Computer network security system employing portable storage device

[0001] The present invention relates generally to computer network security systems. More particularly, the invention relates to a security system that allows secure access by a remote authenticated user via a remote client that utilizes a portable storage device containing user-generated one-time passwords.

[0002] With the proliferation of computer network-propagated viruses and worms, and with the increasing frequency at which computer networks are being broken into, there is considerable interest today in computer security. Sophisticated network administrators construct secure firewalls to prevent such attacks. Less sophisticated system administrators, including most home network computer users, employ security measures that are far less robust. At present, many home computer networks are simply unprotected. Computer network security is a complex issue, and many home network users simply do not have the skill or training to ensure that their networks are free from attack.

[0003] The present invention provides a network security solution that is both highly secure and easy to use. The invention is thus ideal for home network security applications, where the network “administrator” may not necessarily have a great deal of training or experience in security issues.

[0004] The invention employs a portable storage device that maintains a set of one-time passwords. Using system software from a secure vantage point within the home network, the user generates a set of one-time passwords that are stored on the portable storage device. The portable storage device may then be installed in or connected to any remote client computer, giving that remote client computer the ability to establish and authenticate a secure connection with the home network. Each password is used only once, and session management software within the home network has the ability to limit a session to a predetermined length of time (e.g., 30 minutes). Although communication between home network and remote client is preferably over a secure channel, communication of the one-time password over this channel is further protected by using an encrypted version of the user's PIN number. The PIN number is encrypted at the remote client using a plug-in module that accesses a protected area within the portable storage device to retrieve the key used for this encryption.

[0005] The preferred embodiment takes the form of a home gateway that includes a firewall which functions as a screening router. The screening router screens out all requests to access content on the home network. All URLs associated with the home network are unreachable directly from the outside, and are thus maintained as protected URLs. The remote client, even after authentication, cannot issue URLs for the home network directly. Rather, upon authentication, a web proxy system is employed to communicate with the home network on behalf of the authenticated remote client. The proxy system works in conjunction with URL modification and URL verification processes. The URL verification process verifies the authenticity of the client, while the URL modification process gives the web proxy system the correct reference for the trusted domain resource. The URL modification process is unique for each authenticated client and for each authenticated session. Thus URLs that have been modified for a given authenticated client and for a given session cannot be re-used by other clients, or even for the same client during a later session.

[0006] In the preferred embodiment the authentication function is performed by a bastion host system forming part of the home gateway. The bastion host has software to perform the remote key authentication process by which the remote client authenticates itself using the one-time password obtained from the portable storage device. The bastion host also performs the URL verification and modification functions mentioned above.

[0007] The invention thus affords a high level of security in an easy-to-administer package. Everything a user needs to gain access to the home network from a remote client computer (except for knowledge of the user's personal identification number) is stored on the portable storage device. In a presently preferred embodiment the storage device also includes suitable browser plug-in software that supplies a remote client computer's browser with the capability of performing the authentication process, including the process of accessing and using the appropriate one-time password.

[0008] Unlike other security systems that rely on a trusted third party source for key distribution, the invention allows the user to create his or her own keys by operating configuration software at the trusted home network site. The user thus configures the portable storage device, supplying it with a set of one-time passwords using the configuration software.

[0009] The configuration software also installs a corresponding set of authentication codes in a secure database associated with the gateway. In this way, both the gateway and the portable storage device are provided with the corresponding keys needed to perform authentication. This solves the problem of how to securely distribute keys to a remote client, so that the remote client can then gain access to the home network.

[0010] For a more complete understanding of the invention, its objects and advantages, refer to the following specification and to the accompanying drawings.

[0024] The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

[0025] An exemplary implementation of the secure network access architecture is illustrated in FIG. 1. The remote client 20 communicates with the home network 22 via a hostile network, such as the internet 24. The home network 22 can be as simple as a single computer or as complex as a full scale local area network. Because the invention is particularly well suited for use by unsophisticated users, the home network illustrated in FIG. 1 comprises a fairly simple local area network including a server 26, a printer 28 and a pair of desktop or laptop computers 30. While a simple home network has been illustrated here, it will be appreciated that the principles of the invention can readily be extended to larger networks such as those found in offices, public agencies, universities and the like.

