PATENT DOCUMENT

Publication Number: US-7735120-B2
Application Number: US-74379603-A
Country: US
Kind Code: B2

Title: Server computer issued credential authentication

Abstract:
A method for authenticating computers is disclosed. The method comprises issuing a credential from a first computer to a second computer. When the second computer authenticates to the first computer, the second computer transmits the credential and a first challenge to the first computer. The first computer determines whether the credential is valid, computes a first response to the first challenge, and generates a second challenge. The first computer transmits the first response and the second challenge to the second computer. The second computer determines whether the first response is valid and computes a second response to the second challenge. The second computer transmits the second response to the first computer in order to verify and authenticate the computers.

Claims:
1. A method for authenticating a computing device, the method comprising the following steps:
 issuing a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from a first computing device to a second computing device; 
 upon a loss of connection, transmitting said credential and a computer challenge from the second computing device to the first computing device; 
 transmitting a response to said computer challenge from said first computing device to said second computing device; and 
 verifying said response with said second computing device in order to authenticate and verify said computing devices and reestablish said connection, 
 wherein:
 the challenge is a random number generated by the second computing device, and 
 the first computing device computes the response to the challenge by performing a predetermined function on the random number. 
 
 
     
     
       2. The method of  claim 1  wherein the second computing device determines whether the first computing device response is valid by performing the predetermined function on the random number and comparing the result to the response. 
     
     
       3. The method of  claim 2  wherein the predetermined function is a hash function. 
     
     
       4. The method of  claim 1  wherein the second computing device establishes a connection with the first computing device when the response is valid. 
     
     
       5. The method of  claim 1  wherein the first computing device determines whether the credential transmitted from the second computing device is valid by determining whether the expiration time of the credential has been exceeded. 
     
     
       6. A system for authenticating a computer, the system comprising:
 a first computer; and 
 a second computer in communication with the first computer; 
 wherein the first computer and the second computer are configured to execute the following instructions:
 issue a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from the first computer to the second computer; 
 upon a loss of connection, transmit the credential and a challenge from the second computer to the first computer; 
 transmit a response to the challenge from the first computer to the second computer; and 
 verify the response with the second computer in order to authenticate and verify the computers and re-establish said connection, 
 
 wherein:
 the second computer is configured to generate a challenge that is a random number, and 
 the first computer is configured to generate a response to the challenge by performing a predetermined function on the random number. 
 
 
     
     
       7. The system of  claim 6  wherein the second computer is configured to determine whether the response is valid by performing the predetermined function on the random number and comparing the result to the response. 
     
     
       8. The system of  claim 7  wherein the predetermined function is a hash function. 
     
     
       9. The system of  claim 6  wherein the second computer establishes a connection with the first computer when the response is valid. 
     
     
       10. The system of  claim 6  wherein the first computer determines whether the credential transmitted from the second computer is valid by determining whether the expiration time of the credential has been exceeded. 
     
     
       11. A method for authenticating a computer, the method comprising the steps:
 issuing a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from a first computer to a second computer; 
 generating with the second computer a first challenge; 
 transmitting the credential and the first challenge from the second computer to the first computer; 
 determining with the first computer whether the credential is valid; 
 computing a first response to the first challenge and generating a second challenge with the first computer; 
 transmitting the first response and the second challenge from the first computer to the second computer; 
 determining with the second computer whether the first response is valid; 
 computing a second response to the second challenge with the second computer; 
 transmitting the second response from the second computer to the first computer; and 
 determining with the first computer whether the second response is valid to verify and authenticate the computers. 
 
     
     
       12. The method of  claim 11  wherein the second computer encrypts the credential before transmitting the credential to the first computer. 
     
     
       13. The method of  claim 11  wherein the first computer challenge is a random number generated by the second computer and the first computer computes a first response to the first challenge by performing a predetermined function on the random number. 
     
     
       14. The method of  claim 13  wherein the second computer determines whether the first response is valid by performing the predetermined function on the random number and comparing the result to the first response. 
     
     
       15. The method of  claim 13  wherein the predetermined function is a hash function. 
     
     
       16. The method of  claim 11  wherein the second challenge is a random number generated by the first computer and the second computer computes a second response to the second challenge by performing a predetermined function on the random number. 
     
     
       17. The method of  claim 16  wherein the first computer determines whether the second response is valid by performing the predetermined function on the random number and comparing the result to the second response. 
     
