Signature verification failed: incorrect signature

A principal that is not listed in the allowed signers file for this key fails with only "Could not verify signature." (no preceding detail line).

OpenSSH ssh-keygen -Y verify (SSHSIG) — also used by Git for SSH-signed commits

What it means

`ssh-keygen -Y verify` checked an SSHSIG signature and the cryptographic verification failed: the data read from standard input is not exactly what was signed, or the signature was made by another key. SSH signatures also bind a namespace (`file`, `git`, `email`…); a signature verified under a different namespace fails with "namespace does not match" before the cryptographic check. Each failure is followed by "Could not verify signature.".

Why it happens

• The verified data differs from the signed data

  common

  A modified file, a different line ending (`\r\n` vs `\n`), a trailing newline added by an editor, or verifying the wrong file. SSHSIG signs a hash of the exact bytes.

• Wrong namespace

  common

  Signatures created with `-n file` cannot be verified with `-n git`, and Git always uses `git`. Namespaces stop a signature made for one purpose from being reused for another.

• Identity or key not in allowed_signers

  occasional

  `-I` names a principal that the allowed signers file does not associate with the signing key, or the key was rotated and the file still lists the old one.

How to fix it

1. 1.

   Verify the exact bytes with the right namespace

   Pipe the original file into `ssh-keygen -Y verify` and use the namespace from signing. `find-principals` shows which identity the key maps to.

   # Verify exactly the bytes that were signed, with the namespace used at signing time
   ssh-keygen -Y verify -f allowed_signers -I release@example.com -n file -s release.txt.sig < release.txt
   # Which principal does the allowed_signers file map this signature's key to?
   ssh-keygen -Y find-principals -f allowed_signers -s release.txt.sig

2. 2.

   Separate cryptography from trust

   `check-novalidate` verifies the signature without consulting allowed signers. If it passes, the problem is identity mapping; if it fails, the data or the signature is wrong.

   ssh-keygen -Y check-novalidate -n file -s release.txt.sig < release.txt   # signature valid at all (no trust check)?

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