Implement signing

[Vincinator]

Requirements

• Use SSL key
• sign layer + metadata
• place signature in annotation inside manifest
• place what has been signed in annotation inside manifest
• Check if there is an emerging standard for signatures in OCI
  • Requirements: Artifact Signing opencontainers/distribution-spec#70
  • Define structure of signatures in OCI opencontainers/image-spec#400

Thanks @nkraetzschmar

Integration into Pipelines

• Use KMS signing
• For development and local testing add oci-sign.crt creation in Makefile of gardenlinux/gardenlinux cert/ folder

[Vincinator]

Checked Annotation Rules. I see no issue with using annotations for storing signed object* and signature.

*signed object: the string that contains target digest + metadata

[Vincinator]

I could not find consensus in the oci-spec repository on signing, could not find a clear favourite that will emerge in the next time.

We may consider also using https://github.com/notaryproject/notary, but for now we do not plan to use this.

[Vincinator]

TODO: need to input version as this should be part also of string that should be signed

[Vincinator]

TODO: we also need to add verification of signatures before we edit add a manifest. For example, if we attach a layer, we need to make sure that the old signature of the manifest entry was valid. Otherwise, we would make something that was invalid before valid without directly checking it.

e.g.:

• we have a manifest A with an invalid signature in the oci-index
• we attach a layer with test results for example
• attaching a layer changes the manifest and its digest, so the signature of the manifest in the oci-index must be re-calculated.
• if we did not verify signature of that manifest entry, we would make a modified manifest with invalid signature valid and allow any other compromised layer to be signed by us

EDIT: done ✅

[Vincinator]

TODO: we also need to add verification of digests. The signed-data contains a digest, which is a sha256 hash of the respective blob (if layer) or json (if manifest).

If the blob or manifest is modified, the digests also changes in the registry. gl-oci should also verify digest where possible, but in theory it should not be required, because the oct-registry would not allow to host mismatching digests, with the potential implementation dependent exception of entries in the oci-index.

Therefore we just make sure to verify digests wherever we use the digest.

EDIT: done ✅

[Vincinator]

TODO: verification of signatures fails