     
       18. The method of  claim 17  wherein the predetermined function is a hash function. 
     
     
       19. The method of  claim 11  wherein the first computer determines whether the credential transmitted from the second computer is valid by determining whether the expiration time of the credential has been exceeded. 
     
     
       20. The method of  claim 11  further comprising the steps of:
 encrypting the first challenge with the second computer before transmitting to the first computer; 
 decrypting the first challenge with the first computer before determining whether the first response is computed; 
 encrypting the first response and the second challenge with the first computer before transmitting; 
 decrypting the first response and the second challenge with the second computer before determining whether the first response is valid and the second response is computed; 
 encrypting the second response with the second computer before transmitting; and 
 decrypting the second response with the first computer before determining whether the second response is valid. 
 
     
     
       21. The method of  claim 20  wherein the credential is encrypted before issuing the credential to the second computer and the credential is decrypted by the first computer when returned by the second computer. 
     
     
       22. A computer-readable storage medium containing a program with instructions that execute the following procedure:
 issue a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from a first computer to a second computer; 
 generate a first challenge with the second computer; 
 transmit the credential and the first challenge from the second computer to the first computer; 
 determine with the first computer whether the credential is valid; 
 compute a first response to the first challenge and generate a second challenge with the first computer; 
 transmit the first response and the second challenge from the first computer to the second computer; 
 determine with the second computer whether the first response is valid to verify the first computer; 
 compute a second response to the second challenge with the second computer; 
 transmit the second response from the second computer to the first computer; and 
 determine with the first computer whether the second response is valid to verify and authenticate the computers. 
 
     
     
       23. The computer-readable storage medium of  claim 22  having instructions for the second computer to encrypt the credential before transmitting the credential to the first computer. 
     
     
       24. The computer-readable storage medium of  claim 22  having instructions for the second computer to generate the first challenge that is a random number and the first computer computes a first response to the first challenge by performing a predetermined function on the random number. 
     
     
       25. The computer-readable storage medium of  claim 24  wherein the second computer determines whether the first response is valid by performing the predetermined function on the random number and comparing the result to the first response. 
     
     
       26. The computer-readable storage medium of  claim 25  wherein the predetermined function is a hash function. 
     
     
       27. The computer-readable storage medium of  claim 22  having instructions for the first computer to generate a second challenge that is a random number and the second computer computes a second response to the second challenge by performing a predetermined function on the random number. 
     
     
       28. The computer-readable storage medium of  claim 27  wherein the first computer determines whether the second response is valid by performing the predetermined function on the random number and comparing the result to the second response. 
     
     
       29. The computer-readable storage medium of  claim 28  wherein the predetermined function is a hash function. 
     
     
       30. The computer-readable storage medium of  claim 22 , wherein the first computer determines whether the credential transmitted from the second computer is valid by determining whether the expiration time of the credential has been exceeded. 
     
     
       31. The computer-readable storage medium of  claim 22  further comprising instructions for:
 encrypting the first challenge with the second computer before transmitting to the first computer; 
 decrypting the first challenge with the first computer before the first response is computed; 
 encrypting the first response and the second challenge with the first computer before transmitting; 
 decrypting the first response and the second challenge with the second computer before determining whether the first response is valid and the second response is computed; 
 encrypting the second response with the second computer before transmitting; and 
 decrypting the second response with the first computer before determining whether the second response is valid. 
 
     
     
       32. The computer-readable storage medium of  claim 31  wherein the instructions further comprise encrypting the credential before issuing the credential to the second computer and decrypting the credential with the first computer when returned from the second computer. 
     
     
       33. A system for authenticating a computer, the system comprising:
 a first computer; and 
 a second computer in communication with the first computer; 
 wherein the first computer and the second computer are configured to execute the following instructions:
 issue a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from the first computer to the second computer; 
 generate a first challenge with the second computer; 
 transmit the credential and the first challenge from the second computer to the first computer; 
 determine with the first computer whether the credential is valid; 
 compute a first response to the first challenge and generate a second challenge with the first computer; 
 transmit the first response and the second challenge from the first computer to the second computer; 
 determine with the second computer whether the first response is valid; 
 
 compute a second response to the first challenge with the second computer;
 transmit the second response from the second computer to the first computer; and 
 determine with the first computer whether the second response is valid to authenticate and verify the computers. 
 
