System and method for secure replacement of high level cryptographic keys in a personal security device

[0001] The present invention relates to a data processing system and method for replacing root cryptographic keys installed inside a personal security device without requiring the use of potentially compromised root keys.

[0002] There is considerable knowledge in the current art involving cryptographic key replacement strategies for issued personal security devices (PSD) such as smart cards, subscriber identification modules (SIM), wireless identification modules (WIM), identification tokens, integrated circuit cards (IC cards) and related devices. Most replacement strategies rely on the use of the installed keys in order to perform key replacements, which may allow a fraudulent key holder to monitor the key replacement and thus potentially compromise the replacement keys. For example, U.S. Pat. Nos. 6,240,187 and 5,761,306 by Lewis describe sophisticated mechanisms to perform asymmetric key replacements incorporating sequential key generation and cryptographic techniques in order to securely send the replacement keys over an open network to a PSD. The sequential key replacements are dependent on currently installed keys.

[0003] In another approach, U.S. Pat. No. 6,230,267 by Richards, et al. describes a secure data loading process which could be extended to installing asymmetric keys and the preferable use of properly authenticated digital certificates to ensure that data stored inside a PSD has not been compromised. The approach taught by the Richards patent is effective for domain level key replacements but is not intended for use in replacing compromised high level key sets such as root keys or master keys. Again, the data transfer arrangement relies on the integrity of existing keys in order to ensure a secure data transfer.

[0004] In a third approach, U.S. Pat. No. 4,972,472 by Brown, et al. incorporates three separate cryptographic keys having active, retired and replacement status respectively. This key replacement strategy requires a secure channel in order to perform the key replacements and could allow a compromised key set to remain active for a predetermined period before being “retired.” Another limitation in employing this approach is the lack of a secure key replacement mechanism, which ensures the integrity of the installed keys. The intent of the Brown patent is to provide a transition period for normal key replacements.

[0005] A fourth approach is described in U.S. Pat. No. 5,680,458 by Spelman et al. where notification and replacement of a compromised asymmetric root key are performed using an out-of-band mechanism and having each PSD holder enter a key sequence which forms a partial replacement key. This viable approach relies on time consuming and potentially costly out-of-band notifications and end user intervention in order to change a potentially compromised root key.

[0006] Other common alternatives include returning the PSDs to the issuer for key exchanges; however, this removes the PSDs from service and impacts the PSD's end users. The final alternative is to simply dispose of the current PSDs and issue replacements, which is an expensive solution and may result in loss of customized data stored in the issued PSDs.

[0007] Little is disclosed in the current or prior art concerning secure recovery from the loss or compromise of a hardware security module (HSM), which may contain the master keys or the ability to generate master keys for issued PSDs. The loss or compromise of an active HSM is a particularly difficult and costly situation to remedy. Thus it is apparent that a secure root level key replacing technique which does not rely on currently active key sets or user intervention and can be performed using existing communications infrastructures is highly desirable. A method and system is described herein, which solves several of the limitations described above.

[0008] Cryptographic key replacement attempts to securely replace key sets already deployed with newly generated key sets in the event that the currently deployed key sets becomes compromised. A distinction is made from normal updating of keys before a pre-determined key's valid period expires. In the later case, a new set of keys can be securely installed using the expiring keys. For the former, it is not be appropriate to replace the compromised key using the compromised key itself due to the ability of the fraudulent key holder to monitor the key replacement process.

[0009] This invention provides a system and method for generating, installing and activating a high level key set, which is intended to replace a currently active high level key set without requiring the use of the currently active high level key to perform the key replacement.

[0010] To practice this invention, two high level keys are initially installed and registered with the internal PSD security executive during the PSD's initial personalization phase; one key is used as a master key, the second is used as a master key replacement key. The master key replacement key allows access to the PSD in order to securely replace a potentially compromised master key.

