Patent Publication Number: US-6993652-B2

Title: Method and system for providing client privacy when requesting content from a public server

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to network security, and more specifically to a method and system for providing client privacy when requesting content from an application server. 
   2. Discussion of the Related Art 
   The Internet is an insecure network. Many of the protocols used on the Internet do not provide any security. Data that is transmitted over the Internet without using encryption or any other type of security scheme is said to be transmitted “in the clear”. Tools are readily available that allow hackers to “sniff” data, such as passwords, credit card numbers, client identity and names, etc., that is transmitted over the Internet in the clear. Thus, applications that send unencrypted data over the Internet are extremely vulnerable. 
   Kerberos is an example of a known network authentication protocol that is designed to provide authentication for client/server applications by using secret-key cryptography. The Kerberos protocol, which is available from the Massachusetts Institute of Technology, uses cryptography so that a client can purportedly prove its identity to a server (and vice versa) across an insecure network connection. After a client and server have used Kerberos to prove their identity, they can also encrypt all of their communications to purportedly assure privacy and data integrity as they conduct their business. 
   It is with respect to these and other background information factors relevant to the field of network security that the present invention has evolved. 
   SUMMARY OF THE INVENTION 
   The present invention provides a method of providing client privacy when requesting content from an application server. The method includes the steps of: receiving a request for a ticket granting ticket (TGT ticket) from a client; generating the TGT ticket with an identity of the client encrypted therein; sending the TGT ticket to the client; receiving a request for a service ticket (ST ticket) for the application server from the client that includes the TGT ticket and that does not provide the identity of the client in the clear; generating the ST ticket with the identity of the client encrypted therein; and sending the ST ticket to the client without providing the identity of the client in the clear. 
   In another embodiment, the invention can be characterized as a system for providing client privacy when requesting content from an application server. The system includes an authentication server configured to receive a request for a TGT ticket from a client, generate the TGT ticket with an identity of the client encrypted therein, and send the TGT ticket to the client. A ticket granting server is configured to receive a request for an ST ticket for the application server from the client that includes the TGT ticket and that does not provide the identity of the client in the clear, generate the ST ticket with the identity of the client encrypted therein, and send the ST ticket to the client without providing the identity of the client in the clear. 
   A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth an illustrative embodiment in which the principles of the invention are utilized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating a system made in accordance with an embodiment of the present invention; and 
       FIG. 2  is a flow chart illustrating a method of providing client privacy when requesting content from an application server in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Kerberos suffers from the disadvantage that a key distribution center (KDC) reply to a ticket request from a client for a particular application server includes the client name in the clear. Because Kerberos specifies that in such replies the particular application server&#39;s identity is also provided in the clear, the client&#39;s identity can be easily linked to the content. This means that the client&#39;s (i.e. the user&#39;s) privacy is severely compromised because somebody can easily identify the particular servers from which the client is requesting content. Network users requesting content from a public server may not desire to be associated with the content they request. The present invention provides a method and system that overcomes these and other disadvantages and provides improved user privacy when requesting content from a server, such as a public server. 
   The present invention is well-suited to key management protocols that utilize the concept of tickets, which are authentication tokens encrypted with a symmetric key that allow a client to authenticate to a specific server. In accordance with an embodiment of the present invention, the client name or identity is encrypted in all key management messages where the client is either requesting a ticket for a specific application server (e.g. content provider) or is talking directly to the content provider. The user (client) name is encrypted in all key management messages that are either directly addressed to an application server or that contain the server name in the clear. These key management messages are between the client and the KDC and between the client and an application server. The present invention overcomes the disadvantages of standard Kerberos, where standard Kerberos tickets carry the client name in encrypted form but KDC replies to ticket requests for a particular server include the client name in the clear. 
