The hash value calculation unit 244 outputs information indicating the calculated n natural numbers β_i and the generated n combined ciphertexts, by using a processing device, such as the CPU 911.

[0742] The ciphertext verification text generating unit 245 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the information indicating the n integers s_i output by the random number generating unit 222, and the information indicating the n natural numbers β_i output by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0743] Based on the pairing e, the element g of the multiplicative group G1, and the element h2 of the multiplicative group G2 indicated by the input public encryption parameter, the ciphertext verification text generating unit 245 calculates an element e(g,h2) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element h2 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0744] Based on the pairing e, the element g of the multiplicative group G1, and the element h3 of the multiplicative group G2 indicated by the input public encryption parameter, the ciphertext verification text generating unit 245 calculates an element e(g,h3) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element h3 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0745] Since the elements e(g,h2) and e(g,h3) of the multiplicative group GT are constant regardless of the integer s_i, they may be calculated beforehand and stored by using a storage device, such as the magnetic disk drive 920.

[0746] Based on the multiplicative group GT indicated by the input public encryption parameter, the n integers s_i indicated by the input information, and the calculated element e(g,h2) of the multiplicative group GT, the ciphertext verification text generating unit 245 calculates n elements e(g, h2)^s_i of the multiplicative group GT (i being an integer greater than or equal to 1 and less than or equal to n), which is the s_i-th power of the element e(g,h2) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0747] Based on the n integers s_i indicated by the input information and the n natural numbers β_i indicated by the input information, the ciphertext verification text generating unit 245 calculates n integers (s_i·β_i) respectively (i being an integer greater than or equal to 1 and less than or equal to n), which is a product of the integer s_i and the natural number β_i, by using a processing device, such as the CPU 911.

[0748] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated element e(g,h3) of the multiplicative group GT, and the calculated n integers (s_i·β_i), the ciphertext verification text generating unit 245 calculates n elements e(g, h3)^(s_i·β_i) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT, which is the (s_i·β_i) th power of the element e(g,h3) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0749] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated n elements e(g,h2)^s_i of the multiplicative group GT, and the calculated n elements e(g,h3)^(s_i·β_i) of the multiplicative group GT, the ciphertext verification text generating unit 245 calculates n elements y_i=e(g,h2)^s_i·e(g,h3)^(s_i·β_i) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT, which is a product of the element e(g,h2)^s_i of the multiplicative group GT and the element e(g,h3)^(s_i·β_i) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0750] The ciphertext verification text generating unit 245 outputs bit sequences respectively indicating the calculated n elements y_i of the multiplicative group GT, as n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n), by using a processing device, such as the CPU 911.

[0751] The ciphertext combining unit 246 inputs the n combined ciphertexts output by the hash value calculation unit 244 and the n ciphertext verification texts CC_i output by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0752] The ciphertext combining unit 246 generates one key ciphertext C by combining all the input n combined ciphertexts and the input n ciphertext verification texts, by using a processing device, such as the CPU 911.

[0753] The ciphertext combining unit 246 outputs the generated one key ciphertext C, by using a processing device, such as the CPU 911.

[0754] In this example, although the ciphertext combining unit 246 inputs the n combined ciphertexts output by the hash value calculation unit 244, since the combined ciphertext is made by combining the first ciphertext C1_i, the second ciphertext C2_i, and the third ciphertext C3_i, consequently it means the ciphertext combining unit 246 inputs the n first ciphertexts C1_i, the n second ciphertexts C2_i, and the n third ciphertexts C3_i.

[0755] FIG. 11 shows an example of the flow of information exchanged in the ciphertext generating apparatus 200 which is a comparative example of the present Embodiment.

[0756] The recipient identification input unit 232 inputs n recipient identification information ID₁, ID₂, . . . , ID_n. The random number generating unit 222 generates n integers s₁, s₂, . . . , s_n whose number is the same as that of the recipient identification information ID_i, ID₂, . . . , ID_n input by the recipient identification input unit 232.

[0757] The first ciphertext generating unit 241 generates a first ciphertext C1₁ based on the plaintext M and the integer s₁, a first ciphertext C1₂ based on the plaintext M and the integer s₂, . . . , and a first ciphertext C1_n based on the plaintext M and the integer s_n.

[0758] The second ciphertext generating unit 242 generates a second ciphertext C2_i based on the recipient identification information ID₁ and the integer s₁, a second ciphertext C2₂ based on the recipient identification information ID₂ and the integer s₂, . . . , and a second ciphertext C2_n based on the recipient identification information ID_n and the integer s_n.

[0759] The third ciphertext generating unit 243 generates a third ciphertext C3_i based on the integer s₁, a third ciphertext C3₂ based on the integer s₂, . . . , and a third ciphertext C3_n based on the integer s_n.

[0760] The hash value calculation unit 244 generates a combined ciphertext (C1₁|C2₁|C3₁) by combining the first ciphertext C1₁, the second ciphertext C2₁, and the third ciphertext C3₁, generates a combined ciphertext (C1₂|C2₂|C3₂) by combining the first ciphertext C1₂, the second ciphertext C2₂, and the third ciphertext C3₂, . . . , and generates a combined ciphertext (C1_n|C2_n|C3_n) by combining the first ciphertext C1_n, the second ciphertexts C2_n, and the third ciphertext C3_n.

[0761] “|” as used herein denotes combining. For example, (C1₁|C2₁|C3₁) indicates a bit sequence made by combining a bit sequence C2₁ to the end of a bit sequence C1₁, and further combining a bit sequence C3₁ to the end of that. Combining order different from this may be used.

[0762] The hash value calculation unit 244 calculates a hash value H₁ based on the generated combined ciphertext (C1₁|C2₁|C3₁), a hash value H₂ based on the generated combined ciphertext (C1₂|C2₂|C3₂), . . . , and a hash value H_n based on the generated combined ciphertext (C1_n|C2_n|C3_n).

[0763] The ciphertext verification text generating unit 245 generates a ciphertext verification text CC₁ based on the integer s₁ and the hash value H₁, a ciphertext verification text CC₂ based on the integer s₂ and the hash value H₂, . . . , and a ciphertext verification text CC_n based on the integer s_n and the hash value H_n.

[0764] The ciphertext combining unit 246 generates one key ciphertext C=(C1₁|C2₁|C3₁|CC₁|C1₂|C2₂|C3₂|CC₂| . . . |C1_n|C2_n|C3_n|CC_n) by combining all the n combined ciphertexts (C1₁|C2₁|C3₁), (C1₂|C2₂|C3₂), . . . , (C1_n|C2_n|C3_n) and the n ciphertext verification texts CC_i, CC₂, CC_n. Although, in this example, the key ciphertext C is generated by combining the ciphertexts in the order of collecting texts from the first ciphertext to the ciphertext verification text corresponding to one recipient in one, combining order different from this may be used. For example, it is also acceptable to perform combining in the order of collecting the first ciphertexts in one, and the second ciphertexts in one, like (C1₁|C1₂| . . . |C1_n|C2₁|C2₂| . . . |C2_n|C3₁|C3₂| . . . |C3_n|CC₁|CC₂| . . . CC_n). The combining order of collecting the texts from the first ciphertext corresponding to the ciphertext verification text corresponding to one recipient in one, like the example above, is more preferable because the key ciphertext C is generated by combining the combined ciphertexts combined by the hash value calculation unit 244, as they are. Moreover, there is also an advantage that the ciphertext receiving apparatus can easily find a ciphertext corresponding to the recipient corresponding to the apparatus.

[0765] Next, the ciphertext receiving apparatuses 301 to 303 will be described. Since the configuration of the functional block of the ciphertext receiving apparatuses 301 to 303 is also the same as that explained in Embodiment 1, different points from Embodiment 1 will be explained with reference to FIG. 6.

[0766] The ciphertext decomposition unit 346 inputs one key ciphertext C output by the ciphertext receiving unit 351, by using a processing device, such as the CPU 911. The ciphertext decomposition unit 346 acquires the corresponding first ciphertext C1_ID, the corresponding second ciphertext C2_ID, the corresponding third ciphertext C3_ID, and the corresponding ciphertext verification text CC_ID which are corresponding to the recipient concerned, from the input one key ciphertext C, by using a processing device, such as the CPU 911.

[0767] The ciphertext decomposition unit 346 outputs the acquired corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID, corresponding third ciphertext C3_ID, and corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0768] For example, it is assumed that one key ciphertext C is made by combining ciphertexts in the order of (C1₁|C2₁|C3₁|CC₁|C1₂|C2₂|C3₂|CC₂| . . . C1_n|C2_n|C3_n|CC_n) as shown in FIG. 11. The ciphertext decomposition unit 346 separates one key ciphertext C into several ones, for example, by sum of bit lengths of from the first ciphertext to the ciphertext verification text to be (C1₁|C2₁|C3₁|CC₁), (C1₂|C2₂|C3₂|CC₂), . . . , (C1_n|C2_n|C3_n|CC_n), and acquires the corresponding ciphertext (C1_ID|C2_ID|C3_ID|CC_ID) concerned from the separated ones.

[0769] For example, the ciphertext decomposition unit 346 judges at what position the corresponding ciphertext concerned exists, based on other portions of the notification data 720 received by the ciphertext receiving unit 351, and acquires the corresponding ciphertext depending upon the judgment result.

[0770] Next, the ciphertext decomposition unit 346 separates the acquired corresponding ciphertext (C1_ID|C2_ID|C3_ID|CC_ID), and obtains a combined verification ciphertext (C1_ID|C2_ID|C3_ID) and a corresponding ciphertext verification text CC_ID.

[0771] Furthermore, the ciphertext decomposition unit 346 separates the acquired combined verification ciphertext (C1_ID|C2_ID|C3_ID), and obtains a corresponding first ciphertext C1_ID, a corresponding second ciphertext C2_ID, and a corresponding third ciphertext C3_ID.

[0772] The ciphertext decomposition unit 346 outputs the acquired corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID, corresponding third ciphertext C3_ID, and corresponding ciphertext verification text CC_ID. Moreover, in this example, in order to save the time and effort of the verification hash value calculation unit 344's generating a combined verification ciphertext (C1_ID|C2_ID|C3_ID), the ciphertext decomposition unit 346 also outputs the acquired combined verification ciphertext (C1_ID|C2_ID|C3_ID) in addition to them, by using a processing device, such as the CPU 911.

[0773] The verification hash value calculation unit 344 inputs the corresponding first ciphertext C1_ID, the corresponding second ciphertext C2_ID, and the corresponding third ciphertext C3_ID output by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911. The verification hash value calculation unit 344 generates a combined verification ciphertext (C1_ID|C2_ID|C3_ID) by combining the input corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID and corresponding third ciphertext C3_ID, by using a processing device, such as the CPU 911. The order of combining may be different from this as long as it is the same as that used when the hash value calculation unit 244 of the ciphertext generating apparatus 200 generates a combined ciphertext.

