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Verifying integrity of CNG images

To ensure the CNG images aren’t tampered with after they are pushed to the registry, their digests are signed using cosign. cosign uses ECDSA-P256 keys and SHA256 hashes. Keys are stored in PEM-encoded PKCS8 format.

These digests can be verified using cosign verify command as described below:

The images are signed using a private key and can be only verified locally using the corresponding public key. Moving to a keyless signing/verification with GitLab.com OIDC provider is being discussed in issue 638.

  1. Download the public key used for signing from https://charts.gitlab.io/cosign.pub:

    wget https://charts.gitlab.io/cosign.pub

  2. Verify the CNG image:

    $ cosign verify --key cosign.pub registry.gitlab.com/gitlab-org/build/cng/gitlab-workhorse-ee:v16.9.0 | jq -r

Verification for registry.gitlab.com/gitlab-org/build/cng/gitlab-workhorse-ee:v16.9.0 --
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - Existence of the claims in the transparency log was verified offline
  - The signatures were verified against the specified public key
[
  {
    "critical": {
      "identity": {
        "docker-reference": "dev.gitlab.org:5005/gitlab/charts/components/images/gitlab-workhorse-ee"
      },
      "image": {
        "docker-manifest-digest": "sha256:218a67cc46b49ba0563dbdc83618bf11fa5453577a4aa75475823e315952ea79"
      },
      "type": "cosign container image signature"
    },
    "optional": {
      "Bundle": {
        "SignedEntryTimestamp": "MEUCIQCDj2Ffe8Qll9clqAKoBA8wTwg2NrzMLvpVMkw61qdhmAIgQgLYCT7IdGwVEp5UrQjN67Zt9CTATQpi08+CrGgqnxw=",
        "Payload": {
          "body": "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",
          "integratedTime": 1707955615,
          "logIndex": 71468664,
          "logID": "c0d23d6ad406973f9559f3ba2d1ca01f84147d8ffc5b8445c224f98b9591801d"
        }
      }
    }
  }
]