[0026] From a security standpoint, the home network 22 is a trusted network. In contrast, the internet 24 is a hostile network. The system operator of the home network has the ability to configure the home network so that only authorized users can access it and use its services. The home administrator does not have similar control over the internet 24. Therefore, to separate the trusted side from the hostile side, the present invention employs a gateway device 32. The details of the gateway device will be described more fully below. Essentially, the gateway functions as a gatekeeper, blocking all unauthorized access to the home network. The gateway device is also responsible for mediating the authentication process by which the remote client 20 authenticates itself so that it can gain access to the server 26 on the home network 22.

[0027] A key component of the authentication mechanism of the invention is a portable storage device 34. Two such devices are shown in FIG. 1, one being configured at A and one being used by the remote client at B. The gateway device includes a suitable memory recording apparatus 36 to initialize and configure the portable storage device. The portable storage device 34 can take a variety of forms, including CD ROM, DVD, CD-R. DVD-RAM, flash memory, and other hard disk-based media or solid state memories. A CD ROM embodiment has been illustrated in FIG. 1

[0028] According to one aspect of the invention, any portable storage device can act as a remote key storage mechanism. However, in the presently preferred embodiment, an additional level of protection is provided by including within the portable storage device a secure protected area. The details of this protected area will be discussed more fully below.

[0029] The portable storage device 34 functions as a secure key distribution mechanism. The user initializes and configures the portable storage device within the perimeter of the home network. More specifically, in the preferred embodiment the user accomplishes this using a secure interface to the portable storage device, defined within the gateway, and using secure software modules running on the gateway in a tightly controlled, secure fashion. In the preferred embodiment, the gateway includes a secure database in which the keys are also stored. This database is likewise a tightly controlled, secure entity that may be accessed only by secure software modules running on the gateway.

[0030] In the future, if secure components are developed to maintain interface and software module security across a distributed environment, the portable storage device initialization and configuration functions, and the secure database maintenance functions, may be implemented using computer systems within the trusted domain, other than the gateway.

[0031] After initialization, the portable storage device may then be taken by the user to any computer anywhere in the world. By installing the portable storage device in that computer (such as in the remote client 20), the computer is rendered capable of authenticating itself for communication with the home network 22. In the presently preferred embodiment the portable storage device is provided with one-time passwords during initialization. Thereafter, a plug-in program accesses these one-time passwords as will be more fully explained. The plug-in program may be a program module or applet suitable for use by web browser software already resident on the remote client 20. The plug-in software enables the remote client 20 to participate in the exchange of messages needed for authentication. The preferred embodiment uses the secure sockets layer (SSL) to establish a secure pipeline 38 through the internet 24.

[0032] Referring to FIG. 2, the gateway device 32 implements a firewall 40 that employs a packet filtering screening router 42. This router is configured to prohibit all remote clients from accessing any host on the home network 22 directly. The screening router in turn communicates with a bastion host 44 which performs the remote key authentication functions 46 and the web proxy functions 48, as will be more fully discussed below. Essentially, the bastion host 44 requires authentication, using the appropriate one-time password obtained from the portable storage device. To ultimately obtain information through the firewall, the remote client must successfully participate in the remote key based authentication process shown diagrammatically at 21.

[0033] Once authentication has been successfully completed, the bastion host performs URL address translation and client verification services as part of the web proxy functions 48. Specifically, URLs arriving from the remote client are verified as coming from the authenticated client, and then modified specifically for that client. The web proxy system consults active state middleware (ASMM 72, FIG. 4) to determine if there is an active state for that authorized client. If so, the web proxy system accesses the trusted home network on behalf of the remote client. Notably, the address translation function is specific for each client. Re-use of URLs is prohibited, thus thwarting a system attack where URLs for an authenticated client are intercepted and reused by an impostor. Further details of this process are described below in connection with FIG. 9.

