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Cosign v3 bundle-format signatures are classified as attestations, so signed images score "unsigned" #281

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@mikecardwell

Cosign 3 attaches signatures as Sigstore bundle referrers, artifact type application/vnd.dev.sigstore.bundle.v0.3+json with annotation dev.sigstore.bundle.predicateType: https://sigstore.dev/cosign/sign/v1, and no longer writes the sha256-<digest>.sig tag by default. classifyArtifactType in registry/client.go recognises a signature only by an artifact type containing "signature", "notary" or "cosign", or by that tag, and the predicate-type branch checks only vex, sbom, provenance and vuln. Such images get a red X on Signature and 8/10. Example: index.docker.io/grepular/pdns-recursor-trimmed:latest, whose two referrers are the cosign signature bundle and an OpenVEX bundle, and which cosign verify accepts. Suggested fix: treat a predicate type containing sigstore.dev/cosign/sign as a signature.

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