 
     
     
       34. The system of  claim 33  wherein the second computer is configured to encrypt the credential before transmitting the credential to the first computer. 
     
     
       35. The system of  claim 33  wherein the second computer is configured to generate a first challenge that is a random number and the first computer is configured to compute a first response to the first challenge by performing a predetermined function on the random number. 
     
     
       36. The system of  claim 35  wherein the second computer is configured to determine whether the first response is valid by performing the predetermined function on the random number and compare the result to the first response. 
     
     
       37. The system of  claim 36  wherein the predetermined function is a hash function. 
     
     
       38. The system of  claim 33  wherein the first computer is configured to generate a second challenge that is a random number and the second computer is configured to compute a second response to the second challenge by performing a predetermined function on the random number. 
     
     
       39. The system of  claim 38  wherein the first computer is configured to determine whether the second response is valid by performing the predetermined function on the random number and comparing the result to the second response. 
     
     
       40. The system of  claim 39  wherein the predetermined function is a hash function. 
     
     
       41. The system of  claim 33  wherein the first computer is configured to determine whether the credential transmitted from the second computer is valid by determining whether the expiration time of the credential has been exceeded. 
     
     
       42. The system of  claim 33  wherein the first computer and the second computer are configured to:
 encrypt the first challenge with the second computer before transmitting to the first computer; 
 decrypt the first challenge with the first computer before the first response is computed; 
 encrypt the first response and the second challenge with the first computer before transmitting; 
 decrypt the first response and the second challenge with the second computer before determining whether the first response is valid and the second response is computed; 
 encrypt the second response with the second computer before transmitting; and 
 decrypt the second response with the first computer before determining whether the second response is valid. 
 
     
     
       43. A method of authentication performed between a first user with a first computer and a second user with a second computer, the method comprising the steps of:
 issuing a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from the first user to the second user; 
 generating a first challenge with the second user; 
 transmitting the credential and the first challenge to the first user; 
 determining with the first user whether the credential is valid; 
 generating with the first user a first response to the first challenge and a second challenge; 
 transmitting the first response and the second challenge to the second user; 
 determining with the second user whether the first response is valid; 
 generating with the second user a second response to the second challenge; 
 transmitting the second response to the first user; and 
 determining with the first user whether the second response is valid in order to authenticate and verify the first and second users. 
 
     
     
       44. A method for authenticating a computer, the method comprising the following steps:
 issuing a credential based on session information, a hash seed, a maximum iterative value, credential information and an expiration time from a first computer to a second computer; 
 in response to a connection between the first computer and the second computer being terminated, transmitting said credential and a computer challenge from the second computer to the first computer when the second computer is to be authenticated; 
 transmitting a response to said computer challenge from said first computer to said second computer; and 
 verifying at said second computer whether said response is valid, wherein said second computer re-establishes a connection with the first computer when the response is valid, 
 wherein:
 the challenge comprises a random number generated by the second computer, 
 the first computer generates the response to the challenge by calculating a predetermined function of the random number, and 
 the second computer verifies whether the response is valid by calculating the predetermined function of the random number and comparing the result of the calculation to the response. 
 
 
     