[0034] To better envision how a remote client is able to gain access to information stored on the home network, refer to FIG. 3. FIG. 3 presents a data flow diagram illustrating how the access to information on the home network is effected. The remote client establishes a secure channel 50, such as an SSL connection over the internet. The screening router 42 serves as a firewall that always blocks URLs associated with the home network as depicted at 52. The screening router 42 does allow certain communications to pass through the firewall, namely those needed for authentication and subsequent communication using the web proxy functions. Thus the screening router communicates with the bastion host 44, first, for session management and authentication, and second, for web proxy services after authentication.

[0035] The bastion host includes a session manager that requires authentication using a one-time password. For added security, the session manager also controls the length of the session by terminating the session after a predetermined time (e.g., 30 minutes). These control functions are illustrated at 54. The session manager performs the authentication function by interaction with the plug-in software within the portable storage device as depicted at 56.

[0036] The portable storage device is provided with one-time passwords as part of the user configuration depicted at 58. Specifically, the gateway generates random numbers and random keys which are then personalized for a specific user, using the user's PIN. The PIN thus becomes a factor in the encryption equation. Specifically, the PIN is exclusive-ORed (XOR) with the key K_i. To prevent discovery of the user's PIN through brute force attack, the preferred embodiment uses an encrypted version of the user's PIN as one of the factors needed to generate the one-time password. The value used to encrypt the user-supplied PIN is a special symmetric key K_s. K_s is generated by the gateway and stored in a protected area within the portable storage device. Only a properly authenticated storage device, and a properly authenticated user of the portable storage device can access the protected area to retrieve K_s.

[0037] Specifically, the portable storage device must be authenticated by the reading device into which it is installed, inserted or attached, and the user must be authenticated by supplying the PIN—after being prompted by the plug-in module to enter it.

[0038] Once the session manager establishes an authenticated session, the requests for URL access are processed by URL verification and modification functions used in conjunction with the web proxy functions depicted at 60. The web proxy then communicates with the trusted home network, to obtain information or resources on behalf of the remote client, using the modified URLs, as verified and customized for the particular authenticated client.

[0039] The details of the gateway's proxying and client authentication module's interaction are shown in FIG. 4. In the presently preferred embodiment, the gateway 32 is the only machine with a valid certificate and the only host equipped with the SSL web proxy server 48. This SSL proxy server is the only entity used in all remote client accesses of the home content. The packet filtering policies deployed at the screening router are configured appropriately: only the https traffic directed at the web proxy is passed by a packet filter, all other traffic is rejected.

[0040] The process begins with the remote client 20 issuing the direct https URL request to a gateway. This request includes a User ID. At this point the remote client and gateway's remote key authentication module 70 take part in the authentication process based on use of a one-time password. The details of this authentication will be discussed in detail below.

[0041] Successful authentication creates a “client-gateway session state” (CGSS) in the active session management middleware (ASMM) 72 of the gateway. At the beginning of each session, a session duration timer associated with ASMM 72 is reset to zero (e.g., not to exceed W=30 minutes). The ASMM 72 then retrieves the user-specific parameters from a secure user database 73. This secure user database is secretly populated with information about the one-time passwords when the user configures the portable storage device (as at 58 in FIG. 3). This secret information and other CGSS parameters are used during the authentication and URL validation processes.

[0042] Specifically, after authentication ASMM 72 instructs the SSL web proxy components of the web proxy server 48 to do the appropriate web proxying combined with URL validation as at 74 and to perform the bi-directional URL modification processes as at 76. Those processes are described more fully below in connection with FIG. 10.

[0043] After the expiration of a user session (suggested 30 minutes), the web proxy system sends a custom page (from the templates page database 78) to the remote client, indicating that a new portable storage device-based authentication must take place. The remote user is also able to explicitly tear down the client-gateway session (CGSS), by issuing a request of the type:

• • [0044] https://gw_host/?UserID=db&TearDown=on
    The ASMM 72 then tears down the client-gateway session state (CGSS) and instructs the web proxy system not to resume the “old” SSL session. A full SSL handshake has to be performed again by the client and the proxy to establish the new SSL session, perform the full round of portable storage device-based authentication, and create the new CGSS. It is important to note that once the CGSS is torn down, no proxying of the web content may occur in response to an “outside” request, since the requested URLs would fail their authenticated validations.

