feat: add support for referrers attestations

[arewm]

Changes

While the backwards compatible tag-based attestation method works, it can result in a lot of additional tags being pushed to OCI registries. In order to maintain fewer tags, Chains should be able to use the referrer's API when pushing signatures and attestations.

Co-Authored-By: Claude [email protected]
Signed-off-by: arewm [email protected]

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Release Notes

Add support for pushing OCI signatures and provenance with the referrer's API

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[anithapriyanatarajan]

@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

[arewm]

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 created sigstore/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).

[arewm]

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 the referrers API is available or fallback to updating the image index pushed to a tag described by the referrers tag schema. A client querying the referrers API and receiving a 404 Not Found MUST fallback to using an image index pushed to a tag described by the referrers tag schema.

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

[arewm]

This is something that Claude created. I'll leave it for now, but will plan to remove it before merging.

[arewm]

I just reworked this implementation to include the changes made in sigstore/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

[tekton-robot]

@arewm: PR needs rebase.

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