[APPROVALNOTIFIER] This PR isNOT APPROVED

This pull-request has been approved by:
To complete thepull request process, please assignpriyawadhwa after the PR has been reviewed.
You can assign the PR to them by writing/assign @priyawadhwa in a comment when ready.

The full list of commands accepted by this bot can be foundhere.

Hi@arewm. Thanks for your PR.

I'm waiting for atektoncd member to verify that this patch is reasonable to test. If it is, they should reply with/ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors shouldjoin the org to skip this step.

Once the patch is verified, the new status will be reflected by theok-to-test label.

I understand the commands that are listedhere.

@arewm - Could you clarify if all the OCI registries support the referrer API end point? If not could we include validation and default fall back option if the end point is not supported for the given registry. Also, Recommend to create an upstream issue for this with label kind/feature with more details on why we are incorporating this enhancement.Thanks

All registries might not support the referrers API but this is also why the configuration can be defined.

After creating this PR, I investigated the gap more and createdsigstore/cosign/issues/4335. I used cosign's support for the new bundle format in the PR. I don't know if there are any implications with this change, but I suspect that we might actually want that to be configured separately from the use of the referrer's API.

My understanding is that Cosign should handle the fallback, but it does make sense to add that as a test case here too.

After I do a little more investigation with cosign, I will create an issue in Chains to further describe the challenge I am trying to resolve (the motivation is similar to the cosign issue).

I found the spec:https://github.com/opencontainers/distribution-spec/blob/v1.1.0/spec.md#unavailable-referrers-api

A client that pushes an image manifest with a defined subject field MUST verify thereferrers API is available or fallback to updating the image index pushed to a tag described by thereferrers tag schema. A client querying thereferrers API and receiving a 404 Not Found MUST fallback to using an image index pushed to a tag described by thereferrers tag schema.

So we should expect cosign to fall back appropriately. The configuration change is just toattempt to use the referrer's API in the first place instead of only using the legacy option.

I just reworked this implementation to include the changes made insigstore/cosign#4357. It is not currently possible for cosign to download all of these attestations, but that is also something that I am working on. I have been using Tekton as the attestation generator for testing Cosign, so as I continue the investigation, this PR may change.

I have some sample images that I generated attestations and signatures for here:https://quay.io/repository/arewm/test-repo?tab=tags