Sign git commits with a gated ssh key¶

When: You sign your git commits with an ssh key, so GitHub shows them as Verified. You want an AI harness to ask you before it signs a commit in your name. Your own terminal signs with no popup.

Git signs a commit with ssh-keygen -Y sign. After cw harden ssh, that sign goes through the CmdWarden ssh gate, like a push over ssh (UC13). The gate reads it as class write.

Step 1: Gate the ssh agent¶

Do UC13 first. Your key must be in the OpenSSH agent:

cw harden ssh

Open a new terminal. Then list the keys through the gate:

ssh-add -L

Expect: one line for each key, for example ssh-ed25519 AAAAC3Nza... you@laptop. Copy the line of the key that signs.

Step 2: Tell git to sign with that key¶

git config --global gpg.format ssh
git config --global gpg.ssh.program "C:/Windows/System32/OpenSSH/ssh-keygen.exe"
git config --global user.signingkey "key::ssh-ed25519 AAAAC3Nza... you@laptop"
git config --global commit.gpgsign true

Put the line from Step 1 after key::. With key::, git asks the agent to sign, and no key file is read.

gpg.ssh.program must be the Windows OpenSSH ssh-keygen. The ssh-keygen of Git for Windows cannot read the gate pipe.

Step 3: Add the key to GitHub as a signing key¶

On GitHub, open Settings, then SSH and GPG keys, then New SSH key. Set Key type to Signing Key, and paste the line from Step 1. A key that you use to push must also be there as an Authentication Key.

Step 4: Check a signed commit¶

Make a file that tells git which key is yours, so git can check the signature:

"you@example.com $(ssh-add -L | Select-Object -First 1)" | Set-Content "$HOME\.ssh\allowed_signers"
git config --global gpg.ssh.allowedSignersFile "$HOME/.ssh/allowed_signers"

Use the e-mail address of your git commits. Then make a commit in a test repo:

git commit --allow-empty -m "Test the signature"
git verify-commit HEAD

Expect: Good "git" signature for you@example.com with ED25519 key SHA256:....

Who gets the card¶

Who commits	Default level	Result
Your terminal	Trusted	Git signs with no popup
An AI harness	Read	You get the card for each signed commit

The card shows the ssh-keygen -Y sign command and the key under KEYS. Allow for session lets the harness sign more commits until the session ends. A Deny stops the commit. Git shows agent refused operation and fatal: failed to write commit object.

Each sign writes an audit row with tool ssh, purpose sign, and the key name. To set a level for ssh only, run cw policy set <policyKey> ssh <level>.

Limits¶

The limits of UC13 apply. A key file with no passphrase in ~/.ssh lets a program sign with no gate. Keep the key in the agent only.
Git for Windows 2.34 or later has ssh signing. Run git --version to check.

Undo¶

git config --global --unset commit.gpgsign
cw unharden ssh