Patent ID: 7934100

Claim:
An information processing system, comprising: a plurality of time stamp authorities that generate time stamps in accordance with time stamp requests and return the time stamps to sources that issue the requests; a selection unit that selects, among the plurality of time stamp authorities, a time stamp authority to which a request for generation of a time stamp to be applied to digital data is to be transmitted, the time stamp authority selected being different from a time stamp authority selected in a last selection process; an acquisition unit that acquires the time stamp to be applied to the digital data by transmitting the time stamp request including a hash value calculated based on the digital data to the selected time stamp authority; a generation unit that generates link information specifying an order in which time stamps have been acquired by the acquisition unit; a storage controller that causes verification information including the time stamp, identification information of the time stamp authority from which the time stamp is acquired, and link information generated by the generation unit based on the time stamp, that are associated with each other, to be stored in a memory; a reception unit that receives a time stamp verification request, regarding the digital data, for verification of an invalidated time stamp, wherein the invalidated timestamp is known to be generated by an invalidated time stamp authority; and a verification unit that specifies a time stamp in the memory that matches the invalidated time stamp, wherein the verification unit uses times certified by valid time stamps that were last acquired before the specified time stamp and first acquired after the specified time stamp, respectively, to verify a start and end of a time range in which the invalidated time stamp was applied.