     
       45. The method of  claim 44 , the method further comprising:
 checking at the first computer whether the credential transmitted from the second computer is valid by determining whether the expiration time of the credential has been exceeded.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to computer network security and more particularly to a system and method for providing a limited credential for authenticating access to network resources. 
     BACKGROUND OF THE INVENTION 
     In order to access network resources, a user enters identification and password information into a client computer that is transmitted to a server computer over a computer network for authentication. In turn, the server computer authenticates the client computer using the password and identification information, thereby allowing access to network resources. If the connection with the server computer is terminated, the user must re-enter the identification and password information into the client computer in order to re-authenticate and reconnect with the server computer. The user must re-enter the information because the password is not stored on the client computer for security reasons. 
     A “credential” can be issued to the client computer for facilitating the reconnect procedure. The credential is data that is used to prove the identity of the subject. In this instance, the credential is used by the client computer to authenticate the client computer to the server computer such that the user does not need to re-enter or store on the client computer the password information. There is no need for the client computer to transmit the identification and password information to the server computer if the client computer has the credential because the credential ensures with a high degree of reliability that the client computer should have access. 
     The credential is generated by a trusted third party (TTP) such as the type used with the Kerberos system. The TTP issues the credential that allows the client computer to authenticate itself to the server computer. The credential may be time limited and encrypted by the TTP using a symmetric algorithm and decrypted/verified by the server computer using the same. The TTP is used for a large aggregation of machines and contains all of the keys used for authentication by both users and machines. 
     A drawback with the TTP is that it requires significant infrastructure and is a separate entity that must be configured. Furthermore, the configuration information must be present on all of the server computers. Therefore, in order to implement a change, all of the machines (i.e. client and server computers), as well as the TTP, must be re-configured. Another drawback of the TTP is that it is a high value target because it contains all of the keys used for authentication. 
     Another type of authentication mechanism is pretty good privacy (PGP Ticket). A TTP (e.g., server administrator) issues a credential to a client computer that allows the server computer to authenticate the client computer. The credential issued to the client computer is time limited and digitally signed by the TTP using commonly known public key technology. The credential is interpreted/verified by the server computer. 
     A drawback with PGP Ticket is that the security and verifiability of the TTP&#39;s public key is weak. Furthermore, a change to the TTP&#39;s key requires revoking all of the old keys and updating all of the server computers and client computers with new keys. Additionally, if the TTP&#39;s key is compromised, then all of the server computers that rely on that key are also compromised. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a method for authenticating computers. The method comprises a first computer (e.g., a server computer) issuing a credential to a second computer (e.g., a client computer). When the second computer attempts to authenticate with the first computer, the second computer generates a first challenge and transmits the credential and the first challenge to the first computer. The first computer determines whether the credential is valid and computes a first response to the first challenge: Additionally, the first computer generates a second challenge. The first computer transmits the first response and the second challenge to the second computer. In order to authenticate the first computer, the second computer determines whether the first response is valid. The second computer also computes a second response to the second challenge. The second computer transmits the second response to the first computer which then determines whether the second response is valid in order to authenticate the second computer and establish a connection. 
     The credential may be encrypted before issuing it to the second computer and transmitting it from the second computer to the first computer. Similarly, the first challenge, the first response, the second challenge, and the second response may be encrypted before transmission. Each of the first challenges and responses, as well as the second challenges and responses, are decrypted upon receipt by the first or second computers. 
     The second challenge may be a random number such as a nonce generated by the first computer. The second computer computes a second response to the first computer challenge by performing a predetermined function on the random number. The first computer determines whether the second response is valid by performing the predetermined function on the random number and comparing the result to the second response. The predetermined function may be a hash function. 
     Similarly, the first challenge may be a random number such as a nonce. The first computer computes a first response to the first challenge by performing a predetermined function on the random number. The second computer determines whether the first response is valid by performing the predetermined function on the random number and comparing the result to the first response. 
     The credential may be issued with an expiration time. Once the expiration time has been reached, the credential is no longer valid. 
