Description

I have a private Sigstore instance, in which I attempted to rotate the TSA certificate chain. When trying to verify any blob/image after the rotation, I get the following error:

$ cosign --verbose verify-blob --certificate-identity=kilgore@kilgore.trout --bundle signed.bundle to-sign --rfc3161-timestamp=timestamp.txt --use-signed-timestamps
Error: unable to load TSA certificates: TSA certificate chain must contain exactly one leaf certificate
main.go:74: error during command execution: unable to load TSA certificates: TSA certificate chain must contain exactly one leaf certificate

This is because the GetTSACerts function in pkg/cosign/tsa.go only expects to parse out a single certificate chain, because of the assumption that these individual chains are provided as individual TUF targets.

However in order to be able to properly rotate the TSA certificate chain, one needs to provide the whole TSA certificate chain as a single TUF target (which is possible and I do it). However, when I attempt to introduce a new TSA chain as a new TUF target and keeping the old one as "Expired", I get the above error when verifying any artifact.

I think the GetTSACerts function should behave more like GetRekorPubs, which returns multiple keys and all of them are considered valid for verification.

I think this will actually work fine in the future with #3844 because of how the timestamp verification function from sigstore-go works. I can submit a PR to fix this if you think it's still worth it before #3844 gets merged - I think it's worth because of private Sigstore deployments that might be slower to upgrade.

Version

2.4.1, but applies to latest main branch as well AFAICS