[Rpm-maint] [rpm-software-management/rpm] Ignore subkeys that are expired or cannot be used for signing (Issue #1911)

[Justus Winter]

> This alone isn’t sufficient for primary keys. The person with a copy of the secret key can just make a signature that claims to be from when the key was still valid. They could also create a new self-signature that makes the key no longer expired.

While that is true, I would assume that the set of trusted OpenPGP certificates on old installations is fixed and won't be updated.  Under this assumptions, the attacker can not extend the expiration time.  Thus, honoring the expiration closes an attack vector by invalidating newer signatures.

There is the caveat that an attacker can backdate the signature creation time.  To protect against that, the creation time must be corroborated by other means, the OpenPGP implementation cannot do that alone, one needs to consider context.

Invalidation of cryptographic material, either via expiration or revocation, is tricky.  But, ignoring expiration times certainly doesn't help.

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