     In accordance with the present invention, there is provided a computer-readable medium containing a program with instructions to execute the authentication between a first computer and a second computer. The instructions issue a credential from the first computer to the second computer. The credential and a first challenge are transmitted from the second computer to the first computer when the second computer is to be authenticated. The first computer determines whether the credential is valid and computes a response to the first challenge. Furthermore, the first computer generates a second challenge that is transmitted to the second computer with the first response. The second computer determines whether the first response is valid and computes a second response to the second challenge. The second response is transmitted to the first computer and the first computer determines whether the second response is valid in order to authenticate the second computer. 
     A system for authenticating computers has a first computer and a second computer in communication with each other. The first and second computers are configured to execute instructions which authenticate the second computer. Specifically, the first computer issues a credential to the second computer. In order to be authenticated, the second computer transmits the credential along with a first challenge to the first computer. The first computer is configured to determine whether the credential is valid and computes a first response to the first challenge. The first computer generates and transmits a second challenge and the first response to the second computer which verifies the first response and generates a second response. The second response is transmitted from the second computer to the first computer in order to authenticate the second computer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
         FIG. 1  is a network diagram; 
         FIG. 2  is a flowchart illustrating how a reconnect credential is used by a server computer to authenticate a client computer; 
         FIG. 3  is a flowchart illustrating how a reconnect credential is issued by the server computer; 
         FIGS. 4-6  are flowcharts illustrating how a client computer uses the reconnect credential to authenticate with the server computer; and 
         FIG. 7  is a flowchart illustrating how a reconnect credential is reissued to a client computer. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects will now be described in connection with exemplary embodiments, including certain aspects described in terms of sequences of actions that can be performed by elements of a computer system. For example, it will be recognized that in each of the embodiments, the various actions can be performed by specialized circuits, circuitry (e.g., discrete and/or integrated logic gates interconnected to perform a specialized function), program instructions executed by one or more processors, or by any combination. Thus, the various aspects can be embodied in many different forms, and all such forms are contemplated to be within the scope of what is described. The instructions of a computer program as illustrated in  FIGS. 2-7  for issuing and reconnecting with an authentication credential can be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer based system, processor containing system, or other system that can fetch the instructions from a computer-readable medium, apparatus, or device and execute the instructions. 
     As used herein, a “computer-readable medium” can be any means that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-readable medium can be, for example but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non exhaustive list) of the computer readable-medium can include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read only memory (ROM), an erasable programmable read only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read only memory (CDROM). 
     Referring now to the drawings wherein the showings are for purposes of illustrating preferred embodiments of the present invention only, and not for purposes of limiting the same,  FIG. 1  illustrates client computers  12   a ,  12   b ,  12   c  connected to a network  18 . Server computers  14   a ,  14   b , and  14   c  are also connected to the network  18 . In order for a client computer  12  to access a server computer  14 , the client computer  12  transmits user identification and password information to the desired server  14 . The server computer  14  uses this information to authenticate the client computer  12  and establish a connection. 
       FIG. 2  is a flowchart showing how the reconnect credential is used is shown. In step  202 , the server computer  14  issues a credential. Referring to  FIG. 3 , one embodiment of a method of initially issuing the credential from the server computer  14  to the client computer  12  is shown. The client computer  12  already has an agreed upon initial session key (k 1 ), while the server computer  14  has a long term server key (ks) and the initial session key (k 1 ). In step  302 , the client computer sends the initial timestamp (t 1 ) encrypted using the initial session key (k 1 ) to the server computer  14 . In step  304 , the server computer chooses security parameters and an expiration time (exp) for the credential. In the illustrated embodiment, the security parameters include a hash seed (s), and a maximum number of times (m) to run a hash function. In step  306 , the server computer  14  calculates a time interval (t 3 ) that is the time interval between the clock of the server computer  14  and initial timestamp (t 1 ). Information that uniquely identifies the session (sessioninfo) is generated in step  308  by the server computer  14  in order to distinguish the session from other sessions. 
     The server computer  14  generates a credential (cred) for the client computer  12  in step  310 . As previously discussed, the credential proves the identity of the client computer  12 . For the embodiment shown in  FIG. 3 , the credential (cred) is generated using the long term server key (ks) and the seed (s), the maximum number of times (m) to run the hash function, the expiration time (exp), the time interval (t 3 ), and username (user/domain) information to uniquely determine the client computer  12 . It will be recognized by those of ordinary skill in the art that there may be other methods of generating the credential in order to prove the identity of the client computer  12 . In step  312 , the server computer  14  transmits the session information (sessioninfo), seed (s), maximum times (m) to run the hash function, the expiration time (exp), and the credential (cred) encrypted by the initial session key (k 1 ) to the client computer  12  in order to issue the credential. 