[0774] Moreover, as the example mentioned above, when the ciphertext decomposition unit 346 outputs the combined verification ciphertext (C1_ID|C2_ID|C3_ID), the verification hash value calculation unit 344 may input the combined verification ciphertext (C1_ID|C2_ID|C3_ID) output by the ciphertext decomposition unit 346 instead of generating the combined verification ciphertext (C1_ID|C2_ID|C3_ID).

[0775] The verification hash value calculation unit 344 inputs a public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0776] Based on the hash function HF indicated by the input public encryption parameter and the generated or input combined verification ciphertext (C1_ID|C2_ID|C3_ID), the verification hash value calculation unit 344 calculates a natural number β′ by converting the combined verification ciphertext (C1_ID|C2_ID|C3_ID) by the hash function HF, to be treated as a hash value H′, by using a processing device, such as the CPU 911.

[0777] The verification hash value calculation unit 344 outputs information indicating the calculated natural number β′, by using a processing device, such as the CPU 911.

[0778] The ciphertext verification unit 345 inputs the public encryption parameter stored by the public parameter storage unit 310, the secret key d_u, stored by the secret key storage unit 323, the corresponding second ciphertext C2_ID, the corresponding third ciphertext C3_ID and the corresponding ciphertext verification text CC_ID output by the ciphertext decomposition unit 346, and the information indicating the natural number β′ output by the verification hash value calculation unit 344, by using a processing device, such as the CPU 911.

[0779] Based on the multiplicative group G2 indicated by the input public encryption parameter, the element h_ID,3 of the multiplicative group G2 indicated by input secret key d_ID, and the natural number β′ indicated by the input information, the ciphertext verification unit 345 calculates an element h_ID,3^β′ of the multiplicative group G2, which is the β′-th power of the element h_ID,3 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0780] Based on the multiplicative group G2 indicated by the input public encryption parameter, the element h_ID,2 of the multiplicative group G2 indicated by the input secret key d_ID, and the calculated element h_ID,3^β′ of the multiplicative group G2, the ciphertext verification unit 345 calculates the element h_ID,2·h_ID,3^β′ of the multiplicative group G2, which is a product of the element h_ID,2 of the multiplicative group G2 and the element h_ID,3^β′ of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0781] Based on the pairing e indicated by the input public encryption parameter, the element u_ID of the multiplicative group G1 indicated by the input corresponding second ciphertext C2_ID, and the calculated element h_ID,2·h_ID,3^β′ of the multiplicative group G2, the ciphertext verification unit 345 calculates an element e(u_ID,h_ID,2·h_ID,3^β′) of the multiplicative group GT, which is a pairing value of the element u_ID of the multiplicative group G1 and the element h_ID,2·h_ID,3^β′ of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0782] Based on the integers r_ID,2 and r_ID,3 indicated by the input secret key d_ID, and the natural number β′ indicated by the input information, the ciphertext verification unit 345 calculates an integer r_ID,2+r_ID,3·β′, which is a sum obtained by adding the product of the integer r_ID,3 and the natural number β′ to the integer r_ID,2, by using a processing device, such as the CPU 911.

[0783] Based on the multiplicative group GT indicated by the input public encryption parameter, the element v_ID, of the multiplicative group GT indicated by the input corresponding third ciphertext C3_ID, and the calculated integer r_ID,2+r_ID,3·^β′, the ciphertext verification unit 345 calculates an element v_ID^(r_ID,2+r_ID,3·^β′) of multiplicative group GT, which is the (r_ID,2+r_ID,3·β′)th power of the element v_ID of multiplicative group GT, by using a processing device, such as the CPU 911.

[0784] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated element e(u_ID,h_ID,2·h_ID,3^β′) of the multiplicative group GT, and the calculated element v_ID^(r_ID,2+r_ID,3·β′) of the multiplicative group GT, the ciphertext verification unit 345 calculates an element y′=e(u_ID, h_ID,2·h_ID,3^β′)·v_ID^(r_ID,2·r_ID,3^β′) of the multiplicative group GT, which is a product of the element e(u_ID,h_ID,2·h_ID,3^β′) of the multiplicative group GT and the element v_ID^(r_ID,2+r_ID,3·β′) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0785] The ciphertext verification unit 345 compares the element y_ID of the multiplicative group GT indicated by the input corresponding ciphertext verification text CC_ID and the calculated element y′ of the multiplicative group GT, and when the element y_ID of the multiplicative group GT accords with the calculated element y′ of the multiplicative group GT, judges that the one key ciphertext C input by the ciphertext receiving unit 351 has consistency, and when they do not accord with each other, judges that the one key ciphertext C input by the ciphertext receiving unit 351 has no consistency, by using a processing device, such as the CPU 911.

[0786] In this case, the element y_i of the multiplicative group GT calculated by the ciphertext verification text generating unit 245 of the ciphertext generating apparatus 200 is y_i=e(g,h2)^s_i·e(g,h3)^(s_i·β_i). Moreover, since the element u_i of the multiplicative group G1 calculated by the second ciphertext generating unit 242 of the ciphertext generating apparatus 200 is u_i=(g1·g^(−ID_i))^s_i and the element g1 of the multiplicative group G1 calculated by the public parameter generating unit 113 of the encryption parameter generating apparatus 100 is g1=g^α, thereby u_i=g^(s_i·(α−ID_i)). The element v_i of the multiplicative group GT calculated by the third ciphertext generating unit 243 of the ciphertext generating apparatus 200 is v_i=e(g,g2)^s_i.

[0787] On the other hand, the element y′ of the multiplicative group GT calculated by the ciphertext verification text generating unit 245 is y′=e(u_ID,h_ID,2·h_ID,3^β′)·v_ID^(r_ID,2+r_ID,3·β′). According to the bilinear of the pairing e, y′=e(u_ID,h_ID,2)·e(u_ID,h_ID,3)^β′·v_ID^(r_ID,2+r_ID,3·β′). Moreover, since the element h_ID,2, h_ID,3 of the multiplicative group G2 calculated by the secret key generating unit 132 of the encryption parameter generating apparatus 100 is h_ID,2=(h2·g2^(−r_ID,2))^(1/(α−ID)), h_ID,3=(h3·g2^(−r_ID,3))^(1/(α−ID)), thereby e(u_ID,h_ID,2)=e(g, h2·g2^(−r_ID,2))^s_ID,e(u_ID, h_ID,3)=e(g,h3·g2^(−r_ID,3))^s_ID. Therefore, y′=e(g,h2)^s_ID·e(g,h3)^(s_ID·β′). When one key ciphertext C has consistency, since β′=β_ID, thereby y′=y_ID.

[0788] When the ciphertext verification unit 345 judges that the one key ciphertext C input by the ciphertext receiving unit 351 has consistency, the ciphertext decryption unit 341 performs the following processing in order to restore the plaintext M.

[0789] The ciphertext decryption unit 341 inputs the public encryption parameter stored by the public parameter storage unit 310, the secret key d_ID stored by the secret key storage unit 323, and the corresponding first ciphertext C1_ID, the corresponding second ciphertext C2_ID, and the corresponding third ciphertext C3_ID output by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0790] Based on the pairing e indicated by the input public encryption parameter, the element h_ID,1 of the multiplicative group G2 indicated by the input secret key d_ID, and the element u_ID of the multiplicative group G1 indicated by the input corresponding second ciphertext C2_ID, the ciphertext decryption unit 341 calculates an element e(u_ID, h_ID,1) of the multiplicative group GT, which is a pairing value of the element u_ID of the multiplicative group G1 and the element h_ID,1 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0791] Based on the multiplicative group GT indicated by the input public encryption parameter, the integer r_ID,1 indicated by the input secret key d_ID, and the element v_ID of the multiplicative group GT indicated by the input corresponding third ciphertext C3_ID, the ciphertext decryption unit 341 calculates an element v_ID^r_ID,1 of multiplicative group GT, which is the r_ID,1-th power of the element v_ID of multiplicative group GT, by using a processing device, such as the CPU 911.

[0792] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated element e(u_ID,h_ID,1) of the multiplicative group GT, and the calculated element v_ID^r_ID,1 of the multiplicative group GT, the ciphertext decryption unit 341 calculates an element e(u_ID,h_ID,1) v_ID^r_ID,1 of the multiplicative group GT, which is a product of the element e(u_ID,h_ID,1) of the multiplicative group GT and the element v_ID^r_ID,1 of multiplicative group GT, by using a processing device, such as the CPU 911.

[0793] Based on the key generation function KDF indicated by the input public encryption parameter and the calculated element e(u_ID,h_ID,1)·v_ID^r_ID,1 of the multiplicative group GT, the ciphertext decryption unit 341 calculates a key bit sequence k′=KDF(e(u_ID,h_ID,1)·v_ID^r_ID,1), which is a bit sequence of predetermined length obtained by converting the element e(u_ID,h_ID,1)·v_ID^r_ID,1 of the multiplicative group GT by the key generation function KDF, by using a processing device, such as the CPU 911.

[0794] Based on the input corresponding first ciphertext C1_ID and the calculated key bit sequence k′, the ciphertext decryption unit 341 generates a bit sequence by decrypting the corresponding first ciphertext C1_ID with the key bit sequence k′, by using a processing device, such as the CPU 911.

[0795] In this case, the key bit sequence k_i calculated by the first ciphertext generating unit 241 of the ciphertext generating apparatus 200 is k_i=KDF (e(g,h1)^s_i).

[0796] On the other hand, the key bit sequence k′ calculated by the ciphertext decryption unit 341 is k′KDF(e(u_ID,h_ID,1)·v_ID^r_ID,1).

[0797] Moreover, the element h_ID,1 of the multiplicative group G2 calculated by the secret key generating unit 132 of the encryption parameter generating apparatus 100 is h_ID,1=(h1·g2^(−r_ID,1)) ^(1/(α−ID)). Therefore, since k′=KDF(e(g,h1)^s_ID), thereby k′=k_ID.

[0798] The ciphertext decryption unit 341 decrypts the corresponding first ciphertext C1_ID by using the same key bit sequence k′ as that used for encrypting the plaintext M by the first ciphertext generating unit 241 of the ciphertext generating apparatus 200, thereby enabling to restore the plaintext M.

[0799] Next, operations will be described.

[0800] FIG. 12 is a flowchart showing an example of the flow of parameter setting processing in which the encryption parameter generating apparatus 100 being a comparative example according to the present Embodiment sets a public encryption parameter.

[0801] The same reference numbers are attached to the steps common to the steps of the parameter setting processing explained in Embodiment 1.

[0802] In the secret information generation step S11, the secret information generating unit 112 randomly generates an integer α being greater than or equal to 1 and less than a natural number r, based on the natural number r indicated by the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0803] The secret information storage unit 121 stores information indicating the integer α generated by the secret information generating unit 112, by using a storage device, such as the magnetic disk drive 920.

[0804] In the encryption parameter generation step S12, the public parameter generating unit 113 determines a key generation function KDF and a hash function HF, based on the multiplicative group GT and the natural number r indicated by the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0805] The public parameter generating unit 113 randomly selects an element g of the legitimate group G1 and four elements g2, h1, h2, and h3 of the multiplicative group G2, based on the multiplicative groups G1 and G2 indicated by the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[0806] The public parameter generating unit 113 calculates an element g1=g^α of the multiplicative group G1, based on the multiplicative group G1 indicated by the group parameter stored by the group parameter storage unit 111, the integer α generated by the secret information generating unit 112 at the secret information generation step S11, and the selected element g of the multiplicative group G1, by using a processing device, such as the CPU 911.