[0045] In the preferred embodiment, the gateway 32, with the proxy service running on it, is the only host which needs a valid IP address. The rest of home hosts may obtain their IP addresses from the internally managed address pool, using such mechanisms as DHCP and network address translation (NAT).

[0046] With the overall architecture of the presently preferred embodiment in mind, details of the preferred remote key authentication process will now be described with reference to FIGS. 5-8. Referring to FIG. 5, authentication is mediated by the remote key authentication module 46. The presently preferred embodiment uses at least 128 bit-strength ciphers, meaning that all session keys (K_i, K_s) should be at least 128 bits long. Prior to use, keys are generated by a key management system detailed below in connection with FIG. 11.

[0047] In the preferred embodiment we require a “true” random number generator for generation of random cryptographic parameters. Many of the security aspects of the proposed system depend on random sequences used as one-time passwords and keys for their encryption. Such generator could take into account, for example, external temperatures, information about networking connections and many other external sources, and combine these together using some strong hashing functions (e.g., MD5).

[0048] The portable storage device-based authentication process is shown in FIG. 6. During the remote key's initialization stage, the “future” remote user must be physically proximate to the home gateway. The user must insert the key card (or CD media) into the gateway for the initialization procedure and subsequent transfer of one-time passwords to the key card. Remote key card initialization process is depicted in FIG. 11 and is described in detail below. During this initialization stage, the gateway simply generates a set number (e.g., N=100) of random passwords (S_i), encrypts those with random keys (K_i) and makes them dependent on the user's knowledge of the 4-digit PIN (which is then protected by a symmetric key (K_s) stored in the protected area). These encrypted passwords (E_i) are then stored in the portable storage device that can be taken by the remote client on a trip. Passwords (S_i) and corresponding keys (K_i) are also stored in the gateway's secure database for access by the remote key authentication module 70 (FIG. 4).

[0049] Referring to FIG. 6, The authentication process then proceeds according to the steps enumerated as follows:

• [0050] Step (1) Remote client submits its user ID to the gateway inside https request.
• [0051] Step (2) Gateway sends the counter's value (i) to the remote client along with the value of corresponding key (K_i).
• [0052] The gateway uses the counter (i) to index the number of successful user authentications. If N=100, the counter goes from 100 down to 1. When the counter reaches 0, no more authentications are allowed for a given user ID without re-initializing the key card.
• [0053] Step (3) Client browser Plugin software decrypts the one time password (E_i) according to the expression: S_i=D_{Ki⊕Eks(PIN)}(E_i). Plugin sends S_i to the gateway's Key Card Authentication module.

[0054] At this point the remote key authentication module 70 (FIG. 4) compares the value S_j received from the client with the one stored in its secure database 73 (FIG. 4). If they match, the ASMM 72 decrements the value of the usage counter (i) and establishes CGSS inside the gateway's ASMM 72 (FIG. 4).

[0055] A denial of service (DOS) attack would occur if an intruder were able to use up all the password space by simply submitting the User ID to the gateway multiple times. To prevent this the preferred embodiment decrements the counter only after a successful user authentication. In addition, we suggest a time window (W), within which a maximum of three authentication attempts is permitted. If the number of unsuccessful attempts reaches three, no further authentication attempts for a given user are permitted within this current W. The proposed length for the window W is 5 minutes.

[0056] For a more complete understanding of the authentication process, refer now to FIGS. 7 and 8. FIG. 7 illustrates the processing performed at the remote client once the appropriate information has been received from the gateway. FIG. 8 illustrates the same process at a somewhat higher level, showing the exchange of messages that are passed among the various entities that participate in the authentication process.

[0057] Communication between the remote client and the gateway begins with the remote client accessing a log-in page generated or served from pages stored in the template page database. This invokes the plug-in module, which then prompts the user to supply his or her log-in name or user ID. The user then enters his or her user ID, and this information is sent to the gateway where the ID is checked by the ASSM middleware and where the secure database is accessed to retrieve the values of i and K_i that are appropriate for that user. Once these values are retrieved, the authentication process is ready to begin.