     The credential received by the client computer  12  is used to re-establish a seamless connection with the server computer  14 . In the reconnect procedure shown in  FIG. 2 , the connection between the server computer  14  and the client computer  12  is terminated in step  204 . The connection may be lost by unexpected session termination. In order to reconnect, the client computer  12  transmits the credential to the server computer  14  in step  206 . In step  208 , the process of authenticating the client computer  12  and server computer  14  with the credential issued from step  202  is performed. Once the authentication is complete, the connection is reestablished in step  210 . 
     Referring to  FIGS. 4A-4B , the method from step  208  for authenticating the client computer  12  and the server computer  14  with the credential issued in step  202  is shown. The client computer  12  has the credential (cred), seed (s), and maximum number of times (m) to perform the hash function. In step  402 , the client computer  12  chooses the number of times (n) to run the hash function. Also, the client computer  12  chooses a client computer challenge such as a random number (client_nonce) for security purposes. The client computer  12  then signs the data using a hashed message authentication code (HMAC) and hashed seed (s) data in step  404 . The data to be signed is the credential (cred), the client computer&#39;s random number (client_nonce), the number of times (n) to run the hash function, the timestamp (t 2 ) and the seed (s) data hashed n and (n−1) times. The data signed by the HMAC is then transmitted from the client computer  12  to the server computer  14  in step  406 . 
     In step  408 , the server computer  14  checks the signatures and decrypts the credential (cred) using the long term server key (ks). If the decrypt fails, then the process exits in step  410 . If the decrypt does not fail, then in step  412 , the server computer  14  retrieves the seed (s), the maximum number of times (m) to perform the hash function, the expiration time of the credential (exp), and the time interval (t 3 ). 
     The server computer  14  hashes the credential (cred) and determines if the credential (cred) is in a revocation list in step  414 . If the hashed credential (cred) is in the revocation list, then the credential (cred) is not valid and the process exits in step  416 . However, if the hashed credential (cred) is not in the revocation list, then the process proceeds to step  418  where the server computer  14  checks to see if the expiration time (exp) of the credential (cred) has been exceeded. If the time has been exceeded then the credential (cred) is not valid and the process exits in step  420 . Furthermore, in step  418 , the server computer  14  determines whether the maximum number of times (m) to perform the hash function is greater than or equal to the number of times (n) to perform the hash function. If m is not greater than or equal to n then the process exits in step  420 . Furthermore, the server computer  14  validates HMAC(s) and the user/domain in step  418 . If either one of these values is invalid, then the process exits in step  420 . 
     The server computer  14  performs two separate hashes. It hashes the seed (s) data n number of times ((s)H(n)) and n−1 times ((s)H(n−1)) in step  422 . In step  424 , the server computer  14  compares the values of (s)H(n) and (s)H(n−1) found in step  422  with the values decrypted in step  408 . If the values do not match, then the process exits in step  426 . 
     Next, in step  428 , the server computer  14  decrypts and hashes the client_nonce to generate a server computer response. The server computer  14  then chooses a server computer challenge such as a random number (server_nonce) in step  430 . In step  432 , the server computer  14  adds the hashed credential (cred) to the revocation list so that it cannot be used again. In order to continue authentication of the credential, in step  434  the server computer  14  encrypts and transmits the server_nonce chosen in step  430 , the hashed client_nonce from step  428  using as a key the seed data (s) hashed n−1 times. 
     Referring to  FIG. 5 , the process of authenticating the client computer  12  is continued. Specifically, in step  502 , the client computer  12  receives the message containing the server_nonce, the hashed client_nonce, encrypted using the seed data (s) hashed n−1 times as the key, transmitted by the server computer  14  in step  434 . In step  504 , the client computer  12  decrypts the message received in step  502 . Next, in step  506 , the client computer  12  verifies the hashed client_nonce by comparing it with the known value. If the values do not match, then the process exits in step  508 . However, if the values do match, then the client computer  12  can be assured that the server computer  14  is authentic because it successfully hashed the client_nonce. In step  510 , the client computer  12  hashes the server_nonce received from step  502  to generate a client computer response. In order to verify to the server computer  14  that the client computer  12  has successfully received the message, the client computer  12  transmits the hashed server_nonce encrypted with the seed (s) hashed n−1 times to the server computer  12  in step  512 . 
     Referring to  FIG. 6 , the server computer  14  receives the message from step  512  and decrypts the message in step  604 . The server computer  14  verifies the hashed server_nonce by comparing it to a known value in step  608 . If the server_nonce does not match the value generated earlier by the server computer  14 , then the process exits in step  609 . However, if the server_nonce is verified, then both the server computer  14  and the client computer  12  are authenticated to each other and a connection can be established in step  610 . 
     It is possible that the credential expires before being used by the client computer  12 . As previously discussed, the credential includes an expiration time (exp) after which the credential cannot be used. If the credential is not used before the expiration time, the credential is invalid. Therefore, it is necessary to re-issue a valid credential to the client computer  12  before the end of the expiration time. Any reasonable rule can be used to determine when to reissue a credential prior to or after its expiration. As seen in  FIG. 2 , the credential is about to expire in step  212 . Accordingly, in step  214 , the server computer  14  re-issues the credential to the client computer  12 . 