[0011] During the personalization stage, a diversification secret is generated and associated with a PSD or group of PSDs by use of the PSD's masked identification number commonly referred to as a PSD identification number or PSD ID. The diversification secret is used to generate a master key replacement key. The diversification secret may be generated using a random number generator or other equivalent means, which produces a sufficiently robust data block.

[0012] Once the master key replacement key has been generated, the diversification secret is encrypted with a key management system public key and the resulting cryptogram is stored on a secure server. The key management system's private key is retained in a secure location usually within the secure domain of the key management system's hardware security module (HSM.)

[0013] In the preferred embodiment of the invention, generation of diversification secrets and encryption with the key management system's public key(s) occur within the secure domain of one or more of the access server's HSM(s). It is also envisioned that the encrypted diversification secret(s) may be stored online on a local hard disk or offline using a compact disk (CD).

[0014] In the event it becomes necessary to perform a master key replacement due to an actual or potential high level key compromise, the appropriate encrypted diversification secret(s) are retrieved from storage using the affected PSD's identification number(s) as a cross reference. The secret(s) are then decrypted using the key management system's private key, used to diversify the recovered master key replacement key data block, which regenerates the master key replacement key(s.) A new master key set is generated using a new master key data block diversified using the PSD's identification number.

[0015] A secure channel is then established between the access server's HSM and the PSD(s) containing the compromised master keys. The master key replacement keys are then used to access the card executive, delete the compromised master key(s) and install the replacement master key(s). It is also envisioned that a new set of key replacement keys may be generated and installed in the PSDs following the master key replacement(s).

[0023] This invention describes a method and system to generate a replacement key used to securely access a personal security device (PSD) in the event a security executive level key becomes compromised, for example a PSD master key.

[0024] In the preferred embodiment of the invention, a first server equipped with a hardware security module (HSM) is connected to one or more clients over a telecommunications network. The first server performs the routine authentication and cryptography service for clients connected over a telecommunications network and is hereinafter referred to as an access server. A second server, equipped with another HSM, performs cryptographic key management and is hereinafter referred to as a key management server. The physical location and arrangement of the servers is intended to minimize theft or compromise of the HSMs.

[0025] Referring to FIG. 1A, the preferred embodiment of the invention is shown where a first HSM 10 associated with an access server 20 has previously been initialized at the secure key management server site 120. The access server HSM 10 contains a public key Kpub 45B, a master key data block MKmas 55B and a master key replacement key data block MKrep 65B. Details of the initialization process are provided in the discussion for FIG. 1B.

[0026] The access server 20 and associated HSM 10 are interconnected over a telecommunications network 60 with a client 90 and associated PSD(i) 100. The telecommunications network 60 includes both open and private network arrangements. During key replacements, the access server 20 and client 90 roles are limited to maintaining a secure communications channel between the HSM 10 and PSD(i) 100, thus maximizing end-to-end security. The access server 20 includes storage 70 capabilities for storing an encrypted diversification secret 85. The storage 70 may include both online and offline mechanisms.

[0027] The PSD(i) 100 includes a unique master key Kmas(i) 55C and a unique key replacement key Krep(i) 65C generated by the access server HSM 10 and securely injected into the PSD100. In the preferred embodiment of the invention, both the master key Kmas(i) 55C and the master key replacement key Krep(i) 65C are injected into the PSD(i) 100 during the pre-issuance personalization stage. It is also envisioned that the master key replacement key Krep(i) 65C may be installed post issuance using Kmas(i) 55C to allow access to the PSD security executive.

[0028] In the preferred embodiment of the invention, the key management server 30 is maintained in a secure location 120 and operates as a centralized offline facility for managing the overall key management system. Associated with the key management system is a second HSM 40 containing a private key Kpri 75A, which is the counterpart to Kpub 45B, a master key data block MKmas 55A and a master key replacement key data block MKrep 65A. The original public key Kpub 45A may also be stored within this HSM 40 or locally stored on the key management server.

