Signing

The Enterprise Contract relies on Sigstore signatures to verify the authenticity of content being verified. Currently, this is integrated with the ec validate image command.

Image Verification

When the Enterprise Contract validates an image, there are certain builtin policy rules that are always applied and cannot be skipped. Most of these rely on cosign to fetch and verify image signatures and attestations. These meta artifacts are associated to the underlying container image by digest. This has two important properties. First, the image and its meta artifacts can be copied to another repository or registry without affecting validation. Second, the meta artifact for one image cannot be used to fulfill the validation of another image.

Sigstore Levels

There are different levels of Sigstore adoption. These can be done incrementally. The Enterprise Contract can be used to validate artifacts regardless of the level.

Long-Lived Keys

Using long-lived keys is the easiest and simplest way to adopt Sigstore. For verification, there are two requirements. The public key itself and a flag to indicate that Rekor integration should be disabled:

ec validate image --public-key=cosign.pub --ignore-rekor --image $IMAGE

Long-Lived Keys with Auditability

This approach uses Rekor to track when the signing key is used. When using the public instance of Rekor, the only requirement is the public key:

ec validate image --public-key=cosign.pub --image $IMAGE

Using an Alternative Rekor instance is also supported.

Identity-Based Short-Lived Keys ("keyless")

This is the strongest and most sophisticated Sigstore level. Here a complete Sigstore deployment is required. Verification is no longer based on a key. Instead, it is based on the signer’s identity. There are two pieces of information required, the certificate identity and the certificate OIDC issuer:

ec validate image --certificate-identity=$IDENTITY --certificate-oidc-issuer=$ISSUER --image $IMAGE

--certificate-identity and --certificate-oidc-issue flags perform an exact value match. Use --certificate-identity-regexp and --certificate-oidc-issuer-regexp to perform a regular expression match if additional flexibility is needed.

Any certificate involved in the signature is also provided as policy input. Use this data to establish a fine-grained verification process by leveraging rego policies. See the GitHub Certificate Checks as an example.

As with the previous level, it is also possible to use an Alternative Rekor instance during verification.

Alternative Rekor

By default, the ec validate image command uses the production public instance of Rekor. This section describes different ways to use an alternative Rekor instance during validation.

The simplest, and preferred method, is to use cosign initialize to populate the Sigstore root from an alternative TUF mirror:

cosign initialize -mirror $TUF_MIRROR -root $TUF_ROOT
ec validate image ...

See cosign initialize --help for details on initializing the Sigstore root.

If the Sigstore deployment does not include a TUF mirror, first delete the Sigstore root, then use the --rekor-url flag:

rm -rf ~/.sigstore/root
ec validate image --rekor-url $REKOR_URL ...