[0807] The parameter storage unit 122 stores information indicating the group parameter stored by the group parameter storage unit 111, the determined key generation function KDF, the determined hash function HF, the selected element g of the multiplicative group G1, the calculated element g1 of the multiplicative group G1, and the selected four elements g2, h1, h2, and h3 of the multiplicative group G2, as public encryption parameters, by using a storage device, such as the magnetic disk drive 920.

[0808] In the encryption parameter publishing step S13, the public parameter publishing unit 114 publishes information indicating the group parameter stored by the group parameter storage unit 111, the key generation function KDF and the hash function HF determined by the public parameter generating unit 113, the element g of the multiplicative group G1 and the four elements g2, h1, h2 and h3 of the multiplicative group G2 selected by the public parameter generating unit 113, and the element g1 of the multiplicative group G1 calculated by the public parameter generating unit 113, as public encryption parameters, by using a processing device, such as the CPU 911.

[0809] The public encryption parameters published by the public parameter publishing unit 114 are acquired by the ciphertext generating apparatus 200 and the ciphertext receiving apparatuses 301 to 303, and stored by the encryption parameter storage unit 210 and the public parameter storage unit 310, by using a storage device, such as the magnetic disk drive 920.

[0810] FIG. 13 is a flowchart showing an example of the flow of secret key setting processing in which the encryption parameter generating apparatus 100 being a comparative example according to the present Embodiment sets a secret key. The same reference numbers are attached to the steps common to the steps of the secret key setting processing explained in Embodiment 1.

[0811] In the identification input step S21, the identification information input unit 131 inputs an integer ID as recipient identification information, by using a processing device, such as the CPU 911.

[0812] In the secret key generation step S22, the secret key generating unit 132 randomly generates three integers r_ID,1, r_ID,2, and r_ID,3, being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0813] Based on the multiplicative group G2 and three elements h1, h2, and h3 of the multiplicative group G2 indicated by the public encryption parameter stored by the parameter storage unit 122, and the generated three integers r_ID,1, r_ID,2, and r_ID,3, the secret key generating unit 132 calculates three elements of the multiplicative group G2, h_ID,1)=(h1·g2^(−r_ID,1))^(1/(α−ID)), h_ID,2=(h2·g2^(−r_ID,2))^(1/(α−ID)), h_ID,3=(h3·g2^(−r_ID,3))^(1/(α−ID)), by using a processing device, such as the CPU 911.

[0814] In the secret key notification step S23, the secret key notification unit 133 secretly notifies the corresponding ciphertext receiving apparatus of the information indicating the three integers r_ID,1, r_ID,2 and r_ID,3 generated by the secret key generating unit 132 at the secret key generation step S22, and the three elements h_ID,1, h_ID,2, and h_ID,3 of the multiplicative group G2 calculated by the secret key generating unit 132 at the secret key generation step S22, as a secret key d_ID, by using a processing device, such as the CPU 911.

[0815] The secret key d_ID notified by the secret key notification unit 133 is acquired by the corresponding ciphertext receiving apparatus, and stored by the secret key storage unit 323 by using a storage device, such as the magnetic disk drive 920.

[0816] FIG. 14 is a flowchart showing an example of the flow of ciphertext generation processing in which the ciphertext generating apparatus 200 being a comparative example according to the present Embodiment generates a ciphertext. The same reference numbers are attached to the steps common to the steps of the ciphertext generation processing explained in Embodiment 1.

[0817] In the recipient identification input step S32, the recipient identification input unit 232 inputs n integers ID_i respectively corresponding to n recipients, as n recipient identification information for respectively identifying the n recipients, by using a processing device, such as the CPU 911.

[0818] In the random number generation step S34, the random number generating unit 222 randomly generates n integers s_i, greater than or equal to 1 and less than the natural number r, respectively corresponding to the n recipients, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0819] In the first ciphertext generation step S35, based on the multiplicative group GT, the pairing e, the element g of the multiplicative group G1 and the element h1 of the multiplicative group G2, and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit 210, and the n integers s_i respectively corresponding to the n recipients generated by the random number generating unit 222 at the random number generation step S34, the first ciphertext generating unit 241 calculates n key bit sequences k_i=KDF(e(g,h1)^s_i) respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0820] Based on the session key K (=plaintext M) generated by the session key generating unit 221 at the session key generation step S33, and the calculated n key bit sequences k_i respectively corresponding to the n recipients, the first ciphertext generating unit 241 generates, by using a processing device, such as the CPU 911, n first ciphertexts C1_i respectively corresponding to the n recipients by respectively encrypting one session key K by using the n key bit sequences k_i respectively corresponding to the n recipients.

[0821] In the second ciphertext generation step S36, based on the multiplicative group G1, the elements g and g1 of the multiplicative group G1 indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers ID_i respectively corresponding to the n recipients input by the recipient identification input unit 232 at the recipient identification input step S32, and the n integers s_i respectively corresponding to the n recipients generated by the random number generating unit 222 at the random number generation step S34, the second ciphertext generating unit 242 calculates n elements u_i=(g1·g^(−ID_i))^s_i of the multiplicative group G1 respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0822] The second ciphertext generating unit 242 treats bit sequences respectively indicating the calculated n elements u_i of the multiplicative group G1 respectively corresponding to the n recipients, as n second ciphertexts C2_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0823] In the third ciphertext generation step S37, based on the multiplicative group GT, the pairing e, the element g of the multiplicative group G1 and the element g2 of the multiplicative group G2 indicated by the public encryption parameters stored by the encryption parameter storage unit 210 and the n integers s_i respectively corresponding to the n recipients generated by the random number generating unit 222 at the random number generation step S34, the third ciphertext generating unit 243 calculates n elements v_i=e(g,g2)^s_i of the multiplicative group GT respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0824] The third ciphertext generating unit 243 treats bit sequences respectively indicating the calculated n elements v_i of the multiplicative group GT respectively corresponding to the n recipients, as n third ciphertexts C3_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0825] In the hash value calculation step S38, based on the n first ciphertexts C1_i respectively corresponding to the n recipients generated by the first ciphertext generating unit 241 at the first ciphertext generation step S35, the n second ciphertexts C2_i respectively corresponding to the n recipients generated by the second ciphertext generating unit 242 at the second ciphertext generation step S36, and the n third ciphertexts C3_i respectively corresponding to the n recipients generated by the third ciphertext generating unit 243 at the third ciphertext generation step S37, the hash value calculation unit 244 generates n combined ciphertexts (C1₁|C2₁|C3₁) respectively corresponding to the n recipients made by respectively combining the n first ciphertexts C1_i respectively corresponding to the n recipients, the n first ciphertexts C2_i respectively corresponding to the n recipients, and the n third ciphertexts C3_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0826] Based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit 210, and the generated n combined ciphertexts (C1_i|C2_i|C3_i) respectively corresponding to the n recipients, the hash value calculation unit 244 calculates n integers β_i=HF (C1_i|C2_i|C3_i) respectively corresponding to the n recipients obtained by respectively converting the n combined ciphertexts (C1_i|C2_i|C3_i) respectively corresponding to n the recipients, by the hash function HF, by using a processing device, such as the CPU 911.

[0827] In the ciphertext verification text generation step S39, based on the multiplicative group GT, the pairing e, the element g of the multiplicative group G1, and two elements h2 and h3 of the multiplicative group G2 indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers s_i respectively corresponding to the n recipients generated by the random number generating unit 222 at the random number generation step S34, and the n integers β_i respectively corresponding to the n recipients calculated by the hash value calculation unit 244 at the hash value calculation step S38, the ciphertext verification text generating unit 245 calculates n elements y_i=e(g,h2)^s_i·e(g,h3)^(s_i·β_i) of the multiplicative group GT respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0828] The ciphertext verification text generating unit 245 treats bit sequences respectively indicating the calculated n elements y_i of the multiplicative group GT respectively corresponding to the n recipients, as n ciphertext verification texts CC_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0829] FIG. 15 is a flowchart showing an example of the flow of ciphertext reception processing in which the ciphertext receiving apparatuses 301 to 303 being comparative examples according to the present Embodiment receive ciphertexts. The same reference numbers are attached to the steps common to the steps of the ciphertext reception processing explained in Embodiment 1.

[0830] In the ciphertext decomposing step S52, the ciphertext decomposition unit 346 acquires one corresponding first ciphertext C1_ID, one corresponding second ciphertext C2_ID, one corresponding third ciphertext C3_ID, and one corresponding ciphertext verification text CC_ID corresponding to the recipient corresponding to the ciphertext receiving apparatus, based on the one key ciphertext C output by the ciphertext receiving unit 351 at the notification data receiving step, by using a processing device, such as the CPU 911.

[0831] In the verification hash value calculation step S53, based on the hash function HF indicated by the public encryption parameter stored by the public parameter storage unit 310, and the one corresponding combined ciphertext (C1_ID|C2_ID|C3_ID) made by combining the one corresponding first ciphertext C1_ID, the one corresponding second ciphertext C2_ID, and the one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, the verification hash value calculation unit 344 calculates one integer β′ by converting the one corresponding combined ciphertext (C1_ID|C2_ID|C3_ID) by the hash function HF, as a verification hash value H′, by using a processing device, such as the CPU 911.

[0832] In the ciphertext verification step S54, based on the multiplicative group GT and the pairing e indicated by the public encryption parameters stored by the public parameter storage unit 310, the two integers r_ID,2 and r_ID,3 and the two elements h_ID,2 and h_ID,3 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit 323, one element u_ID of the multiplicative group G1 indicated by the one verification second ciphertext C2_ID and one element v_ID of the multiplicative group GT indicated by the one verification third ciphertext C3_ID acquired by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, and the one integer β′ calculated by the verification hash value calculation unit 344 at the verification hash value calculation step S53, the ciphertext verification unit 345 calculates one element y′=e(u_ID,h_ID,2·h_ID,3^β′)·v_ID^(r_ID,2+r_ID,3·β′) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0833] The ciphertext verification unit 345 compares the one element y_ID of the multiplicative group GT indicated by the one corresponding ciphertext verification text CC_ID acquired by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52 with the calculated one element y′ of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0834] When the element y_ID of the multiplicative group GT is equal to the element y′ of the multiplicative group GT, the ciphertext verification unit 345 judges that the one key ciphertext C received by the ciphertext receiving unit 351 has consistency, and it goes to the ciphertext decrypting step S55.

[0835] When the element y_ID of the multiplicative group GT is not equal to the element y′ of the multiplicative group GT, the ciphertext verification unit 345 judges that the one key ciphertext C received by the ciphertext receiving unit 351 has no consistency, and the ciphertext reception processing is ended.

