Add support for validating signatures from PGP subkeys #2874
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This change should update the gpgme and openpgp implmenetations to support validating signatures made by signing subkeys. It will close an open issue that was referenced specifically in [podman](containers/podman#16406), but should resolve this across a number of tools that use this library. Signed-off-by: Danny Grove <danny@dannygrove.com>
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containers/podman#16406 contains a long discussion about expired (or revoked) keys, IIRC it’s fairly unclear what is the right thing to do for long-term artifacts like software, and with no trusted timestamps.
Adding what, 150? new lines of code to the security critical signing code path is also a bit unattractive (although that’s, by itself, not at all a reason not to do the right thing).
There’s also a bit of awkward timing in that #2569 will probably replace the default / recommended verification implementation.
OTOH, these design concerns are, right now, also a topic there: #2569 (comment) .
I noticed this as I was finishing out the implementation but based on my understanding of the comments. It appears the GPGPME/Go OpenPGP will still exist, it just won't be the hot path. As the sequoia OpenPGP implementation has also been something that's still being worked though, having any support for subkeys IMO is ideal.
To my understanding, once a key is revoked or expired the signature is no longer valid. Regardless of when it was signed. This default to close IMO is a feature. Revocations are packets within the key just like anything else and generally the systems/users doing validation of OpenPGP signatures understand the implications of this. If a key is revoked and pushed to a keyserver, the user/machine has to pull down the updated packets for it to take effect within the verification pipeline. If you are aware that a revocation has been published but you still need to use a previously signed image it would probably be more ideal for you to mirror the image and sign it yourself than the much less secure option of not importing the updated key. This same problem exists is most signing schemes (key rotation). |
This change should update the gpgme and openpgp implmenetations to support validating signatures made by signing subkeys. It will close an open issue that was referenced specifically in podman, but should resolve this across a number of tools that use this library.