Red Hat Trusted Artifact Signer with Enterprise Contract: Trustable container images

Recently, Red Hat announced the technical preview of Red Hat Trusted Artifact Signer which is a production-ready deployment of the Sigstore project for enterprise use. In this article, we will learn how to use Trusted Artifact Signer when signing, attesting and verifying a container image with cosign and Enterprise Contract (EC).

Before starting, we must deploy Trusted Artifact Signer on our Red Hat OpenShift cluster by following Chapter 1 of the Deployment Guide. Be sure to also run the source ./tas-env-variables.sh script to set up the shell variables (URLs) to the Sigstore services endpoints (Fulcio, Rekor etc).

Once Trusted Artifact Signer is up and running, we no longer need to be logged in to the OpenShift cluster:

oc logout

Next, we will need a container image to play with. This can be any container image in any Open Container Initiative (OCI) registry, e.g. quay.io. The only requirement is that we must have write access to the repository. For convenience, we will set a shell variable with the image reference:

IMAGE=quay.io/lucarval/rhtas-test@sha256:6b95efc134c2af3d45472c0a2f88e6085433df058cc210abb2bb061ac4d74359

That’s it for the prerequisites. Things are about to get exciting.

Let’s get signing

First, let’s tell cosign and EC to use Trusted Artifact Signer instead of the publicly available sigstore deployment:

cosign initialize --mirror=$TUF_URL --root=$TUF_URL/root.json

Now we are ready to sign the image:

cosign sign -y --fulcio-url=$FULCIO_URL --rekor-url=$REKOR_URL \
     --oidc-issuer=$OIDC_ISSUER_URL $IMAGE

The command above will cause your default web browser to open to a login page. This is the Keycloak instance created during the Trusted Artifact Signer deployment. Login with the credentials of an existing user.

The image should now be signed.

Before verifying the image signature, let’s also create a Supply-chain Levels for Software Architects (SLSA) Provenance attestation and associate it with the container image. Usually, the system responsible for building the container image is also responsible for doing this. Here, we simply create a sample SLSA Provenance:

echo '{
 "builder": {
   "id": "https://localhost/dummy-id"
 },
 "buildType": "https://localhost/dummy-type",
 "invocation": {},
 "buildConfig": {},
 "metadata": {
   "buildStartedOn": "2023-09-25T16:26:44Z",
   "buildFinishedOn": "2023-09-25T16:28:59Z",
   "completeness": {
     "parameters": false,
     "environment": false,
     "materials": false
   },
   "reproducible": false
 },
 "materials": []
}
' > predicate.json

Now we sign and attach the predicate above as an attestation to the image.

cosign attest -y --fulcio-url=$FULCIO_URL \
   --rekor-url=$REKOR_URL \
   --oidc-issuer=$OIDC_ISSUER_URL \
   --predicate predicate.json \
   --type slsaprovenance $IMAGE

Just as before, a web browser will appear. Authentication happens automatically as you are already logged in.

Finally, we will use EC to verify the signature and attestation of the image.

ec validate image --image $IMAGE \
   --certificate-identity-regexp '.*' \
   --certificate-oidc-issuer-regexp '.*' \
   --output yaml --show-successes

The command above should display a detailed report of the verifications performed as well as detailed information about the signatures.

NOTE: When verifying a container image, avoid using a loose regular expression like the example above. Instead, be as specific as possible to be sure the signatures match the expected identity.

I hope you enjoyed this high level overview showcasing how to use Red Hat Trusted Artifact Signer with cosign and Enterprise Contract!