[0836] In the ciphertext decrypting step S55, based on the multiplicative group GT, the pairing e and the key generation function KDF stored by the public parameter storage unit 310, the integer r_ID,1 and the element h_ID,1 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit 323, and the one element u_ID of the multiplicative group G1 indicated by one corresponding second ciphertext C2_ID and the one element v_ID of the multiplicative group GT indicated by one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, the ciphertext decryption unit 341 calculates one key bit sequence k′=KDF(e(u_ID,h_ID,1)·v_ID^r_ID,1), by using a processing device, such as the CPU 911.

[0837] Based on the one corresponding first ciphertext C1_ID acquired by the ciphertext decomposition unit 346 at the ciphertext decomposing step S52, and the calculated one key bit sequence k′, the ciphertext decryption unit 341 decrypts the corresponding first ciphertext C1_ID by using the key bit sequence k′, to generate a bit sequence of predetermined length, as a session key K′(=plaintext M), by using a processing device, such as the CPU 911.

[0838] As mentioned above, the session key K′ generated by the ciphertext decryption unit 341 has the same bit sequence as that of the session key K generated by the session key generating unit 221 of the ciphertext generating apparatus 200.

[0839] Next, security of the ID-based cryptosystem used in the ID-based cryptographic communication system which is a comparative example according to the present Embodiment will be discussed. To simplify in the following proving, the multiplicative group G1 and the multiplicative group G2 shall be the same multiplicative group G.

[0840] First, as a mathematical problem to which the ID-based cryptosystem being a comparative example according to the present Embodiment reduces, q-D-tABDHE problem (q Decisional Trancated Augmented Bilinear Diffie-Hellman Exponent Problem) is defined.

[0841] In the q-D-tABDHE problem, a solver B is given q+3 elements g′, g′_q+2, g, g₁, . . . , g_q, of the multiplicative group G and one element Z of the multiplicative group GT. Moreover, there is an integer α greater than or equal to 1 and less than the order of the multiplicative group G, as secret information from the solver B. The elements g and g′ of the multiplicative group G are randomly selected elements of the multiplicative group G. When the i-th power of an integer α is described as an integer α^i, the element g_i (i being an integer greater than or equal to 1 and less than or equal to q) of the multiplicative group G is an element g^(α^i) of the multiplicative group G, which is the α^i-th power of the element g of the multiplicative group G. The element g′_q+2 of the multiplicative group G is similarly defined. That is, the element g′_q+2 of the multiplicative group G is g′_q+2=g′^(α^(q+2)). The solver B replies to the question of judging whether an element e(g_q+1,g′) of the multiplicative group GT, which is a pairing value of an element g_q+1=g^(α^(q+1)) of the multiplicative group G and an element g′ of the multiplicative group G, is equal to the given element Z of multiplicative group GT or not.

[0842] Let's define the distribution where practically Z=e(g_q+1,g′) as P, and the distribution where Z is a randomly selected element of the multiplicative group GT as R.

[0843] The advantage of the solver B is defined as Adv^q-d-tabdhe_k,G,GT(B)=|Pr[B (1^k, I)=1:I←P]−Pr[B(1^k, I)=1:I←R]|. B(1^k,I) as used herein indicates a judgment result of the solver B, and “B(1^k,I)=1” indicates that it has been judged to be Z=e(g_q+1,g′). “I←P” indicates an event where practically Z=e(g_q+1,g′), and “I←R” indicates an event where Z is a randomly selected element of the multiplicative group GT. That is, Adv^q-d-tabdhe_k,G,GT(B) is an absolute value of the difference between the probability that the solver B judges to be Z=e(g_q+1,g′) in the case of practically Z=e(g_q+1,g′) (namely, the judgment of the solver B is right), and the probability that the solver B judges to be Z=e(g_q+1,g′) in the case of Z being a randomly selected element of the multiplicative group GT (that is, the judgment of the solver B is wrong).

[0844] Let's define Adv^q-d-tabdhe_k,G,GT(t)=max (Adv^q-d-tabdhe_k,G,GT(B)). This means the advantage of the solver who has the highest advantage among the solvers B who reply during the time t. (t,ε,q)-D-tABDHE assumption (or just “q-D-tABDHE assumption”) is satisfied with (G,GT) when Adv^q-d-tabdhe_k,G,GT(t)≦ε.

[0845] First, the MU-IND-ID-CCA2 security of the ID-based cryptosystem Π of single recipient, which is a basis of the ID-based cryptosystem Π′ of multiple recipients being a comparative example according to the present Embodiment, will be proved.

[0846] It is assumed that the attacker A has an advantage Adv^mu_n,Π,Γ(A)=|Pr[T₀]−1/2| for the cryptographic system Π. T₀ as used herein indicates an event satisfying b=b′ (namely, the attacker A replies correctly) in a game (hereinafter called “G₀”) which the attacker A challenges in the definition of the MU-IND-ID-CCA2 security. Pr[T₀] indicates probability that the event T₀ occurs. Moreover, it is defined to be q=q_x+n+1. Similarly, hereinafter, the event satisfying b=b′ in the game G_λ (λ (λ being an integer greater than or equal to 1 and less than or equal to 6) is called T_λ.

[0847] The game G₁ differs from the game G₀ in that the object which the attacker A attacks is not the cryptographic system II but the encryption oracle O_E, the key derivation oracle O_X, and the decryption oracle O_D which have been simulated by the solver B.

[0848] First, the solver B simulates the key generation capability S for PKG as follows:

[0849] The solver B randomly generates an integer α greater than or equal to 1 and less than the order r of the multiplicative group G.

[0850] The solver B generates three q-th order polynomials f_j(x)(j being an integer greater than or equal to 1 and less than or equal to 3) with a random integer coefficient greater than or equal to 0 and less than r.

[0851] The solver B randomly generates a key generation function KDF and a hash function HF with collision resistance.

[0852] The solver B randomly selects the element g of the multiplicative group G.

[0853] The solver B calculates an element g1=g^α of the multiplicative group G.

[0854] The solver B calculates three elements h1=g^f₁(α), h2=g^f₂(α), and h3=g^f₃(α) of the multiplicative group G.

[0855] The solver B randomly generates an integer s0 greater than or equal to 1 and less than r.

[0856] The solver B calculates an element g′=g^s0 of the multiplicative group G.

[0857] The solver B outputs a common parameter params=(g,g1,h1,h2,h3,KDF,HF). Since g, α, f_j(x), and s0 are random, the distribution of the common parameter params is the same as that of the practical key generation capability S for PKG.

[0858] Moreover, the solver B simulates the key derivation oracle O_X as follows:

[0859] When the input identification information ID (treated as an integer) of the user is equal to the integer α, the solver B rejects it. This is also performed in the practical cryptographic system Π.

[0860] When ID≠α, the solver B calculates three integers r_ID,j=f_j(ID) (j being an integer greater than or equal to 1 and less than or equal to 3).

[0861] The solver B calculates the (q−1)th order polynomial F_ID,j(x)=(f_j(x)−f_j(ID))/(x−ID).

[0862] The solver B calculates three elements h_ID,j=g^F_ID,j(α) (j being an integer greater than or equal to 1 and less than or equal to 3) of the multiplicative group G.

[0863] The solver B outputs d_ID=(r_ID,1,r_ID,2,r_ID,3,h_ID,1,h_ID,2,h_ID,3) as a secret key d_ID.

[0864] In this case, since h1=g^f_i(α), therefore h_ID,1=g^((f₁(α)−f₁(ID))/(α−ID))=(h1·g^(−f₁(ID)))^(1/(α−ID)). Similarly, h_ID,2, and h_ID,3 are calculated. Since r_ID,j=f_j(ID), the relation between r_ID,j and h_ID,j is the same as that in the practical cryptographic system Π.

[0865] Moreover, the solver B simulates the decryption oracle O_D as follows:

[0866] The solver B simulates the key derivation oracle O_X by the procedure mentioned above, and generates a secret key d_ID.

[0867] Based on the generated secret key d_ID, the solver B decrypts a ciphertext C by the same procedure as that of the practical cryptographic system Π.

[0868] Furthermore, the solver B simulates the encryption oracle O_E as follows:

[0869] When the input user's identification information ID* is equal to the integer α, the solver B generates a ciphertext C by the same procedure as that of the practical cryptographic system Π.

[0870] When ID*·α, the solver B simulates the key derivation oracle O_X by the procedure mentioned above, and generates a secret key d_ID*=(r_ID*_,1, r_ID*_{, 2}, r_ID*_{, 3}, h_ID*_,1, h_ID*_,2, h_ID*_,3.

[0871] The solver B calculates an element Z=e(g,g′)^(α^((q+1)) of the multiplicative group GT.

[0872] Based on the (q+2) th order polynomial φ(x)=x^(q+2), the solver B calculates the (q+1)th order polynomial Φ_ID* (x)=(φ(x)−φ(ID*))/(x−ID). The τ-th order coefficient (τ being an integer greater than or equal to 0 and less than or equal to q) of the calculated (q+1)th order polynomial Φ_ID* (x) is defined to be Φ_ID*_,τ.

[0873] The solver B randomly generates an integer s′* greater than or equal to 1 and less than r.

[0874] The solver B calculates an element u*=(g′^(φ(α)−φ(ID*))^s′* of the multiplicative group G.

[0875] The solver B calculates an element v*=(Z·e(g′,Π(g^(Φ_ID*, α^τ)))^s′* of the multiplicative group GT. The solver B calculates a key bit sequence k*=KDF (e(u*,h_ID*_,1)·v*^r_ID*_,1)

[0876] The solver B encrypts the selected message with the key bit sequence k*.

[0877] The solver B combines the bit sequence (first ciphertext C1) indicating the encrypted message, the bit sequence (second ciphertext C2) indicating the element u* of the multiplicative group G, and the bit sequence (third ciphertext C3) indicating the element v* of the multiplicative group G, and calculates a hash value β* by the hash function HF.

[0878] The solver B calculates an element y*=e(u*,h_ID*_,2·h_ID*_,3^β*)·v*^(r_ID*_,2+r_ID*_,3·β*) of the multiplicative group GT.

[0879] The solver B generates a ciphertext C, and outputs it.

[0880] Now, let s*=s′*·s0·Φ_ID* (α). Since g′=g^s0, φ(α)−φ(ID*)=Φ_ID*(α)·(α−ID*),g1=g^α, therefore u*=g^(s*·(α−ID*))=(g1·g^(−ID*))^s*. Moreover, since Π(g^(φ_ID,·α^τ))=g^Σ(φ_ID,·(α^τ)=g^(φ_ID(α)−α^(q+1)), therefore v*=e(g,g)^s* according to the bilinear of the pairing e.

[0881] Furthermore, since r_ID,j=f_j(ID), h_ID,1=g^((f₁(α)−f₁(ID))/(α−ID)), therefore k*=KDF(e(g^(s*·(α−ID*)),g^((f₁(α)−f₁(ID))/(α−ID)))·e(g,g)^(s*·f₁(ID))=KDF (e(g,g)^(s*·f₁(ID)))=KDF(e(g,h1)^s*).

[0882] Similarly, y*=e(g,h2)^s*·e(g,h3)^(s*·β).