   Referring to  FIG. 1 , there is illustrated a model of a system  100  made in accordance with an embodiment of the present invention. The system  100 , which comprises an example of one possible implementation of the present invention, uses an authentication key management protocol that provides security and privacy on a network, such as the Internet, and that can scale to millions of users. In general, the system  100  involves a client  102  interacting with a centralized Key Distribution Center (KDC)  104  using both public key and symmetric key algorithms, as well as with individual application servers, such as the application server  106 , using only symmetric key algorithms. The protocol is generic and can easily be adapted to different applications that require authentication in a distributed environment. Furthermore, it can be interfaced with a centrally administered user database. 
   The client  102  may comprise a process or device that makes use of a network service on behalf of a user. By way of example, the client  102  may comprise any type of computer, or the client  102  may comprise a “thin client” such as a wireless telephone or home appliance having a low-end microprocessor. Note that in some cases a server may itself be a client of some other server (e.g. a print server may be a client of a file server). The application server  106  provides a resource to network clients. In the illustrated embodiment, the KDC  104  includes an authentication server (AS server)  108  and a ticket granting server (TGS server)  110 . The AS server  108  issues a ticket granting ticket (TGT ticket) to the client  102  after verifying its credentials. The TGS server  110  provides an application server service ticket (ST ticket) to the client  102 . The ST ticket is an end service ticket that the client  102  presents to the application server  106  when the client  102  requests a service. The application server  106  provides various services to the client  102 , when the client  102  authenticates itself using the ST tickets. 
   The basic message types used by the system  100  are as follows: 
   (A) Authentication Server Request message (AS_REQ): Message from the client  102  to request TGT ticket from the AS server  108 ; 
   (B) Authentication Server Reply message (AS_REP): Reply message to the client  102  from the AS Server  108  with the TGT ticket; 
   (C) Ticket Granting Server Request message (TGS_REQ): Message from the client  102  to request an ST ticket from the TGS server  110 ; 
   (D) Ticket Granting Server Reply message (TGS_REP): Reply message from the TGS Server  110  to the client  102  with the ST ticket; 
   (E) Ticket Challenge message (TKT_CHALLENGE): Message that is sent to the client  102  from the application server  106  to initiate key management; 
   (F) Key Request message (KEY_REQ): Message sent from the client  102  to the application server  106  to request security (key management) parameters; 
   (G) Key Reply message (KEY_REP): Reply message from the application server  106  to the client  102  with sub key and application specific data; and 
   (H) Security Established message (SEC_ESTABLISHED): Message from the client  102  to the application server  106  stating that security is established. 
   Each of the messages will typically include a header followed by the body of the message, with the header being common to all the messages. By way of example, the header may include a message type field, a protocol version number field, and checksum. The message type field indicates the message type, such as AS_REQ, AS_REP, etc. Immediately following the message header is the body of the message having the list of attributes preferably in type-length-value format. 
   The client  102  generates an AS_REQ message to initiate the authentication service exchange between the client  102  and the AS server  108  (part of the KDC  104 ) when it wishes to obtain a TGT ticket, which is a ticket for the TGS server  110 , also part of the KDC  104 . In other words, the AS_REQ message is sent by the client  102  to the AS server  108  to obtain the TGT ticket which is used by the client to request ST tickets for specific application servers, such as the application server  106 . By way of example, the AS_REQ message may include the client&#39;s identity (e.g. name), the TGS server  110 &#39;s identity, and a nonce to tie it to a response. It may also include a list of symmetric encryption algorithms that are supported by the client  102 . To check against replays, this message may also include a timestamp, as well as a signature for message integrity. The signature may be a keyed checksum or a digital signature. 
   The public key to verify a signature is preferably kept in the user database. Digital certificates can be optionally included in the AS_REQ message and may be utilized instead of the stored public keys to verify digital signatures. The client  102 &#39;s permanent symmetric key for verifying a keyed checksum is preferably kept in the same user database. The AS_REQ message may also include public key information that is necessary for key agreement (e.g. Elliptic Curve Diffie-Heilman parameters). By way of example, Elliptic Curve may be used for public key encryption because of its processing speed. It is one or two orders of magnitude faster than RSA. The Rijndael encryption standard may be used with the 128-bit key length. 
