Verifying cosign signatures when using podman

Today, I learned that podman supports verifying cosign signatures, if you configure it properly.

Here is how to make it work with a public key, instead of the self-signed certificates that Sigstore uses.

First, I edited the /containers/registries.d/default.yaml file to use sigstore attachments (that comprises signatures, and probably attestations):

docker:
    ghcr.io/freedomofpress/dangerzone/v1:
      use-sigstore-attachments: true

Then, I updated the /etc/containers/policy.json to check for out public key:

   "docker": {
        "ghcr.io/freedomofpress/dangerzone/v1": [
            {
                "type": "sigstoreSigned",
                "keyPath": "/home/alexis/dev/fpf/dangerzone/share/freedomofpress-dangerzone.pub",
                 "signedIdentity": {"type": "matchRepository"}
            }
        ]
    }

Now, when doing a podman pull, it works! If I specify another public key, it fails with:

Error: unable to copy from source docker://ghcr.io/freedomofpress/dangerzone/v1:latest: copying system image from manifest list: Source image rejected: cryptographic signature verification failed: invalid signature when validating ASN.1 encoded signature

With the proper signatures and public keys, a podman pull actually gets and stores the signatures for us:

$ podman pull ghcr.io/freedomofpress/dangerzone/v1
Trying to pull ghcr.io/freedomofpress/dangerzone/v1:latest...
Getting image source signatures
Checking if image destination supports signatures
Copying blob 89b08d9231ff done   |
Copying config 945016a2e0 done   |
Writing manifest to image destination
Storing signatures
<imageID>