Countering credentials copying

Abstract

Systems, methods and computer readable media for enabling a server device ( 5 ) to validate one or more client devices ( 1 ). A shared unpredictable secret ( 50 ) is generated. The shared unpredictable secret ( 50 ) is stored in the client device ( 1 ) and in the server device ( 5 ). The client device ( 1 ) proves possession of a correct shared unpredictable secret ( 50 ) to the server device ( 5 ). The shared unpredictable secret ( 50 ) is replaced by a new shared unpredictable secret ( 54 ) each time the client device ( 1 ) is validated by the server device ( 5 ).

What is claimed is:  
     
         1 . A method for validating a client device by a server device, said method comprising the steps of: 
 generating a shared unpredictable secret;    storing the shared unpredictable secret in the client device and in the server device;    requiring the client device to prove that it holds a correct secret as a precondition to the server device validating the client device; and    replacing the shared unpredictable secret by a new shared unpredictable secret when the server device validates the client device.    
     
     
         2 . The method of  claim 1  wherein an initial shared unpredictable secret is determined in the client device and in the server device during a registration step that occurs prior to a log-in step.  
     
     
         3 . The method of  claim 2  wherein the registration step entails more checking of bona fides of the client device than does a log-in step.  
     
     
         4 . The method of  claim 2  wherein, during the registration step, the client device is required to make a payment to the user device.  
     
     
         5 . The method of  claim 1  wherein the shared unpredictable secret is generated by a generator from the group comprising a random number generator and a pseudo-random number generator.  
     
     
         6 . The method of  claim 1  wherein the shared unpredictable secret comprises an unpredictable component and a fixed component.  
     
     
         7 . The method of  claim 1  wherein a plurality of client devices desire to be validated by the server device; and 
 each client device has a unique unpredictable secret that it shares with the server device.  
 
     
     
         8 . The method of  claim 1  wherein, following a validation of the client device, the server device discards the original shared unpredictable secret and stores within the server device a new shared unpredictable secret that can be generated by applying update data to the original shared unpredictable secret.  
     
     
         9 . The method of  claim 1  wherein: 
 the server device sends update data to the client device;  
 the client device applies the update data to the shared unpredictable secret to generate a new secret; and  
 the client device replaces the shared unpredictable secret with the new secret.  
 
     
     
         10 . The method of  claim 9  wherein: 
 the server device generates the update data using a generator from the group comprising a random number generator and a pseudo-random number generator; and  
 the step of applying the update data to the shared unpredictable secret comprises computing a one-way function of the combination of the shared unpredictable secret and the update data.  
 
     
     
         11 . The method of  claim 9  wherein the client device sends acknowledgement data to the server device to confirm that the client device has replaced the shared unpredictable secret with the new secret.  
     
     
         12 . The method of  claim 11  wherein, in response to the server device receiving the acknowledgement data from the client device, the server device: 
 validates the client device; and  
 discards the shared unpredictable secret and stores within the server device the new secret, which now becomes a new shared unpredictable secret.  
 
     
     
         13 . The method of  claim 11  wherein: 
 the client device sends to the server device proof data demonstrating that the client device holds a correct secret; and  
 the server device is adapted to accept from the client device any proof data that are generated from a secret that is newer than the secret for which the most recent acknowledgment data have been received by the server device.  
 
     
     
         14 . The method of  claim 11  wherein: 
 the client device sends to the server device both the acknowledgment data and proof data derived from the new secret.  
 
     
     
         15 . The method of  claim 14  wherein: 
 the proof data are computed on the new secret; and  
 the proof data serve also as acknowledgment data.  
 
     
     
         16 . The method of  claim 1  wherein: 
 the client device presents proof data to the server device, wherein the proof data are derived from a shared unpredictable secret using a proof data generation algorithm, and the proof data do not divulge the shared unpredictable secret;  
 the server device checks the proof data by using a proof data generation algorithm consistent with the proof data generation algorithm used by the client device; and  
 when the server device determines that the proof data presented by the client device were not generated from the same shared unpredictable secret that is stored in both the client device and in the server device, the server device does not validate the client device.  
 
     
     
         17 . The method of  claim 16  wherein each proof data generation algorithm is a one-way function.  
     
     
         18 . A system for enabling a server device to validate a client device, said system comprising: 
 at least one client device;    a server device;    a shared unpredictable secret;    means for storing the shared unpredictable secret in the client device;    means for storing the shared unpredictable secret in the server device;    coupled to the client device and to the server device, means for determining whether the client device holds a correct secret;    coupled to the determining means, means for allowing the server device to validate the client device when the client device proves that it holds a correct secret; and    coupled to the client device and to the server device, means for replacing the original shared unpredictable secret with a new shared unpredictable secret when the server device validates the client device.    
     
     
         19 . A computer readable medium containing computer program instructions for enabling a server device to validate a client device, said computer program instructions causing the execution of the following steps: 
 generating a shared unpredictable secret;    storing the shared unpredictable secret in the client device and in the server device;    requiring the client device to prove that it holds a correct secret as a precondition to allowing the client device to be validated by the server device; and    replacing the shared unpredictable secret by a new shared unpredictable secret when the client device is validated by the server device.