Normally, a key expires after some period of time, such as one year, and a document signed with an expired key should not be accepted. However, there are many cases where it is necessary for signed documents to be regarded as legally valid for much longer than two years; long-term loses and contrast are examples. By registering the contract with a digital time-stamping service at the time it is singed, the signature can be validated even after the key expires.

If all parties to the contract deep a copy of the time-stamp, each can prove that the contract was singed with valid keys. In fact, the time-stamp can prove the validity of a contract even if one singer’s key gets compromised at some point after the contract was signed. Any digitally signed document can be time-stamped, assuring that the validity of the signature can be verified after the key expires.

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