     Referring to  FIG. 7 , one embodiment of a method for reissuing the credential is shown while the client computer  12  is still connected to the server computer  14 . The client computer  12  has the credential (cred), seed (s) data, and initial session key (k 1 ). The server computer  14  has the long term server key (ks), the initial session key (k 1 ), and the unique name to identify the session (sessioninfo). In step  702 , the client computer  12  transmits the credential (cred) and the new timestamp (t 1 ) encrypted by the initial session key (k 1 ) to the server computer  14 . The server computer  14  receives and decrypts the message from the client computer  12  in step  704 . Next, the server computer  14  determines if the credential (cred) has expired in step  706 . If the time limit for the credential has expired, then the process exits in step  708 . If the time limit has not expired, then the process proceeds to step  710  where both the client computer  12  and the server computer  14  compute a new session key (k 1 ′) that is equal to the hash of the credential (cred) and the seed (s). The server computer  14  creates a new credential (cred′) in step  712  with a new seed (s′), new expiration time (exp′) and new time interval (t 3 ′). The server computer  14  places the old credential (cred) on the revocation list of invalid credentials in step  714 . Once the old credential (cred) is on the revocation list, then the server computer  14  transmits the new credential (cred′), new seed (s′), new maximum times (m′) to perform the hash function, the new expiration time (exp′), and the session information (sessioninfo) to the client computer  12  in step  716 . The message from the server computer  14  to the client computer  12  is encrypted using the new session key (k 1 ′). In step  718 , the client computer  12  decrypts the message using the new session key (k 1 ′) to retrieve the new credential and values. 
     In order to provide added security, the long term server key (ks) can be changed. In order to change the long term server key (ks), the server computer  14  generates a cutoff date (cutoff_date) that is later than or equal to the expiration date (exp). Next, the server computer  14  generates a new long term server key (ks′). The cutoff date (cutoff_date), the original long term server key (ks), and the new long term server key (ks′) are stored at the server computer  14 . When attempting to decrypt a credential, the server computer  14  first uses the original long term server key (ks) and if that fails then uses the new long term server key (ks′). New credentials are issued using the new long term server key (ks′). When the cutoff date (cutoff_date) has passed, then the long term server key (ks) should be set to the new long term server key (ks′) because all of the old credentials issued using the long term server key (ks) have expired. Also, once the cutoff date has passed, the revocation list of invalid credentials can be purged and the server computer  14  can issue a new long term server key using the method just described. 
     The reconnect authentication method described above provides security against common attacks. For example, because the client computer  12  and the server computer  14  transmit encrypted client_nonce and server_nonce which require the knowledge of the seed (s), the ability to stage a Man-in-the-Middle (MitM) attack where an attacker intercepts and possibly alters data traveling along the network is decreased. The method also provides security against reply attacks where an attacker bugs the network data and reuses it when needed because the messages between the client computer  12  and the server computer  14  include a timestamp which prevent reuse at another time. Reflection attacks where an attacker re-transmits a message created by a user and obtains a right of access are thwarted by using chained nonces and having the user information (user/domain) in the credential. Furthermore, reflection attacks are reduced by having each message between the client computer  12  and server computer  14  be non-symmetric and using a unique long term server key (ks). The chained nonces also provide against interleaved attacks where an attacker disguises oneself under a different identity during communication. A chosen text attack where an attacker tampers with the message and attacks the cryptography algorithm itself is reduced by not using the long term server computer key (ks) to encrypt any data that is obtained from the client computer  12 . Finally, forced delay attacks where an attacker intercepts the data in the protocol to use later are reduced by using timestamps and an expiration time for the credential, as well as using a revocation list. 
     It will be appreciated by those of ordinary skill in the art that the concepts and techniques described here can be embodied in various specific forms without departing from the essential characteristics thereof. For example, the authentication between the server computer  14  and the client computer  12  may be a single challenge whereby the client computer  12  does not need to retransmit any more information to the server computer  14  after sending the credential and challenge. The presently disclosed embodiments are considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, rather than the foregoing description, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced.

Metadata:
Filing Date: 20031224
Publication Date: 20100608
Grant Date: 20100608
Priority Date: 20031224
Inventors: WALLACE LELAND A.
O'ROURKE DAVID M.
Assignee: APPLE INC
CPC Classifications: [{"code": "G06F21/33", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F21/33", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L63/08", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L63/0428", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2221/2129", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F2221/2129", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L63/08", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L63/0428", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 34911201