   The AS server  108  processes the AS_REQ message in order to verify it. If the AS_REQ processing does not generate any error, the AS server  108  generates an AS_REP message in response to the AS_REQ message. Specifically, the AS server  108  looks up the TGS server  110 &#39;s and client  102 &#39;s keys in the database and generates a random session key, for subsequent authentication with the KDC  104 . The AS server  108  generates a TGT ticket, which has both a clear and an encrypted part. The TGS server  110 &#39;s identity and the ticket validity period may be provided in the clear inside the issued TGT ticket. The encrypted part of the ticket contains the client  102 &#39;s name, session key and any other data to be kept private. The ticket preferably also provides a list of encryption types and checksum types supported by the KDC  104 . The encrypted part of the ticket may be encrypted using the KDC  104 &#39;s secret key. 
   The AS_REP message should preferably be signed by the KDC  104  using an algorithm that is identical to the one used by the client  102  to generate a signature for the AS_REQ message. This signature can be either a digital signature or a keyed checksum using the client  102 &#39;s secret key. The public key information is the KDC  104 &#39;s public part of the key agreement parameters and should indicate the same key agreement algorithm as the one selected by the client  102 . Finally, the AS_REP message preferably contains the nonce that was copied from the AS_REQ message, to prevent replays. 
   The encrypted part of the AS_REP message preferably contains the same information as is in the TGT ticket so that the client  102  has read-only access to its own authorization-data, but this is not a requirement of the present invention. This optional feature provides a convenience to the user because if the client  102  knows it own authorization data, it is not going to attempt actions that are later going to be rejected by an application server anyway, since an application server will trust only the copy of the client information that is encrypted inside the ticket. Also, for clients with hardware security that prevents a user from hacking and changing its own authorization data, this optional feature could be a security advantage because readable authorization data might also authorize the client for some local actions, such as for example the right to save and replay movies on local disk. The encrypted part of the AS_REP message preferably also contains the client  102 &#39;s identity to verify that this reply was originally constructed by the KDC  104  for this particular client  102 . The data is preferably encrypted with a symmetric key derived from the key agreement algorithm. 
   The client  102  processes the AS_REP message to verify its authenticity and to decrypt the private ticket part in the message to obtain the TGT ticket. If the authenticity of the AS_REP message cannot be verified, the client  102  preferably does not send an error message back to the AS server  108 . In some cases, the client may retry with another AS_REQ message. 
   The present invention optionally allows the passing of digital certificates in both the AS_REQ and AS_REP messages, to allow the client  102  and the KDC  104  to authenticate each other with digital certificates. Without certificates, it is expected that the client  102  is already provisioned with the KDC public key and that the KDC  104  already has the client  102 &#39;s public key in its database. A digital signature on an AS_REQ is verified by the KDC  104  with a client public key that it looks up in its database. The client  102  verifies a digital signature on an AS_REP with a pre-provisioned KDC public key. 
   After the client  102  has obtained a TGT ticket via the AS server  108  exchange, the client  102  initiates the TGS_REQ message exchange between the client  102  and the TGS server  110  when the client  102  wishes to obtain authentication credentials for a given or particular application server, such as the application server  106 . The TGS_REQ message is generated and sent by the client  102  to the TGS server  110  to obtain an application server service ticket (ST ticket) (that can be used in a KEY_REQ message). The client  102  presents the TGT ticket obtained from the AS_REP message as part of the TGS_REQ message. The TGS_REQ message specifies the application server  106 &#39;s identity as well as the client  102 &#39;s identity (which is inside the TGT ticket). The client  102 &#39;s identity is protected because it is in the encrypted part of the TGT ticket and is not included in the clear part of the message. The session key from the TGT ticket may be used for the encryption and decryption in the TGS_REQ exchange. Thus, a snooper is unable to detect which services the client (i.e. user) is requesting. 
