invalid signature when validating ASN.1 encoded signature

Printed as "Error: …" and again as "error during command execution: …". With a bundle (`--bundle`) cosign 3 prefixes "failed to verify signature: could not verify message:". Signatures in IEEE P1363 form fail with "…when validating IEEE_P1363 encoded signature". The core text is the same in cosign 2.6 and 3.1.

Sigstore cosign (verify-blob / verify with --key) — sigstore-go signature verifier

What it means

cosign verified the ECDSA signature (DER/ASN.1 encoded, hence the wording) over the artifact's digest with the public key you supplied, and it did not match. Either the key is not the one that signed, or the artifact differs from what was signed. Unlike keyless verification, nothing about identities or the transparency log is involved — this is the raw signature check failing.

Why it happens

• Verifying with the wrong public key

  common

  A key from another project, environment or release line; or a key that was rotated while old signatures are still being verified.

• The artifact is not the signed one

  common

  A rebuilt binary, a re-compressed archive, a file downloaded through a proxy that changed it, or text with converted line endings.

• Signature file of another artifact

  occasional

  `.sig` files of several platforms (`linux-amd64`, `darwin-arm64`…) mixed up during download or in a release script.

How to fix it

1. 1.

   Compare the digest and the key

   Check the artifact's SHA-256 against the published digest and the public key against the one derived from the signing key.

   sha256sum artifact.txt                             # compare with the digest published with the release
   cosign public-key --key release.key > derived.pub   # (key holder) public half of the signing key
   diff <(openssl pkey -pubin -in derived.pub -outform DER | sha256sum) \
        <(openssl pkey -pubin -in cosign.pub -outform DER | sha256sum)

2. 2.

   Ship bundles instead of loose signatures

   A Sigstore bundle keeps signature and verification material together, which avoids most mix-ups.

   # Prefer bundles: signature, verification material and the log entry travel together
   cosign sign-blob --yes --key release.key --bundle artifact.sigstore.json artifact.txt   # uploads to Rekor
   cosign verify-blob --key release.pub --bundle artifact.sigstore.json artifact.txt

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