[0883] Therefore, the relation between ciphertexts is also the same as that of the practical cryptographic system Π. Moreover, since s* is independent random for each call of the encryption oracle O_E, the distribution of the ciphertext is also the same as that of the practical cryptographic system Π.

[0884] Furthermore, since q=q_x+n+1, therefore {f_j(ID)} and {f_j(α)} used by the key derivation oracle O_X, the encryption oracle O_E and the decryption oracle O_D are all independent random, thereby the simulation by the solver B is perfect. Then, the advantage of the attacker A in the game G₁ is the same as that in the game G₀, that is Pr[T₁]=Pr[T₀].

[0885] A game G₂ is made by changing the game G₁ as follows:

[0886] In simulation of the encryption oracle O_E, the solver B calculates Z beforehand (for example, at the time of simulation of the key generation capability S for PKG) according to the following procedure instead of calculating the element Z=e(g,g′)^(α^q+1) of the multiplicative group GT.

[0887] The solver B randomly generates an integer z greater than or equal to 1 and less than the order r of the multiplicative group G. However, it shall be z·α^(q+1).

[0888] The solver B calculates an element Z=e(g,g′)^z of the multiplicative group GT.

[0889] If there is some difference between the actions of the attacker A in the games G₁ and G₂, the solver B can judge whether Z=e(g,g′)^(α^(q+1)) or not by distinguishing the difference.

[0890] Now, consider the case where the solver B simulates an oracle by utilizing a given parameter in the given q-D-tABHE problem.

[0891] In the given q-D-tABDHE problem, since the solver B knows q+3 elements g′, g′_q+1, g, g₁, . . . , g_q, though not knowing the secret integer α which is unknown to the solver B, he can perform simulation by utilizing them. That is, the solver B treats the element g of the multiplicative group G which was given in the q-D-tABDHE problem, as the element g of the multiplicative group G which is selected in simulation of the key generation capability S for PKG. Moreover, instead of randomly generating the integer s0, the solver B treats the element g′ of the multiplicative group G which was given in the q-D-tABDHE problem, as the element g′(=g^s0) of the multiplicative group G which is calculated in simulation of the key generation capability S for PKG.

[0892] If the τ-th order coefficient of the q-th order polynomial f_j(x) is defined to be f_j,τ(τ being an integer greater than or equal to 0 and less than or equal to q), since the element h1 of the multiplicative group G is h1=g^f_j(α)=Π(g^(f_j,1·α^τ))=Π(g₁^f_j), even if the solver B does not know α, he can calculate the element h1 of the multiplicative group G. Similarly, h2 and h3 can be calculated. Therefore, even if the solver B does not know α, he can simulate the key generation capability S for PKG. Similarly, even if the solver B does not know α, he can simulate the key derivation oracle O_X and the decryption oracle O_D. Moreover, the solver B can also simulate the encryption oracle O_E except for the calculation of the element Z of the multiplicative group GT.

[0893] The solver B simulates the encryption oracle O_E by using the element Z of the multiplicative group GT which was given to him in the q-D-tABDHE problem, as the element Z of the multiplicative group GT. If Z=(g,g′)^(α^(q+1)), it means that the solver B is performing the game G₁. If Z=·(g,g′)^(α^(q+1)), it means that the solver B is performing the game G₂. The solver B discerns this, based on a difference of the action of the attacker A in the game, and judges whether Z=(g,g′)^(α^(q+1)) or not, to reply to the given q-D-tABDHE problem.

[0894] Therefore, |Pr[T₂]−Pr[T₁]|≦Adv^q-d-tabdhe_k,G,GT(t₁)+3/p is satisfied. Here, t₁=t+O(q²·t_G), and t_G is the time spent for exponential operation in the multiplicative group G.

[0895] A game G₃ is made by changing the game G₂ as follows:

[0896] In simulation of the key generation capability S for PKG, the solver B does not calculate elements h1, h2, and h3 of the multiplicative group G by using the q-th order polynomial f_j(x) randomly generated, but randomly selects the elements h1, h2, and h3 of the multiplicative group G. Moreover, in simulation of the key derivation oracle O_X, the solver B randomly generates an integer r_ID,j for ID, and calculates an element h_ID,1=(h1·g^(−r_ID,j))^(1/(α−ID) of the multiplicative group G. Similarly, h_ID,2, and h_ID,3 are calculated.

[0897] The solver B simulates the encryption oracle O_E according to the following procedure:

[0898] The solver B calculates an integer z′*=(z−α^(q+1))/φ_ID*(α).

[0899] The solver B randomly generates an integer s* greater than or equal to 1 and less than the order r of the multiplicative group G.

[0900] The solver B calculates an element u*=g^(s*·(α−ID*)) of the multiplicative group G.

[0901] The solver B calculates an element v*=e(g,g)^(s*·(1+z′*)) of the multiplicative group GT.

[0902] The solver B calculates a key bit sequence k=KDF(e(g,h1·g^(z′*·r_ID,1)^s*).

[0903] The solver B calculates an element y*=e(g,h2·h3^β*)^s*·e(g,g)^(s*·z′*·(r_ID*_,2+r_ID*_,3·β*)) of the multiplicative group GT.

[0904] Respects other than the above are the same as those of the game G₂.

[0905] When seen from the attacker A, since the game G₂ and the game G₃ look the same, therefore Pr[T₃]=Pr[T₂].

[0906] A game G₄ is made by changing the game G₃ as follows:

[0907] In simulation of the decryption oracle O_D, the solver judges whether v=e(u,g)^(1/(α−ID) and y=e(u,h2·h3 ^β) are satisfied or not. When not satisfied, “rejection” is returned. When satisfied, the solver B calculates a key bit sequence k′=KDF(e(u,h1)^(1/(α−ID))), and decrypts the message.

[0908] Thus, the solver B uses the integer α, not the secret key d_ID, for judging the consistency of the ciphertext and generating the key bit sequence k′.

[0909] In judging the consistency of the ciphertext, it is judged whether y=e(u,h_ID,2·h_ID,3^β))·v^(r_ID,2+r_ID,3·β) or not, in the practical cryptographic system E. Since the judgment method is different, a ciphertext which is not rejected in the practical cryptographic system II may be rejected. When this event is represented as R₄, |Pr[T4]−Pr[T3]|≦Pr[R₄].

[0910] A game G₅ is made by changing the game G₄ as follows:

[0911] In the encryption oracle O_E, the solver B randomly generates an integer r* greater than or equal to 1 and less than the order r of the multiplicative group G.

[0912] The solver B calculates a key bit sequence k*=KDF(e(g,g)^r*).

[0913] The solver B treats the calculated key bit sequence k* as it is, as an encrypted message, regardless of the messages M₀* and M_i* input by the encryption oracle OE.

[0914] Since the output of the encryption oracle O_E is independent of b, M₀* and M₁*, the attacker A does not have an advantage. Therefore, Pr[T₅]=1/2.

[0915] As described in the explanation for the game G₄, the solver B does not use the integer r_ID,1 in simulation of the decryption oracle O_D. The first time (and only once) of the r_ID,1 being used is at the time of ID* being input into the encryption oracle O_E.

[0916] Therefore, when (g,h1,z′,*,s*,M*) is fixed, with respect to (r*,r_ID*_,1), to first randomly generate r_ID*_,1 and then calculate r* to be consistent with the generated r_ID*_,1 gives the same distribution as that of to first randomly generate r* and then calculate r_ID*_,1 to be consistent with the generated r*.

[0917] Thus, when seen from the attacker A, the game G₄ and the game G₅ look the same.

[0918] Since r_ID,1 is not referred to except for in the simulation of the encryption oracle OE, it is not necessary to calculate it based on r*.

[0919] As mentioned above, Pr[T₄]=Pr[T₅]=1/2, and Pr[R₄]=Pr[R₅] are satisfied. R₅ indicates an event that a ciphertext which is not rejected in the practical cryptographic system Π is rejected in game G₅.

[0920] A game G₆ is made by changing the game G₅ as follows:

[0921] In simulation of the i-th encryption oracle O_E, the solver B holds a group his*_i(ID*_i,C*_i,β*_i) composed of the input ID*, the generated ciphertext C*, and the calculated hash value β* in a list Clist.

[0922] In simulation of the decryption oracle O_D, the solver B performs the following judgment before judging the consistency of the ciphertext.

[0923] The solver B judges whether there is his*_i where ID*_i is equal to the input ID and β*_i is equal to β calculated from the input ciphertext C, in the held list Clist or not. When there is his*_i with equality, the solver B compares the portion corresponding to the ciphertext body CB*_i (excluding the ciphertext verification text CC*_i) in the ciphertext C*, included in the his*_i with the portion corresponding to the ciphertext body CB (excluding the ciphertext verification text CC) in the input ciphertext C, and when they are not equal, returns “rejection”.

[0924] The event of rejecting the ciphertext by this judgment is defined as C₆. The event that the ciphertext which is not rejected in the practical cryptographic system E nor rejected by this judgment is rejected by the judgment method changed in the game G₄ is defined to be R₆, and thus |Pr[R₆]−Pr[R₅]|≦Pr[C₆].

[0925] The event C₆ is the case of hash values 13 being equal though the bit sequences used as the basis of the hash value calculation are different, therefore Pr[C₆]≦Adv^cr_H(t₂). In addition, Adv^cr_H(t₂) indicates the collision resistance advantage of the hash function HF.

[0926] In order that the event R₆ occurs, it is necessary to input y being y=e(g,h2·h3^β)^s·e(g,g)^(s·z′·(r_ID,2+r_ID,3β)) for z′ which is not 0, into the decryption oracle O_D. Since (r_ID,2,r_ID,3) is random for each ID, the probability of selecting such y before the ID being input into the encryption oracle OE is less than or equal to 1/r (r being the order of the multiplicative group GT). Moreover, after ID=ID*_i is input into the encryption oracle, if β=β*_i, it is rejected by an additional judgment. Therefore, β·β*_i. The probability of selecting y which satisfies the conditions mentioned above is still less than or equal to 1/r. Thus, Pr[R₆]≦q_d/r is satisfied.

[0927] According to what is described above, Adv^mu_n,Π,Γ(t,q_e, q_d, q_x)≦q_e·Adv^q-d-tabdhe_k,G,GT (t₁)+q_e·Adv^cr_H(t₂)+q_e·(q_d+3)/r.

[0928] The ID-based cryptosystem Π′ being a comparative example according to the present Embodiment is what is extended from the cryptographic system Π of a single recipient explained above to multiple recipients by the conjunctive method. As described in Embodiment 1, the SMMR security of the cryptographic system extended to multiple recipients by the conjunctive method is Adv^smmr_n,Π′,Γ(t,q_d,q_x)≦Adv^mu_n,Π,Γ(t,1,q_d,q_x). Therefore, the SMMR security of the ID-based cryptosystem being a comparative example according to the present Embodiment is Adv^smmr_n,Π′,Γ(t,q_d,q_x)≦Adv^q-d-tabdhe_k,G,GT(t₁)+Adv^cr_H(t₂)+(q_d+3)/r.

