Patent ID: 8625803

Claim:
A method implemented by data processing apparatus, the method comprising: obtaining at a first server a first set of three or more keys, each key in the first set of keys being associated with a different respective period of time, and wherein in order to generate information using a key the first server uses a first key in the first set of keys that is associated with a respective time that coincides with a current time on the first server; obtaining at a different second server a second set of three or more keys including the first key, each key in the second set of keys being associated with a different respective period of time, and wherein in order to generate information using a key the second server uses a second key in the second set of keys that is associated with a respective time period that coincides with a current time on the second server; generating, at the first server and using the first key, a first piece of information based on a first request received from a first client device, wherein the first piece of information is a cookie; receiving, at the second server, a second request from the first client device, the second request including the generated first piece of information; and validating, at the second server and using the first key, the first piece of information, wherein the validating is performed during the current period of time on the second server and wherein the first key is different than the second key.