[0058] The authentication process begins with the gateway communicating the index number i and the key value K_i to the remote client, which is operating using the plug-in module. In FIGS. 7 and 8 this first exchange of information (i, K_i) is illustrated at 200. The plug-in module uses the index value i to index into the table of one-time passwords, E₁ . . . E_N. Note that these are encrypted passwords. In FIG. 7 encrypted password E_i is retrieved by the plug-in module at 202.

[0059] Meanwhile, the plug-in module prompts the user to enter his or her secret PIN number as at 204. The user then enters the secret PIN at 206. In the preferred embodiment, the user's PIN is not used in its plaintext form. Instead, the PIN is encrypted and the encrypted PIN is then used as one factor in generating the key needed to unlock the encrypted one-time password. As shown at 208, the user's PIN is encrypted using the value K_s which is stored in the protected area within the portable storage device. Access to the protected area is gained only upon authentication of the portable storage device.

[0060] The plug-in module retrieves the encrypted session key K_s at 210 and then uses it at 208 to encrypt the user-supplied PIN. At this stage in the process the plug-in module thus has the three pieces of information it needs to generate the one-time password, namely K_s, E_i and K_i. The index value i is used to retrieve a selected one of the encrypted one-time passwords E_i at 202. The encrypted PIN E_Ks, is combined through an exclusive-OR (XOR) operation with the value K_i and the result is used to decrypt the encrypted one-time password E_i. The decryption process is shown in FIG. 7 at 214. The decryption process generates a single one-time password S_i, shown in FIG. 7 at 216. This one-time password is then sent back to the gateway at 218. The gateway is then able to compare the one-time password with the stored one-time passwords within its secure database (secure database 73 of FIG. 4) to verify that the remote client is authorized to proceed with secure communication.

[0061] As previously noted, all direct communication with the secure trusted network is blocked by the screening router 40. Only the proxy system has access to the trusted network, and it will obtain information and services on behalf of a remote client, only after proper authentication has occurred. In performing its task, the proxy system uses the URL validation and URL modification services. The details of these validation and modification services are shown in FIGS. 9 and 10.

[0062] Despite the fact that the communication between the remote client and the home host is protected by the SSL-based encryption, we propose to further strengthen security in a way that guarantees that only the properly authenticated client could be the party capable of issuing a valid URL. This approach will “bullet proof” all URLs, even if the SSL channel is compromised by an intruder.

[0063] As each web page passes through a web proxy toward the remote client, each internal to the home site URL href reference is rewritten by the URL modification module 90 FIG. 10 in a fashion shown below:

• • [0064] https://gateway_host/home_host/MSB/₆₄{MD5₁₂₈((Original_URL∥
  • [0065] User_ID)⊕S_i}/Original_URL/?UserID=db, where S_i=D_{Ki⊕Eks(PIN)}(E_i)

[0066] This reference then becomes the URL, which the remote client receives and subsequently would issue within its requests to the web proxy server. The overall process is shown in FIG. 10. When the modified URL arrives from the remote client to the gateway, the URL verification (validation) module 92 verifies its authenticity. The URL modification module then translates the URL into its regular form:

• • [0067] http://home_host/Origina_URL
    This then becomes the request that the web proxy issues over the trusted home network. Note that the URL modification process is bi-directional. Incoming URLs from the remote client are modified specifically for that client (upon authentication). Similarly, outgoing URLs sent to the remote client are also modified using the same modification rules.

[0068] Given that S_i is only valid for a given remote client session (W=30 minutes), even if the underlying SSL connection is compromised, URLs obtained by the intruder are worthless for referencing within any future sessions.

[0069] The portable storage device by which one-time passwords are securely distributed to a remote client forms an important part of the network security system. As noted above, the system is designed to allow the user to conveniently configure his or her own portable storage device using initialization and configuration software deployed at the gateway. The details by which the portable storage device is configured will now be described in connection with FIG. 11.

