Error / feature-request: Signing git commits with SSH hardware key

Hi!

I noticed git commit signing works well with ssh-ed25519 keys, but does fail with sk-ssh-ed25519@openssh.com SSH hardware keys (with can be used to clone / post to github for instance).

Would that be possible to support in a later release? Thus having a smart card can be useful for both ssh and git.

I also noticed a similar error in a previous mail from Cuckoo Aidan <aidancuckoo@gmail.com>, but he doesn't say which type of key he used. In any case, would that be possible to include the info about which type of keys cannot be used to commit in the github guide https://docs.github.com/en/authentication/managing-commit-signature-verification/telling-git-about-your-signing-key#telling-git-about-your-ssh-key) ?

On 2022-10-11 at 18:12:19, Nicolas Graves wrote:

Hey,

I was surprised to hear that, so I just tested on my Debian amd64/sid system, and I was able to sign and verify using an sk-ssh-ed25519@openssh.com SSH key using my YubiKey 5C. I do believe it does work, although when the signature occurs, there's no notice that it's waiting for user interaction, so you just have to look at the lights to determine that the touch is needed.

Could you maybe mention what version of OpenSSH you're using and on what platform? I used 9.0p1, and as I mentioned, it's Linux. The output looks like so:

  $ git verify-commit --raw HEAD
  Good "git" signature for sandals@crustytoothpaste.net with ED25519-SK key SHA256:PNxAWB7cxxxrCTbgsdoDq71o3rCm9O7Er4q+0YrEAdM

Specifically, what error message or other indications of failure do you see when you try to sign?

We don't control the GitHub documentation, since we're independent of GitHub. If there's incorrect information, you'd need to contact GitHub. However, as I mentioned above, I do believe this works at least in some cases.

On 2022-10-11 20:41, brian m. carlson wrote:

I currently use this version on Guix (so Linux): OpenSSH_8.9p1, OpenSSL 1.1.1q 5 Jul 2022

The errors occurs when I try to "really" commit using magit or git through the command line.

hint: Waiting for your editor to close the file... Waiting for Emacs... error: Couldn't load public key sk-ssh-ed25519@openssh.com AAAAG[..]zaDo=: No such file or directory?

fatal: failed to write commit object

Now if I try this command:

I get the following: error: gpg.ssh.allowedSignersFile needs to be configured and exist for ssh signature verification

It's possible that my config is not complete, I remember not having necessarily done the part with allowedsigners (which is not necessary for an ed25519 "simple" key apparently). I'll take a look, and come back to you.

On 2022-10-11 23:54, Nicolas Graves wrote:

And I can confirm that it was this error and bad configuration, sorry!

This error is not very precise though.

Thanks for your help!

Sorry, I still do have this error, even though the previous one disappeared (it verified a commit signed byb another key) and my configuration is OK.

No such file or directory would indicate I don't own the key. I've verified I have the key on my Yubikey5C, in the directory ~/.ssh with the right condition, and additionnaly I've imported it in ssh-agent with ssh-add.

Using the GIT_TRACE=2 env variable, I get this: 08:32:41.916712 git.c:460 trace: built-in: git commit -m test 08:32:41.917380 run-command.c:655 trace: run_command: ssh-keygen -Y sign -n git -f 'sk-ssh-ed25519@openssh.com AAAAG[...]zaDo=' /tmp/.git_signing_buffer_tmp6Dc0Mx

The ssh-keygen manual indicates the following:

-Y sign
             Cryptographically sign a file or some data using a SSH key.  When
             signing, ssh-keygen accepts zero or more files to sign on the
             command-line - if no files are specified then ssh-keygen will
             sign data presented on standard input.  Signatures are written to
             the path of the input file with “.sig” appended, or to standard
             output if the message to be signed was read from standard input.

             The key used for signing is specified using the -f option and may
             refer to either a private key, or a public key with the private
             half available via ssh-agent(1).  An additional signature name‐
             space, used to prevent signature confusion across different do‐
             mains of use (e.g. file signing vs email signing) must be pro‐
             vided via the -n flag.  Namespaces are arbitrary strings, and may
             include: “file” for file signing, “email” for email signing.  For
             custom uses, it is recommended to use names following a NAME‐
             SPACE@YOUR.DOMAIN pattern to generate unambiguous namespaces.

Running ssh-add -L, I can confirm that the private half is available via ssh-agent.

I've also check that the error doesn't come from the missing input file (used to sign), here /tmp/.git_signing_buffer_tmp6Dc0Mx . Re-running the ssh-keygen command with a mock file fails with the same error.

I don't really know where to investigate next. What I get is that the error probably comes from ssh (should I report it there? The problem feels isolated now that I know which command fails). I may try to update openssh to v9, but since I'm using guix that might be quick as well as tedious.

On 12.10.2022 00:17, Nicolas Graves wrote:

I assume you have specified your key in the git config in user.signingkey as the literal keystring? If so, then you'll need a `key::` prefix. Otherwise git will treat it as a file path. See https://git-scm.com/docs/git-config#Documentation/git-config.txt-usersigningKey

In the initial merge of this feature, git would allow ssh-* keys to be specified without the prefix. This was later deprecated and not all Tutorial/Docs you'll find online consider this.

Cheers, Fabian

On 2022-10-12 08:55, Fabian Stelzer wrote:

Our mails crossed :)

This was the actual error, thanks a lot!