[0929] On the other hand, since the SMMR security in the case of extending a general single recipient cryptographic system to multiple recipients by the conjunctive method is Adv^smmr_n,Π′,Γ(t,q_d,q_x)≦n·Adv^mu_1,Π,Γ(t′,1,q_d,q_x), the recipient number n appears in the inequality showing the relation with respect to the advantage of the mathematical problem. That is, compared with the general case, the SMMR security of the ID-based cryptosystem being a comparative example according to the present Embodiment tightly reduces to the mathematical problem (high reduction rate). Thus, the ID-based cryptosystem which is a comparative example according to the present Embodiment can prove better security.

[0930] Next, the ID-based cryptographic communication system 800 according to the present Embodiment will be explained. Since the whole configuration of the ID-based cryptographic communication system 800 and the block configuration of the encryption parameter generating apparatus 100 according to the present Embodiment are the same as those described in the comparative example, their explanation is herein omitted. The ID-based cryptographic communication system 800 according to the present Embodiment is made by increasing the efficiency of the multiple recipient ID-based cryptosystem explained in the comparative example by making the hash value common.

[0931] First, the ciphertext generating apparatus 200 will be explained.

[0932] Since the block configuration of the ciphertext generating apparatus 200 is the same as that described in the comparative example, respects different from the comparative example will be explained with reference to FIG. 4.

[0933] The hash value calculation unit 244 inputs a public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[0934] The hash value calculation unit 244 inputs n first ciphertexts C1_i output by the first ciphertext generating unit 241, n second ciphertexts C2_i output by the second ciphertext generating unit 242, and n third ciphertexts C3_i output by the third ciphertext generating unit 243, as n ciphertext bodies CB_i output by the ciphertext body generating unit 240, by using a processing device, such as the CPU 911.

[0935] The hash value calculation unit 244 generates one combined ciphertext by combining all the input n first ciphertexts C1_i, n second ciphertexts C2_i, and n third ciphertexts C3_i, by using a processing device, such as the CPU 911.

[0936] In addition, the hash value calculation unit 244 may further input n recipient identification information ID_i output by the recipient identification input unit 232, as (a part of) the n ciphertext bodies CB_i, and may generate one combined ciphertext by combining all the input n recipient identification information ID_i, n first ciphertexts C1_i, n second ciphertexts C2_i, and n third ciphertexts C3_i.

[0937] Based on the hash function HF indicated by the input public encryption parameter and the generated one combined ciphertext, the hash value calculation unit 244 calculates one natural number N by converting the one combined ciphertext by the hash function HF, to be treated as one hash value H₀, by using a processing device, such as the CPU 911.

[0938] The hash value calculation unit 244 outputs information indicating the calculated one natural number N, and the generated one combined ciphertext, by using a processing device, such as the CPU 911.

[0939] The ciphertext verification text generating unit 245 inputs the public encryption parameter stored by the encryption parameter storage unit 210, the information indicating the n integers s_i output by the random number generating unit 222, and the information indicating one natural number N output by the hash value calculation unit 244, by using a processing device, such as the CPU 911.

[0940] Based on the pairing e, the element g of the multiplicative group G1, and the element h2 of the multiplicative group G2 indicated by the input public encryption parameters, the ciphertext verification text generating unit 245 calculates an element e(g,h2) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element h2 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0941] Based on the pairing e, the element g of the multiplicative group G1, and the element h3 of the multiplicative group G2 indicated by the input public encryption parameters, the ciphertext verification text generating unit 245 calculates an element e(g,h3) of the multiplicative group GT, which is a pairing value of the element g of the multiplicative group G1 and the element h3 of the multiplicative group G2, by using a processing device, such as the CPU 911.

[0942] Since the elements e(g,h2) and e(g,h3) of the multiplicative group GT are constant regardless of the integer s_i, they may be calculated beforehand and stored by using a storage device, such as the magnetic disk drive 920.

[0943] Based on the multiplicative group GT indicated by the input public encryption parameter, the one natural number β₀ indicated by the input information, and the calculated element e(g,h3) of the multiplicative group GT, the ciphertext verification text generating unit 245 calculates one element e(g,h3)^β₀ of the multiplicative group GT, which is the β₀-th power of the element e(g,h3) of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0944] Based on the multiplicative group GT indicated by the input public encryption parameter, the calculated element e(g,h2) of the multiplicative group GT, and the calculated one element e(g,h3)^β₀ of the multiplicative group GT, the ciphertext verification text generating unit 245 calculates one element e(g,h2)·e(g,h3)^β₀ of the multiplicative group GT, which is a product of the element e(g,h2) of the multiplicative group GT and the element e(g,h3)^β₀ of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0945] Based on the multiplicative group GT indicated by the input public encryption parameter, the n integers s_i indicated by the input information, and the calculated one element e(g,h2)·e(g,h3)^β₀ of the multiplicative group GT, the ciphertext verification text generating unit 245 calculates n elements y_i=(e(g,h2)·e(g,h3)^β₀)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT, which is the s_i-th power of the element e(g,h2)·e(g,h3)^(β₀ of the multiplicative group GT, by using a processing device, such as the CPU 911.

[0946] The ciphertext verification text generating unit 245 outputs n bit sequences respectively indicating the calculated n elements y_i of the multiplicative group GT, as n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n), by using a processing device, such as the CPU 911.

[0947] The ciphertext combining unit 246 inputs the one combined ciphertext output by the hash value calculation unit 244, and the n ciphertext verification texts CC_i output by the ciphertext verification text generating unit 245, by using a processing device, such as the CPU 911.

[0948] The ciphertext combining unit 246 generates one key ciphertext C by combining all the input one combined ciphertext and the input n ciphertext verification texts, by using a processing device, such as the CPU 911.

[0949] The ciphertext combining unit 246 outputs the generated one key ciphertext C, by using a processing device, such as the CPU 911.

[0950] FIG. 16 shows an example of the flow of information exchanged in the ciphertext generating apparatus 200 according to the present Embodiment.

[0951] The recipient identification input unit 232 inputs n recipient identification information ID₁, ID₂, . . . , ID_n.

[0952] The random number generating unit 222 generates n integers s_i, s₂, . . . , s_n whose number is the same as that of recipient identification information ID₁, ID₂, . . . , ID_n input by the recipient identification input unit 232.

[0953] The first ciphertext generating unit 241 generates a first ciphertext C1₁ based on the plaintext M and the integer s₁, a first ciphertext C1₂ based on the plaintext M and the integer s₂, . . . , and a first ciphertext C1_n based on the plaintext M and the integer s_n.

[0954] The second ciphertext generating unit 242 generates a second ciphertext C2_i based on the recipient identification information ID₁ and the integer s₁, a second ciphertext C2₂ based on the recipient identification information ID₂ and the integer s₂, . . . , and a second ciphertext C2_n based on the recipient identification information ID_n and the integer s_n.

[0955] The third ciphertext generating unit 243 generates a third ciphertext C3₁ based on the integer s_i, a third ciphertext C3₂ based on the integer s₂, . . . , and a third ciphertext C3_n based on the integer s_n.

[0956] The flow up to this point is the same as that of the comparative example.

[0957] The hash value calculation unit 244 generates one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) by combining all the first ciphertext C1₁, the second ciphertext C2₁, the third ciphertext C3₁, the first ciphertext C1₂, the second ciphertext C2₂, the third ciphertext C3₂, . . . , the first ciphertext C1_n, the second ciphertext C2_n, and the third ciphertext C3_n. The order of combining may be different from this.

[0958] The hash value calculation unit 244 generates one hash value H₀, based on the generated one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n).

[0959] The ciphertext verification text generating unit 245 generates a ciphertext verification text CC₁ based on the integer s₁ and the hash value H₀, a ciphertext verification text CC₂ based on the integer s₂ and the hash value H₀, . . . , and a ciphertext verification text CC_n based on the integer s_n and the hash value H₀.

[0960] The ciphertext combining unit 246 generates one key ciphertext C=(C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n|CC₁|CC₂| . . . CC_n) by combining all the one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), the ciphertext verification text CC_i, the ciphertext verification text CC₂, . . . , and the ciphertext verification text CC_n.

[0961] In this example, since the combined ciphertext generated by the hash value calculation unit 244 is used as it is, the combining order of the ciphertext differs from that of the comparative example, and, however, it is not limited to this order. The combining order of the ciphertext may be the same as the one explained in the comparative example, or may be other order.

[0962] Next, the ciphertext receiving apparatuses 301 to 303 will be explained. Since the block configuration of the ciphertext receiving apparatuses 301 to 303 is also the same as that described in the comparative example, respects different from the comparative example will be explained with reference to FIG. 6.

[0963] The ciphertext decomposition unit 346 inputs one key ciphertext C output by the ciphertext receiving unit 351, by using a processing device, such as the CPU 911.

[0964] The ciphertext decomposition unit 346 acquires a corresponding first ciphertext C1_ID, a corresponding second ciphertext C2_ID, a corresponding third ciphertext C3_ID, and a corresponding ciphertext verification text CC_ID which are corresponding to the recipient concerned, from the input one key ciphertext C, by using a processing device, such as the CPU 911.

[0965] The ciphertext decomposition unit 346 outputs the acquired corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID, corresponding third ciphertext C3_ID, and corresponding ciphertext verification text CC_ID, by using a processing device, such as the CPU 911.

[0966] For example, it is assumed that one key ciphertext C is made by combining ciphertexts in the order of (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n|CC₁|CC₂| . . . |CC_n) as shown in FIG. 16.

[0967] The ciphertext decomposition unit 346 separates one key ciphertext C into, for example, n times of the sum of the bit lengths of from the first ciphertext to the third ciphertext, and n times of the bit length of the ciphertext verification text, and acquires a combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) and a combined ciphertext verification text (CC₁|CC₂| . . . |CC_n).

[0968] The ciphertext decomposition unit 346 further separates the combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) into (C1₁|C2₁|C3₁), (C1₂|C2₂|C3₂), . . . , and (C1_n|C2_n|C3_n), and acquires the corresponding ciphertext (C1_ID|C2_ID|C3_ID) concerned from them.

[0969] The ciphertext decomposition unit 346 separates the acquired corresponding ciphertext (C1_ID|C2_ID|C3_ID), and acquires the corresponding first ciphertext C1_ID, the corresponding second ciphertext C2_ID, and the corresponding third ciphertext C3_ID.

[0970] Moreover, the ciphertext decomposition unit 346 separates the acquired combined ciphertext verification text (CC₁|CC₂| . . . |CC_n) into CC₁, CC₂, . . . , CC_n, and acquires the corresponding ciphertext verification text CC_ID concerned.

[0971] The ciphertext decomposition unit 346 outputs the acquired corresponding first ciphertext C1_ID, corresponding second ciphertext C2_ID, corresponding third ciphertext C3_ID, and corresponding ciphertext verification text CC_ID. Moreover, in this example, in order to save the time and effort of the verification hash value calculation unit 344′ s generating a combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), the ciphertext decomposition unit 346 also outputs the acquired combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) in addition to them, by using a processing device, such as the CPU 911.

[0972] The verification hash value calculation unit 344 inputs the combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) output by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911.

