Patent ID: 8392709

Claim:
A computer-implemented method, comprising: performing, by one or more computing devices: receiving by a server computer system a plurality of single request messages from a plurality of different client computer systems, wherein each single request message comprises a respective nonce, a timestamp based on a time at which that single request message was generated, and a digital signature of that single request message; creating a record of previously received nonces, wherein at a given time the record comprises a plurality of nonces received within a valid period of time prior to the given time, wherein each of the plurality of nonces is a nonce of one of said plurality of single request messages; validating a given single request message of the plurality of single request messages, where said validating comprises verifying the digital signature of the given single request message, determining that the timestamp of the given single request message indicates a time that is within said valid period of time prior to a current time, and determining that the nonce of the given single request message is not present within said record of previously received nonces at the current time; in response to validating the given single request message, sending a single response message comprising the nonce of the given single request message and information associated with the fulfillment of the given single request message; and modifying the created record to remove one or more of the previously received nonces that were received longer ago than a rolling window of time with respect to the current time, wherein a length of the rolling window of time is as long as the valid period of time.