Verifying OCI image index offline

[xavier-dfns]

Description

I've been trying to sign & verify offline with key. I'm using sign / save / verify.
The verify step fails with:

Error: no signatures associated with the image saved in img
error during command execution: no signatures associated with the image saved in img

This is the content of the index:

{
   "schemaVersion": 2,
   "mediaType": "application/vnd.oci.image.index.v1+json",
   "manifests": [
      {
         "mediaType": "application/vnd.oci.image.index.v1+json",
         "size": 856,
         "digest": "sha256:8f76951b91383d78e515159899508a92ffdd4d56c814a7c380ac13a919d089e0",
         "annotations": {
            "kind": "dev.cosignproject.cosign/imageIndex"
         }
      }
   ]
}

I'm wondering if the issue doesn't come from #4626 which only look for dev.cosignproject.cosign/image or am I doing something wrong?

What I tried that work:

• verify directly from registry
• sign with --recursive, recover the hash of the platform leaf, save with that platform leaf and then verify

Version: 3.1.0