[0070] Before the user can login from the remote location, he or she needs to initialize the portable storage device (remote key) by inserting it into a the recording apparatus 36 on gateway 32. CD/DVD media is being used as portable storage, the gateway must be equipped with a CD/DVD writer capable of storing information into “user” and “protected” areas on the portable media. The process is shown in FIG. 11. The remote key management module 70 (FIG. 4) performs the remote key initialization process.

[0071] During each portable storage device initialization process, the gateway (FIG. 11, step (1)) generates a set number (N) of user one-time passwords (S_i). These are simply the random numbers obtained from the random number generator. In addition, the gateway generates N random keys (K_i) and forms pair associations between each one-time password and each key (FIG. 11, step (2)).

[0072] In step (3), the gateway's remote key management module obtains a User PIN from the authentication plug-in software at the user side, generates random K_s (step (4)), authenticates the key card, and stores K_s within the protected area of the portable storage device (step (5)).

[0073] In step (6) the gateway's remote key management module 75 (FIG. 5) computes the value of Ei=E_{Ki⊕Eks(PIN)}(S_i) for each pair association (S_i, K_i). These values are then stored in the user data area of the portable storage device in step (7).

[0074] Finally, the user's ID, user's PIN, K_s, and OTP key-password pairs (S_i, K_i) are stored in the secure database 73 (FIG. 4) for future reference by the Remote Key Authentication 70 and ASMM 72 modules (see FIG. 4). If any of the above steps fails for any reason, the overall portable storage device initialization process fails.

[0075] Once the portable storage device has been initialized with keys, the user can take it to any suitably equipped computer, anywhere in the world, and engage in secure client-server communication with the gateway. In the presently preferred embodiment, the remote client computer uploads or installs a plug-in software module (carried by the portable storage device, for example). The plug-in software module configures the client computer to participate in the exchange of messages needed to effect authentication. The plug-in software module also allows the remote client computer to generate the appropriate one-time password by selecting and decrypting one of the passwords stored on the portable storage device using a key generated using information from the gateway and the user's PIN. In the preferred embodiment an encrypted version of the user's PIN is used to generate the key needed to decrypt the selected one time password. The user's PIN, supplied as plaintext by the user, is encrypted using a session key stored in the protected area within the portable storage device. Basing the authentication on an encrypted PIN makes the preferred embodiment more secure, by reducing system vulnerability to a brute force attack upon information communicated over the secure SSL channel.

[0076] The preferred embodiment is designed to prevent any modifications to the remote client's Web browser. However, in order for a client to perform the portable storage device-based authentication process described earlier, the browser would require the help from a plug-in module, capable of accessing the portable storage device and performing some auxiliary cryptographic computations. FIG. 12 shows the browser-Plugin-gateway interaction in detail.

[0077] In FIG. 12, step (1) the remote client submits the query to the gateway, indicating its need to establish the SSL session for a subsequent OTP exchange and including the client's User ID.

• • [0078] https://gw_host/?UserID=user_name

[0079] In step (2), the remote key authentication module 70 FIG. 4 responds with the page containing the <OBJECT> tag, indicating the Plugin's name and the values of (i) and (K_i), passed as plug-in's parameter strings. At this point the Plugin “pops up” a user widget, asking the client for his or her PIN. It then accesses the values of E_i and K_s from the protected area of the portable storage device, computes the value of the one-time password:

• • [0080] S_i=D_{Ki⊕Eks(PIN)}(E_i)
    This value is send back to the remote key authentication module via the same SSL channel in step (3):
  • [0081] https://gw_host/?UserID=db&Si=123456789ABCDEF0FEDCBA9876543210.

[0082] Preferably, the plug-in module should be signed by the manufacturer of the gateway, and be tamper-resistant. Use of cookies is also possible. The cookie, for example, may be modified by the client's helper application, which participates in the user authentication.

[0083] From the foregoing, it will be appreciated that the invention provides a user-friendly, convenient and yet highly secure system for effecting secure communication with a computer system or computer network. The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.