The validation of Electronic Signatures poses several difficult and
interesting problems, in particular when it is necessary to validate
an Electronic signature for example 20 years later, when the signer
key has been compromised and revoked, the CA keys have been
compromised, the crypto algorithm or key length used at the
signature time is no longer secure, and the hash function
used at at the signature time now exhibits message collisions.
The validation problem changes over the different time frames:
- near term, when all involved certificates are still valid and
generally available,
- long term, when one or more of the involved certificates have expired,
- archival, when the initial cryptography used is no longer secure.
Another interesting point to consider is how it is possible to know and
trust the signing time as indicated by the signer.
A "white paper" dealing with these kinds of situations has been written and
demonstrates the use of time-stamping.
This paper can serve as a tutorial to this difficult subject, but also as
input for the discussion of a common validation model for electronic
signatures. The paper can be found at the following addresses:
http://www.openmaster.com/whitepapers/es_validation.pdf
http://www.id2tech.com/news/pdf/ES_validation.pdf
Denis Pinkas