[0029] In an alternate embodiment (not shown) of the invention, the access server 20 performs the same role as the key management server 30. In this alternate embodiment of the invention, a key management HSM 40 generates the Kpri 75A, Kpub 45A, MKmas 55A and Mkrep 65A. The key information Kpub 75A MKmas 55A and MKrep 65A are then securely transferred to an access server HSM 10 and stored in non-volatile memory. The key management HSM 40 is then removed from the access server 20 and securely stored under lock and key until required for master key replacement.

[0030] Referring to FIG. 1B, the access server HSM 10 initialization process is depicted where a copy of the following information is transferred from the key management server HSM 40: a public key Kpub 45A, the master key data block MKmas 55A and the master key replacement key data block MKrep 65A. The private key Kpri 75A remains inside the secure domain of the key management system HSM 40 until required for master key replacement. The information transferred to the access server HSM 10 are identified as public key Kpub 45B, master key data block MKmas 55B and master key replacement key data block MKrep 65B respectively.

[0031] In FIG. 2, the initial PSD personalization process is shown which incorporates an initial master key Kmas(i) 55C and a master key replacement key Krep(i) 65C inside the secure domain of a PSD(i) 100. The master key Kmas(i) 55C is generated by diversifying the master key data block MKmas 55B using the PSD identification number PID(i) 110B. Once generated, Kmas(i) 55C is then securely transferred from the access server HSM 10 through the access server 20 to a PSD writer 50 where Kmas(i) 55C is securely injected 245 into the PSD(i) 100. Depending on the entity performing the PSD personalization, the PSD manufacturer's master key may be required in order to inject Kmas(i) 55C into the PSD(i) 100. In this case, the process reverts to a normal master key replacement, which is known in the art.

[0032] The personalization process includes generation of a secret 85A by the access server HSM 10. In the preferred embodiment, the secret 85A is generated using a random number generator. The master key replacement key Krep(i) 65C is generated by diversifying the master key replacement key data block MKrep 65B with the secret 85A. Krep(i) 65C is then securely transferred from the access server HSM 10 through the access server 20 to the PSD writer 50 where Krep(i) 65C is securely injected 245 into the PSD(i) 100. As before, depending on the entity performing the PSD personalization, the PSD manufacturer's master key may be required in order to inject Krep(i) 65C into the PSD(i) 100. In this case, the process reverts to a normal master key replacement, which is known in the art. The order of injection of Kmas(i) 55C or Krep(i) 65C is not critical to the invention.

[0033] Once the master key replacement key Krep(i) 65C has been generated, the secret (random number) 85A is encrypted by the access server HSM 10 using the public key Kpub 45B and cross-referenced with the PSD identification number PID(i) 110B. Once the secret 85A has been encrypted using Kpub 45B, the secret is deleted without leaving the secure domain of the access server HSM 10. The encrypted secret 85B is transferred 255 to the access server 20 and stored in a secure storage location 70. Cross-referencing the PSD's identification number PID 110A is used to retrieve the encrypted secret 85B.

[0034] Referring to FIG. 3A, a post issuance replacement hardware security module initialization process is shown. In the preferred embodiment of the invention, new replacement master key generating data MKmas(n) 355A is generated by the key management server HSM 40. A replacement access server HSM 310 is initialized by the key management server 30 by transferring a copy of the new master key data block MKmas(n) 355A, a copy of the original master key replacement key data block MKrep 65A and a copy of the private key Kpri 75A from the key management server HSM 40 to the replacement access server HSM 310. Optionally, a new public and private key pair and a new master key replacement key data block may be generated as well to replace the existing key pairs and data blocks following the master key replacement process. The initialized replacement access server HSM 310 is then connected to the access server 20.