   After the client  102  sends out the TGS_REQ message it preferably saves the nonce value in order to later validate the matching TGS_REP message from the KDC  104 . The client  102  preferably keeps the nonce value until a configurable time out value expires. After the time out, the client  102  will no longer be able to process the corresponding TGS_REP and must retry. 
   The TGS server  110  verifies the TGS_REQ message and processes the TGT ticket. The TGS server  110  then generates the TGS_REP message in response to the TGS_REQ message. The TGS_REP message includes the ST ticket (which is the end service ticket) issued by the KDC  104 , which the client  102  presents to the application server  106  when it needs to request a service. The application server  106 &#39;s identity and the ticket validity period may be provided in the clear inside the issued ST ticket. The encrypted part of the ST ticket contains the client  102 &#39;s name and a session key encrypted with a key shared by the application server  106  and the KDC  104 . Any additional client data that needs to be private could be included as part of the encrypted part of the ST ticket. The TGS_REP message is signed by the KDC  104  with a keyed checksum using the TGT ticket session key. Finally, the TGS_REP message contains the nonce that was copied from the TGS_REQ message, to prevent replays. 
   By way of example, the TGS server  110  may generate the TGS_REP message using the following procedure. First, the nonce from the TGS_REQ message is included in the TGS_REP message to tie it to the request. Next, the KDC  104  assigns the type of the random (service ticket) session key. If more than one encryption algorithm is available, the KDC  104  preferably selects the strongest one. The KDC  104  then generates the ST ticket. The application server  106 &#39;s secret key is used to encrypt the encrypted ticket part and also generate a keyed checksum over the whole ST ticket. The end time of the ST ticket is preferably determined by the KDC  104 . The client  102  may specify a shorter lifetime, if it wishes. The encrypted part of the ST ticket contains the client  102 &#39;s identity, session key and other private data. The TGT ticket session key is used to generate the encrypted data portion of the TGS_REP message, and a keyed checksum (using the TGT session key) is added over the TGS_REP message. Again, this is just one example of a procedure that the TGS server  110  may use to generate the TGS_REP message. 
   Because the client  102 &#39;s name is contained in the encrypted part of the ST ticket in the TGS_REP message and is not sent in the clear, the client&#39;s identity is hidden and cannot be linked with the content that the client  102  will request from the application server  106 . This way a snooper cannot determine with which application server the client  102  wishes to communicate. The present invention differs from Kerberos where a KDC reply to a ticket request from a client for a particular application server includes the client name in the clear in addition to the client name being encrypted in the ticket. In fact, with the present invention the only message in which the client  102 &#39;s name is provided in the clear is the AS_REQ message, which is not a problem because no security has been established yet and the client  102  has not asked for or identified a specific application server yet. 
   By way of example, the client  102  may use the following procedure to process the TGS_REP message. First, the client  102  parses the TGS_REP message header. If the header parsing fails, then the client  102  will act as if the TGS_REP was never received. The client  102  preferably does not send an error message back to the TGS server  110 . In some cases, the client  102  will retry with another TGS_REQ message. If there are any outstanding TGS_REQ messages, the client  102  may continue waiting for a reply until a time out and then retry. Next, the client  102  verifies the protocol version number in the header. If this protocol version is not supported, the client  102  will act as if the TGS_REP message was never received. The client  102  then parses the rest of the message. If the message format is found to be illegal, the client  102  will act as if the TGS_REP message was never received. 
   Next, the client  102  looks for an outstanding TGS REQ message with the same nonce. If there is no match, the client proceeds as if the message was never received. If there is a match, then the client  102  verifies the checksum (using the TGT ticket session key). If the checksum does not verify, this message is dropped and the client  102  proceeds as if the message was never received. 
