Provenance attestation does not contain subjects for each tag created.

[marcofranssen]

Description

https://github.com/philips-labs/slsa-provenance-action generates provenance where each tag is captured as a subject in the provenance.

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "subject": [
    {
      "name": "philipssoftware/slsa-provenance:dddb40e199ae28d4cd2f17bad7f31545556fdd3d",
      "digest": {
        "sha256": "e3378aef23821fd6e210229e5b98b5bead2858581b2d590d9e3b49d53c3f71e7"
      }
    },
    {
      "name": "philipssoftware/slsa-provenance:v0.7.2",
      "digest": {
        "sha256": "e3378aef23821fd6e210229e5b98b5bead2858581b2d590d9e3b49d53c3f71e7"
      }
    }
  ],
  "predicateType": "https://slsa.dev/provenance/v0.2",
  "predicate": {
    …
  }
}

Cosign is currently only capturing the repository as a subject.

{
  "_type": "https://in-toto.io/Statement/v0.1",
  "subject": [
    {
      "name": "philipssoftware/slsa-provenance",
      "digest": {
        "sha256": "e3378aef23821fd6e210229e5b98b5bead2858581b2d590d9e3b49d53c3f71e7"
      }
    }
  ],
  "predicateType": "https://slsa.dev/provenance/v0.2",
  "predicate": {
    …
  }
}

See philips-labs/slsa-provenance-action#159 for more details on how we use both slsa-provenance and cosign in our workflow.

Currently there is no way to instruct cosign to take all the tags for a given digest. The downside of this is that you can't easily relate from the build provenance which tags have been released for a given digest/image.

Is this a feature that can be added to cosign? Or maybe should we drop this feature?