[0035] Referring to FIG. 3B, a post issuance master key replacement process is shown. The PSD(i) 100 is connected to the client 90 which causes the PSD's identification number PID(i) 110A to be retrieved and to be returned 1108 to the access server 20. A secure communications channel is then established between the replacement access server HSM 310 and the PSD(i) 100. The access server 20 then retrieves 325 the appropriate encrypted secret 85B from storage 70 for the target PSD(i) 100 by the PSD's associated internal identification number PID(i) 110B. The encrypted secret 85B is then transferred 330 into the replacement access server HSM 310 where the encrypted secret 85B is decrypted using the private key Kpri 75B. The resultant secret is then used to diversify the master key replacement key data block MKrep 365B, regenerating the master key replacement key Krep(n). A new master key is generated as before by diversifying the new master key data block MKmas(n) 355B with the PSD's associated internal identification number PID(i) 110B resulting in the new master key Kmas(n) 355.

[0036] The master key replacement key Krep(n) 365B and the new master key Kmas(n) 355B are then securely sent 375 to the PSD(i) 100. To delete the existing master key Kmas(i) 55C, the regenerated key replacement key Krep(n) 365C is validated 385 by the existing replacement key Krep(i) 65C. If Krep(n) 365C matches Krep(i) 65C, access is granted to the PSD's security executive where the existing master key Kmas(i) 55C is deleted and replaced 395 by Kmas(n) 355C, otherwise the attempted replacement process terminates unsuccessfully. Once the new master key Kmas(n) 355C is installed and registered with the PSD's security executive, the PSD(i) 100 is relocked preventing further access and the secure channel 410 is terminated. Optionally, a new key replacement key may be installed before the security executive is relocked.

[0037] In FIG. 4, a flow chart is shown which describes the initial key replacement key generation process. The process is initiated 400 by generation 410 of an asymmetric key pair by the key management server (KMS). In the preferred embodiment of the invention, the key pair generation process occurs within the secure domain of an HSM. The resulting public key is transferred 420 to an access server and preferably imported into the secure domain of second HSM associated with the access server. A random number is generated by the second HSM associated with the access server, which will be used as a diversification secret 430.

[0038] In a parallel operation, an initial master key is generated 425 at the same time as the initial replacement key generation. This operation may occur within the HSM associated with the access server, HSM associated with the key management server or from another secure server. Once the initial master key is generated, it is then injected 480 into the secure domain of the target PSD and registered with the PSD's security executive.

[0039] The random number generated at 430 is used to diversify a master replacement key data block, which produces 440 the replacement key. The diversification secret is encrypted 450 by the HSM associated with the access server using the public key and stored locally 460 on the access server and associated with the target PSD by its internal identification number or some other unique attribute. The original diversification secret is then deleted 470 without leaving the secure domain of the first HSM.

[0040] Once the initial key replacement key is generated, it is then injected 480 into the secure domain of the target PSD and registered with the PSD's security executive. The PSD is released and the process ends 490.

[0041] In FIG. 5, a flow chart is shown which describes the post issuance key replacement process. The process is initiated 500 by the retrieval 510 of the PSD's unique identification number by an access server. The ID number is used to retrieve the encrypted secret 520 from storage by cross-referencing the PSD ID with the file containing the applicable cryptogram. A new master key is generated at this time as well 525. The encrypted secret is decrypted 530 using a private key previously received from a key management server, revealing a random number. The resulting random number is used to diversify a master replacement key data block, which regenerates a symmetric replacement key 540.

[0042] A secure channel is then established 550 between an access server HSM and the target PSD where the regenerated replacement key is used to unlock the PSD's security executive 560. The existing master key is then deleted 570 and a new master key is injected and registered with the PSD's security executive 580. The security executive is then closed and the secure communications channel with the PSD is terminated 590, which ends the replacement process 595.

[0043] The foregoing described embodiments of the invention are provided as illustrations and descriptions. They are not intended to limit the invention to precise form described. In particular, it is contemplated that functional implementation of the invention described herein may be implemented equivalently in hardware, software, firmware, and/or other available functional components or building blocks.

[0044] Other variations and embodiments are possible in light of above teachings, and it is not intended that this Detailed Description limit the scope of invention, but rather by the claims following herein.