   The client then decrypts the private ticket part in the TGS_REP message, using the TGT ticket session key. If the private ticket part cannot be decrypted because the TGT ticket session key type and the type of the encrypted data do not match, a fatal error is reported to the user and the client  102  does not retry. If the resulting clear text contains formatting errors, contains a session key with the type that is not supported by this client  102 , or contains a client identity that does not match the request, a fatal error is also reported to the user and the client  102  does not retry. 
   The client  102  then processes the ST ticket. If there is an error in the ST ticket, it is reported to the user as a fatal error and the client  102  does not retry with another TGS_REQ message. If no errors in the TGS_REP message are detected, the client  102  saves the full ST ticket and the clear text private ticket part in a new entry in its ticket cache. 
   The application server  106  utilizes the TKT_CHALLENGE message whenever it wants to initiate key management. To prevent denial of service attacks, this message includes a server-nonce field, which is a random value generated by the application server  106 . The client  102  preferably should include the exact value of this server-nonce in the subsequent KEY_REQ message. This TKT_CHALLENGE message also preferably includes the application server  106 &#39;s realm and principal name, which is used by the client  102  to find or to obtain a correct ticket for that application server. 
   The KEY_REQ and KEY_REP messages are used for key management and authentication between the client  102  and the application server  106 . The KEY_REQ message is sent by the client  102  to the application server  106  in order to establish a new set of security parameters. Preferably, any time the client  102  receives a TKT_CHALLENGE message, it responds with a KEY_REQ message. The KEY_REQ message can also be used by the client  102  to periodically establish new keys with the application server  106 . The client  102  starts out with a valid ST ticket, previously obtained in a TGS_REP message. The application server  106  starts out with its service key that it can use to decrypt and validate tickets. The KEY_REQ message includes the ST ticket and keyed checksum needed to authenticate the client  102 . The KEY_REQ message preferably also contains a nonce (to tie it to the response KEY_REP message) and the client timestamp (to prevent replay attacks). 
   When the client  102  generates the KEY_REQ message, the client  102 &#39;s identity is in the encrypted part of the ST ticket so it is not included in the clear part of the message. After the client  102  sends out the KEY_REQ message, it saves the client nonce value in order to later validate the matching KEY_REP message from the application server  106 . The client  102  keeps the client nonce value until a configurable time out value expires. After the time out, the client  102  will no longer be able to process the corresponding KEY_REP message. If the KEY_REQ message was sent unsolicited by the client  102 , the client  102  may retry after this time out. 
   The KEY_REP message is sent by the application server  106  in response to the KEY_REQ message. By way of example, the KEY_REP message may include a randomly generated subkey, encrypted with the session key shared between the client  102  and the application server  106 . The KEY_REP message may also include additional information that is needed to establish security parameters. 
   Finally, a SEC_ESTABLISHED message is sent by the client  102  to the application server  106  to acknowledge that it received a KEY_REP message and successfully set up new security parameters. 
   Referring to  FIG. 2 , there is illustrated a method  200  of providing client privacy when requesting content from an application server. By way of example, the method  200  may be implemented by the KDC  104  and the appropriate message types described above. In step  202  a request for a TGT ticket is received from a client, such as the client  102 . In step  204  the TGT ticket is generated with an identity of the client encrypted therein. Step  204  may be performed, for example, by the AS server  108 . In step  206  the TGT ticket is sent to the client. This step may also be performed by the AS server  108 . In step  208  a request for an ST ticket for a particular application server is received from the client. The request for the ST ticket includes the TGT ticket and does not provide the identity of the client in the clear. In step  210  the ST ticket is generated with the identity of the client encrypted therein, which by way of example, may be performed by the TGS server  110 . In step  212  the ST ticket is sent to the client without providing the identity of the client in the clear, which may also be performed by the TGS server  110 . 
   Thus, the present invention provides a method and system that provides improved user privacy when requesting content from a server, such as a public server. Privacy is improved because the client name or identity is encrypted in all key management messages where the client is requesting a ticket for a specific application server (e.g. a content provider), which overcomes the disadvantages of standard Kerberos. 
   While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.