[0973] Alternatively, the verification hash value calculation unit 344 may combine the ciphertexts and generate a combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), by using a processing device, such as the CPU 911.

[0974] The verification hash value calculation unit 344 inputs public encryption parameters stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[0975] Based on the hash function HF indicated by the input public encryption parameter, and the generated or input combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), the verification hash value calculation unit 344 calculates a natural number β′ by converting the combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1₁|C2_n|C3_n) by the hash function HF, to be treated as a hash value H′, by using a processing device, such as the CPU 911.

[0976] The verification hash value calculation unit 344 outputs information indicating the calculated natural number β′, by using a processing device, such as the CPU 911.

[0977] Next, operations will be described.

[0978] FIG. 17 is a flowchart showing an example of the flow of ciphertext generation processing in which the ciphertext generating apparatus 200 according to the present Embodiment generates a ciphertext. The same reference numbers are attached to the steps common to the steps of ciphertext generation processing explained in the comparative example.

[0979] In the hash value calculation step S38, based on the n first ciphertexts C1_i respectively corresponding to the n recipients generated by the first ciphertext generating unit 241 at the first ciphertext generation step S35, the n second ciphertexts C2_i respectively corresponding to the n recipients generated by the second ciphertext generating unit 242 at the second ciphertext generation step S36, and the n third ciphertexts C3_i respectively corresponding to the n recipients generated by the third ciphertext generating unit 243 at the third ciphertext generation step S37, the hash value calculation unit 244 generates one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) by combining all the n first ciphertexts C1_i respectively corresponding to the n recipients, the n first ciphertexts C2_i respectively corresponding to the n recipients, and the n third ciphertexts C3_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0980] Based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit 210, and the generated one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), the hash value calculation unit 244 calculates one integer β₀=HF(C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) by converting the one combined ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) by the hash function HF, by using a processing device, such as the CPU 911.

[0981] In the ciphertext verification text generation step S39, based on the multiplicative group GT, the pairing e, the element g of the multiplicative group G1, and two elements h2 and h3 of the multiplicative group G2 indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n natural numbers s_i respectively corresponding to the n recipients generated by the random number generating unit 222 at the random number generation step S34, and the one integer β₀ calculated by the hash value calculation unit 244 at the hash value calculation step S38, the ciphertext verification text generating unit 245 calculates n elements y_i=e(g,h2)^s_i·e(g,h3)^(s_i·β₀) of the multiplicative group GT respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0982] The ciphertext verification text generating unit 245 treats bit sequences respectively indicating the calculated n elements y_i of the multiplicative group GT respectively corresponding to the n recipients, as n ciphertext verification texts CC_i respectively corresponding to the n recipients, by using a processing device, such as the CPU 911.

[0983] FIG. 18 is a flowchart showing an example of the flow of ciphertext reception processing in which the ciphertext receiving apparatuses 301 to 303 according to the present Embodiment receive a ciphertext. The same reference numbers are attached to the steps common to the steps of ciphertext reception processing explained in the comparative example.

[0984] In the verification hash value calculation step S53, based on the hash function HF indicated by the public encryption parameter stored by the public parameter storage unit 310, and the combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n), the verification hash value calculation unit 344 calculates an integer β′=HF (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2_n|C3_n) by converting the combined verification ciphertext (C1₁|C2₁|C3₁|C1₂|C2₂|C3₂| . . . |C1_n|C2₁|C3_n) by the hash function HF, by using a processing device, such as the CPU 911.

[0985] Since the steps other than the one described above are the same as those explained in the comparative example, their explanation is herein omitted.

[0986] As mentioned above, in the ID-based cryptographic communication system 800 of the present Embodiment, compared with the ID-based cryptographic communication system 800 explained in the comparative example, although the length of one key ciphertext C is the same, the amount of calculation required for ciphertext generation processing decreases, thereby reducing the cost of the cryptographic communication. Particularly, at the hash value calculation step S38, only one time of hash value calculation is required in the present method, whereas n times of hash value calculation is required in the method explained in the comparative example.

[0987] Moreover, at the ciphertext verification text generation step S39, although 2n times of exponential operation in the multiplicative group GT and n times of multiplication in the multiplicative group GT are required in the method explained in the comparative example, only (n+1) times of exponential operation in the multiplicative group GT and one time of multiplication in the multiplicative group GT are required in the present method. Therefore, it is possible to considerably reduce the amount of calculation in the case of a large number of recipients.

[0988] Next, the SMMR security of the ID-based cryptosystem used by the ID-based cryptographic communication system 800 according to the present Embodiment will be explained. In the description below, the ciphertext body CB_i shall include recipient identification information ID_i for identifying the corresponding recipient.

[0989] The SMMR security of the ID-based cryptosystem according to the present Embodiment can be proved by discussing in parallel to the proof of the MU security of the ID-based cryptosystem of single recipient that is the basis of the ID cryptographic system explained in the comparative example. Explanation for respects common in the proof is omitted, and different respects will be described.

[0990] The decryption oracle O_D, inputs a user's identification information ID and a (separated) ciphertext C_ID corresponding to the ID.

[0991] This (separated) ciphertext C_ID is composed of n ciphertext bodies CB_i, information indicating which of the n ciphertext bodies CB_i is a corresponding ciphertext body CB_ID corresponding to the ID_i and a corresponding ciphertext verification text CC_ID.

[0992] The decryption oracle O_D′ combines the input n ciphertext bodies CB_i to generate a combined verification ciphertext, and calculates a hash value β′.

[0993] The decryption oracle O_D verifies the ciphertext, based on the corresponding ciphertext body CB_ID indicated by the input information, the input corresponding ciphertext verification text CC_ID, the calculated hash value β′, and the secret key d_u, generated by key derivation capability X′.

[0994] When failed in the verification, the decryption oracle O_D′ returns “rejection”.

[0995] When succeeded in the verification, the decryption oracle O_D′ performs decryption processing, and returns the decrypted message.

[0996] A game G₆ is made by changing the game G₅ as follows:

[0997] In simulation of the encryption oracle O_E′, the solver B holds the input ID*=(ID*₁, ID*₂, . . . , ID*_n), the generated n ciphertext bodies CB*=(CB*₁, CB*₂, . . . , CB*_n), and the calculated hash value β*₀.

[0998] In simulation of the decryption oracle O_D′, the solver B judges whether the holding β*₀ is equal to the hash value β′ calculated based on the input n ciphertext bodies CB_i or not. When they are equal, the solver B judges whether there is an ID*_i equal to the input ID among the holding ID*. When there is an ID*_i equal to the input ID, the solver B compares the ciphertext body CB*_i corresponding to the ID*_i with the corresponding ciphertext body CB_ID indicated by the input information, among the holding n ciphertext bodies CB*. When they are not equal, “rejection” is returned.

[0999] Since the composition of the main part of the proof is not different except for the difference above stated, the SMMR security of the ID-based cryptosystem according to the present Embodiment is Adv^smmr_n,Π′,Γ(t,q_d,q_x)≦Adv^q-d-tabdhe_k,G,GT(t₁)+Adv^cr_H(t₂)+(q_d+3)/r.

[1000] Since it is possible to prove the SMMR-IND-ID-CCA2 security by the proof tightly reducing to a mathematical problem, by the ID-based cryptographic communication system 800 according to the present Embodiment, there is an effect that secure cryptographic communication can be realized by using the ID-based cryptographic communication system 800.

[1001] The ciphertext generating apparatus 200 according to the present Embodiment has the following features:

[1002] The encryption parameter storage unit 210 stores information, as public encryption parameters, indicating a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, a key generation function KDF for calculating a bit sequence of predetermined length from the element of the multiplicative group GT, a hash function HF for calculating a natural number being less than the natural number r from a bit sequence of arbitrary length, an element g of the multiplicative group G1, an element g1 of the multiplicative group G1, an element g2 of the multiplicative group G2, an element h1 of the multiplicative group G2, an element h2 of the multiplicative group G2, and an element h3 of the multiplicative group G2, by using a storage device, such as the magnetic disk drive 920.

[1003] The recipient identification input unit 232 inputs n integers ID_i as n recipient identification information, by using a processing device, such as the CPU 911.

[1004] The random number generating unit 222 randomly generates the n integers s_i, being greater than or equal to 1 and less than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[1005] The first ciphertext generating unit 241 calculates n key bit sequences k_i=KDF(e(g,h1)^s_i)(i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the element g of the multiplicative group G1, the element h1 of the multiplicative group G2, the pairing e, and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit 210, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911, and generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients by respectively encrypting the plaintext M input by the plaintext input unit 231, with the n key bit sequences k_i calculated, by using a processing device, such as the CPU 911.

[1006] The second ciphertext generating unit 242 calculates n elements u_i=(g1·g^−ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipients, based on the element g of the multiplicative group G1 and the element g1 of the multiplicative group G1 indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers ID_i input by the recipient identification input unit 232, and the n integers s_i generated by the random number generating unit 222, and treats each bit sequence, indicating each of the calculated n elements u_i of the multiplicative group G1, as the n second ciphertexts (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1007] The third ciphertext generating unit 243 calculates n elements v_i=e(g,g2)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipients, based on the element g of the multiplicative group G1, the element g2 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit 210 and the n integers s_i generated by the random number generating unit 222, and treats each bit sequence, indicating each of the calculated elements v_i of the multiplicative group GT, as the n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1008] The hash value calculation unit 244 calculates one natural number β₀ to be treated as the hash value H₀, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[1009] The ciphertext verification text generating unit 245 calculates n elements y_i=e(g,h2)^s_i·e(g,h3)^(s_i·β₀) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipients, based on the element g of the multiplicative group G1, the element h2 of the multiplicative group G2, the element h3 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers s_i generated by the random number generating unit 222, and the one natural number β₀ calculated by the hash value calculation unit 244, and treats each bit sequence, indicating each of the calculated n elements y_i of the multiplicative group GT, as the n ciphertext verification text CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1010] The ciphertext generating apparatus 200 according to the present Embodiment has the following effects:

[1011] It is possible to perform encryption processing with a small amount of resources, and to realize the cryptographic communication utilizing the ID-based cryptosystem where the SMMR-IND-ID-CCA2 security can be mathematically proved.

[1012] The ID-based cryptographic communication system 800 (cryptographic communication system) according to the present Embodiment has the following features:

[1013] The encryption parameter generating apparatus 100 further includes a group parameter storage unit 111.

[1014] The group parameter storage unit 111 stores information, as group parameters, indicating a natural number r, a multiplicative group G1 whose order is the natural number r, a multiplicative group G2 whose order is the natural number r, a multiplicative group GT whose order is the natural number r, and a pairing e for calculating an element of the multiplicative group GT from an element of the multiplicative group G1 and an element of the multiplicative group G2, by using a storage device, such as the magnetic disk drive 920.

[1015] The secret information generating unit 112 randomly generates an integer α being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the group parameter stored by the group parameter storage unit, by using a processing device, such as the CPU 911.

[1016] The secret information storage unit 121 stores information indicating the integer α generated by the secret information generating unit 112, as secret information, by using a storage device, such as the magnetic disk drive 920.

