3.3.17 How can signatures remain valid beyond the expiration dates of their keys, or, How do you verify a 20-year-old signature?

  Normally, a key expires after, say, two years 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 leases and contracts are examples. How should these cases be handled? Many solutions have been suggested but it is unclear which will prove the best. Here are some possibilities.

One can have special long-term keys as well as the normal two-year keys. Long-term keys should have much longer modulus lengths and be stored more securely than two-year keys. If a long-term key expires in 50 years, any document signed with it would remain valid within that time. A problem with this method is that any compromised key must remain on the relevant CRL until expiration (see Question 3.3.11); if 50-year keys are routinely placed on CRLs, the CRLs could grow in size to unmanageable proportions. This idea can be modified as follows. Register the long-term key by the normal procedure, i.e., for two years. At expiration time, if it has not been compromised, the key can be recertified, that is, issued a new certificate by the certifying authority, so that the key will be valid for another two years. Now a compromised key only needs to be kept on a CRL for at most two years, not fifty.

One problem with the previous method is that someone might try to invalidate a long-term contract by refusing to renew his key. This problem can be circumvented by registering the contract with a digital time-stamping service (see Question 3.3.18) at the time it is originally signed. If all parties to the contract keep a copy of the time-stamp, then each can prove that the contract was signed with valid keys. In fact, the time-stamp can prove the validity of a contract even if one signer's key gets compromised at some point after the contract was signed. This time-stamping solution can work with all signed digital documents, not just multi-party contracts.