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The first example involved a signing key that expired. What would be the point of expiring a signing key? What would that mean? In a paper context, a signature

The first example involved a signing key that expired. What would be the point of expiring a signing key? What would that mean? In a paper context, a signature or seal is still considered valid even if the technical means to create such marks no longer exists. Even if you lose your pen or stamp, signatures made with the pen or stamp are still binding. Implementations should probably just ignore the expiry of a key used for signing when checking a signature...

layer8 3y ago
For long-term signatures, the designated way to handle this is using cryptographic timestamps that establish when the signature was created. That proof-of-existence time is then compared to the validity period of the certificate associated with the key. This allows successfully validating signatures even after certificate expiration. Certificates have an expiration date because keys can be compromised, and cryptographic algorithms can become weak and be broken. The expiration date imposes a time limit for possible fraudulent use of the key. When using certificates, it’s also the certificate that expires, not the key. A new certificate for the same key can be issued (certificate renewal).
jagged-chisel 3y ago
If the signature creation date is before the key expiration date, the signature is valid. If the signature creation date is after expiration, it is possible (more likely than before) that someone else discovered the private key and has signed the document. Signing key expiration exists for the same reason certificate expiration exists: we create keys that are infeasible to discover with current methods, and create replacements as techniques advance. This encourages us to create stronger keys over time. Had you failed to notice a key expiration from the 80s, you would assume the signature is valid - but most computing devices these days could have just generated the signature without the original key.
- upofadown 3y ago
  Why would an attacker with access to the private key specify a signature creation date after key expiry? They can specify whatever date they want. >Had you failed to notice a key expiration from the 80s, you would assume the signature is valid ... If the algorithm is broken then the implementation should not support verification using that algorithm no matter the expiration date. An attacker can update the key's expiration date to whatever they want if the algorithm is broken.

layer8 3y ago

For long-term signatures, the designated way to handle this is using cryptographic timestamps that establish when the signature was created. That proof-of-existence time is then compared to the validity period of the certificate associated with the key. This allows successfully validating signatures even after certificate expiration. Certificates have an expiration date because keys can be compromised, and cryptographic algorithms can become weak and be broken. The expiration date imposes a time limit for possible fraudulent use of the key. When using certificates, it’s also the certificate that expires, not the key. A new certificate for the same key can be issued (certificate renewal).

jagged-chisel 3y ago

If the signature creation date is before the key expiration date, the signature is valid. If the signature creation date is after expiration, it is possible (more likely than before) that someone else discovered the private key and has signed the document. Signing key expiration exists for the same reason certificate expiration exists: we create keys that are infeasible to discover with current methods, and create replacements as techniques advance. This encourages us to create stronger keys over time. Had you failed to notice a key expiration from the 80s, you would assume the signature is valid - but most computing devices these days could have just generated the signature without the original key.

upofadown 3y ago
Why would an attacker with access to the private key specify a signature creation date after key expiry? They can specify whatever date they want. >Had you failed to notice a key expiration from the 80s, you would assume the signature is valid ... If the algorithm is broken then the implementation should not support verification using that algorithm no matter the expiration date. An attacker can update the key's expiration date to whatever they want if the algorithm is broken.