[1017] The public parameter generating unit 113 determines a key generation function KDF for calculating a bit sequence of predetermined length from an element of the multiplicative group GT, and a hash function HF for calculating a natural number being less than the natural number r from a bit sequence of arbitrary length, by using a processing device, such as the CPU 911, randomly selects an element g of the multiplicative group G1, an element g2 of the multiplicative group G2, an element h1 of the multiplicative group G2, an element h2 of the multiplicative group G2, and an element h3 of the multiplicative group G2, based on the multiplicative group G1 and the multiplicative group G2 indicated by the group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911, calculates an element g1=g^α of the multiplicative group G1, based on the integer α indicated by the secret information generated by the secret information generating unit 112 and the selected element g of the multiplicative group G1, by using a processing device, such as the CPU 911, and treats information indicating the group parameter stored by the group parameter storage unit 111, the determined key generation function KDF and hash function HF, the selected element g of the multiplicative group G1, element g2 of the multiplicative group G2, element h1 of the multiplicative group G2, element h2 of the multiplicative group G2, and element h3 of the multiplicative group G2, and the calculated element g1 of the multiplicative group G1, as public encryption parameters.

[1018] The identification information input unit 131 inputs an integer ID_i as recipient identification information, by using a processing device, such as the CPU 911.

[1019] The secret key generating unit 132 randomly generates an integer r_ID,1 being greater than or equal to 1 and less than the natural number r, an integer r_ID,2 being greater than or equal to 1 and less than the natural number r, and an integer r_ID,3 being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the public encryption parameter generated by the public parameter generating unit 113, by using a processing device, such as the CPU 911, calculates an element h_ID,1=(h1·g2^(−r_ID,1))^(1/(α−ID)) of the multiplicative group G2, an element h_ID,2=(h2·g2^(−r_ID,2))^(1/(α−ID)) of the multiplicative group G2, and an element h_ID,3=(h3·g2^(−r_ID,3))^(1/(α−ID)) of the multiplicative group G2, based on the element g2 of the multiplicative group G2, the element h1 of the multiplicative group G2, the element h2 of the multiplicative group G2, and the element h3 of the multiplicative group G2 indicated by the public encryption parameters generated by the public parameter generating unit 113, the integer α stored by the secret information storage unit 121, the integer ID input by the identification information input unit 131, and the generated integer r_ID,1, integer r_ID,2 and integer r_ID,3, by using a processing device, such as the CPU 911, and treats information indicating the generated integer r_ID,1, integer r_ID,2 and integer r_ID,3 and the calculated element h_ID,1 of the multiplicative group G2, element h_ID,2 of the multiplicative group G2, and element h_ID,3 of the multiplicative group G2, as the secret key d_ID.

[1020] The recipient identification input unit 232 inputs n integers ID_i, as n recipient identification information, by using a processing device, such as the CPU 911.

[1021] The random number generating unit 222 randomly generates n integers s_i, being greater than or equal to 1 and less than the natural number r, (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the natural number r indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[1022] The first ciphertext generating unit 241 calculates n key bit sequences k_i=KDF(e(g,h1)^s_i) (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, based on the element g of the multiplicative group G1, the element h1 of the multiplicative group G2, the pairing e, and the key generation function KDF indicated by the public encryption parameters stored by the encryption parameter storage unit 210, and the n integers s_i generated by the random number generating unit 222, by using a processing device, such as the CPU 911, and generates n first ciphertexts C1_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients by respectively encrypting the plaintext M input by the plaintext input unit 231, with the n key bit sequences k_i calculated, by using a processing device, such as the CPU 911.

[1023] The second ciphertext generating unit 242 calculates n elements u_i=(g1·g^−ID_i)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group G1 corresponding to the n recipients, based on the element g of the multiplicative group G1 and the element g1 of the multiplicative group G1 indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers ID_i input by the recipient identification input unit 232, and the n integers s_i generated by the random number generating unit 222, and treats each bit sequence, indicating each of the calculated n elements u, of the multiplicative group G1, as the n second ciphertexts C2_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1024] The third ciphertext generating unit 243 calculates n elements v_i=e(g,g2)^s_i (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipients, based on the element g of the multiplicative group G1, the element g2 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit 210 and the n integers s_i generated by the random number generating unit 222, and treats each bit sequence, indicating each of the calculated n elements v_i of the multiplicative group GT, as the n third ciphertexts C3_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1025] The hash value calculation unit 244 calculates one natural number β₀ to be treated as the hash value H₀, based on the hash function HF indicated by the public encryption parameter stored by the encryption parameter storage unit 210, by using a processing device, such as the CPU 911.

[1026] The ciphertext verification text generating unit 245 calculates n elements y,=e(g,h2)^s_i·e(g,h3)^(s_i·β₀) (i being an integer greater than or equal to 1 and less than or equal to n) of the multiplicative group GT corresponding to the n recipients, based on the element g of the multiplicative group G1, the element h2 of the multiplicative group G2, the element h3 of the multiplicative group G2, and the pairing e indicated by the public encryption parameters stored by the encryption parameter storage unit 210, the n integers s_i generated by the random number generating unit 222, and the one natural number β₀ calculated by the hash value calculation unit 244, and treats each bit sequence, indicating each of the calculated n elements y_i of the multiplicative group GT, as the n ciphertext verification texts CC_i (i being an integer greater than or equal to 1 and less than or equal to n) corresponding to the n recipients, by using a processing device, such as the CPU 911.

[1027] The verification hash value calculation unit 344 calculates a natural number β′ to be treated as the verification hash value H′, based on the hash function HF indicated by the public encryption parameter stored by the public parameter storage unit 310, by using a processing device, such as the CPU 911.

[1028] The ciphertext verification unit 345, calculates an element y′=e(u_ID, h_ID,2·h_ID,3^β′)·v_ID^(r_ID,2+r_ID,3·β′) of the multiplicative group GT, based on the pairing e indicated by the public encryption parameter stored by the public parameter storage unit 310, the integer r_ID,2, the integer r_ID,3, the element h_ID,2 of the multiplicative group G2, and the element h_ID,3 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit 323, an element u_ID of the multiplicative group G1 indicated by the one corresponding second ciphertext C2_ID and an element v_ID of the multiplicative group GT indicated by the one corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit 346, and the natural number β′ calculated by the verification hash value calculation unit 344, by using a processing device, such as the CPU 911, compares the calculated element y′ of the multiplicative group GT with an element y_ID of the multiplicative group GT indicated by the corresponding ciphertext verification text CC_ID acquired by the ciphertext decomposition unit 346, by using a processing device, such as the CPU 911, and when the element y′ of the multiplicative group GT accords with the element y_ID of the multiplicative group GT, judges that the one ciphertext C (key ciphertext C) received by the ciphertext receiving unit 351 has consistency.

[1029] When the ciphertext verification unit 345 judges that the one ciphertext C received by the ciphertext receiving unit 351 has consistency, the ciphertext decryption unit 341 calculates a key bit sequence k′=KDF(e(u_ID,h_ID,1)·v_ID^r_ID,1), based on the pairing e and the key generation function KDF indicated by the public encryption parameters stored by the public parameter storage unit 310, the integer r_ID,1 and the element h_ID,1 of the multiplicative group G2 indicated by the secret key d_ID stored by the secret key storage unit 323, the element u_ID, of the multiplicative group G1 indicated by the one corresponding second ciphertext C2_ID, and the element v_ID of the multiplicative group G2 indicated by the corresponding third ciphertext C3_ID acquired by the ciphertext decomposition unit, by using a processing device, such as the CPU 911, and restores the plaintext M by decrypting the corresponding first ciphertext C1_ID acquired by the ciphertext decomposition unit 346, with the key bit sequence k′ calculated, by using a processing device, such as the CPU 911.

[1030] The ID-based cryptographic communication system 800 according to the present Embodiment has the following effect:

[1031] It is possible to perform encryption processing with a small amount of resources, and to realize the cryptographic communication utilizing the ID-based cryptosystem where the SMMR-IND-ID-CCA2 security can be mathematically proved.

[1032] Referring to FIGS. 18 to 24, Embodiment 3 will now be explained.

[1033] Before explaining the ID-based cryptographic communication system 800 according to the present Embodiment, an ID-based cryptographic communication system, which is a comparative example, will be described first.

[1034] Since the whole configuration of the ID-based cryptographic communication system which is a comparative example according to the present Embodiment is the same as that described in the Embodiment 1, its description is omitted herein.

[1035] First, the encryption parameter generating apparatus 100 will be described. Since the block configuration of the encryption parameter generating apparatus 100 is the same as that described in Embodiment 1, points different from Embodiment 1 will be explained with reference to FIG. 4.

[1036] The group parameter storage unit 111 stores information indicating a natural number r, a multiplicative group G1, a multiplicative group G2, a multiplicative group GT and a pairing e, as group parameters, by using a storage device, such as the magnetic disk drive 920.

[1037] The natural number r is a large prime number based on which it is difficult to solve the DH problem.

[1038] The multiplicative group G1, the multiplicative group G2, and multiplicative group GT are multiplicative groups whose order is the natural number r.

[1039] The multiplicative group G1 and the multiplicative group G2 may be the same multiplicative groups or different multiplicative groups. When the multiplicative group G1 and the multiplicative group G2 are the same multiplicative groups, the group parameter storage unit 111 does not need to store the information indicating the multiplicative group G2.

[1040] Pairing e is a mapping which calculates an element of the multiplicative group GT, from the element of the multiplicative group G1 and the element of the multiplicative group G2. Pairing e shall be bilinear pairing and shall be calculated within a practical time by using a processing device, such as the CPU 911 of a computer.

[1041] The secret information generating unit 112 inputs a group parameter stored by the group parameter storage unit 111, by using a processing device, such as the CPU 911.

[1042] The secret information generating unit 112 randomly generates an integer α being greater than or equal to 1 and less than the natural number r, based on the natural number r indicated by the input group parameter, by using a processing device, such as the CPU 911.

[1043] The secret information generating unit 112 outputs information indicating the generated integer α, by using a processing device, such as the CPU 911.

[1044] The public parameter generating unit 113 inputs the group parameter stored by the group parameter storage unit 111, and the integer α output by the secret information generating unit 112, by using a processing device, such as the CPU 911.

[1045] The public parameter generating unit 113 determines a key generation function KDF, based on the multiplicative group GT indicated by the input group parameter, by using a processing device, such as the CPU 911.

[1046] The key generation function KDF is a function for calculating a bit sequence of predetermined length from an element of the multiplicative group GT.

[1047] The public parameter generating unit 113 determines a hash function HF by using a processing device, such as the CPU 911.

[1048] The hash function HF is a function for calculating a bit sequence of predetermined length from a bit sequence of arbitrary length.

[1049] The hash function HF shall have a very low probability of calculating the same bit sequence from different bit sequences.

[1050] Hereafter, the length of the bit sequence output by the hash function HF shall be m bits.

[1051] The public parameter generating unit 113 determines two functions V1 and V2, based on the multiplicative group G2 indicated by the input group parameter, by using a processing device, such as the CPU 911.