Yuji Shinozaki wrote:
> You can still verify a document signed by an expired key: they are
> simply cryptographically-related numbers. The math will never change.
> It is just that you have to verify that the document was signed within
> the certificate's validity dates ( and within the validity dates of its
> certificate chain ). The problem here will be bookkeeping: keeping a
> record of all the necessary certs throughout the "lifetime" of these
> documents.
>
> [ It just occurred to me: isn't it possible that if a private key is
> compromised, a forger could "back date" documents? A CRL really has
> the effect of invalidating ALL the signatures past, present and future
> that use that cert. Makes you realize how vital private key security
> is. ]
What is crucial to reliance on signatures is the existence of a Time
Stamping Authority to prove the existence of a document (in this case
a signature) at a certain date. See the IETF PKIX discussion on this.
Revocation should not invalidate any private key usage prior to
the time of revocation -- otherwise we lose entirely the non-repudiation
quality of digital signatures.
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