Prove who built it

If two people build two images, each image has a digest. Pulling by digest protects you from tag movement, but it does not say whether the digest was produced by your release workflow, a laptop, or an attacker with push access. Signatures add an identity claim over that digest.

Traditional signing requires storing and rotating a private key. Cosign keyless signing asks an OIDC provider, such as GitHub, to issue a short-lived identity certificate. The signature can then be verified against the certificate subject and issuer. There is still trust, but not a long-lived key sitting in CI like a loaded mousetrap.

Rekor records signing events in an append-only log. That means signatures can be audited after the fact, and suspicious duplicate or unexpected signatures are visible. A transparency log does not prevent every attack. It changes the attack from silent to observable, which is often the difference between incident and mystery.

A signature says an identity signed a digest. An attestation says something more: this build used this workflow, this SBOM belongs to the image, these materials were inputs. Tools can verify attestations separately from signatures. That is how supply-chain policy grows beyond "was it signed?" into "was it built the way we require?"