Feature Overview

• oc-mirror by default leverages OCI 1.1 referrers or its fallback (tag-based discover) to discover related image signatures for any image that it mirrors
• this feature is enabled by default and can be disabled globally
• Optionally, oc-mirror can be configured to include other referring artifacts, e.g. SBOMs or in-toto attestations referenced by their OCI artifact media type

Goals

• As part of OCPSTRAT-918 and OCPSTRAT-1245 we are introducing broad coverage in the OpenShift platform for signatures produced with the SigStore tooling, which allow for scalable and flexibly validation of the signatures, incl. offline environments
• In order to enable offline verification, oc-mirror needs to detect whether any image that is in scope for its mirroring operation has one or more related SigStore signatures referring to, by using the OCI 1.1 referrers API or it's fallback, or cosigns tag naming convention for signatures and mirror those artifacts as well

Requirements

• SigStore-style signature should be mirrored by default, but opt-out has to be available
• The public key from Red Hat and the public Rekor key from Red Hat used to sign products images needs to be available offline 
• SigStore-style attachments should optionally be able to be discovered and mirrored as well as an opt-in, the user should be able to supply a list of OCI media types they are interested in (e.g. text/spdx or application/vnd.cyclonedx for SBOMs)

Background, and strategic fit

OpenShift is planning to ship all payload and layered product images signed consistently via cosign with OpenShift 4.17. oc-mirror should be able to leverage this to provide a seamless signature verification experience in an offline environment by automatically making all required signature artifacts available in the offline registry.

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