Sign commits with SSH keys

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Introduced in GitLab 15.7 with a flag named ssh_commit_signatures. Enabled by default.

Generally available in GitLab 15.8. Feature flag ssh_commit_signatures removed.

When you sign commits with SSH keys, GitLab uses the SSH public keys associated with your GitLab account to cryptographically verify the commit signature. If successful, GitLab displays a Verified label on the commit.

You may use the same SSH keys for git+ssh authentication to GitLab and signing commit signatures as long as their usage type is Authentication & Signing. It can be verified on the page for adding an SSH key to your GitLab account.

For more information about managing the SSH keys associated with your GitLab account, see Use SSH keys to communicate with GitLab.

Configure Git to sign commits with your SSH key

After you create an SSH key and add it to your GitLab account or generate it using a password manager, configure Git to begin using the key.

Prerequisites:

Git 2.34.0 or newer.
OpenSSH 8.1 or newer.

NOTE: OpenSSH 8.7 has broken signing functionality. If you are on OpenSSH 8.7, upgrade to OpenSSH 8.8.
A SSH key with the usage type of either Authentication & Signing or Signing. The SSH key must be one of these types:
- ED25519 (recommended)
- RSA

To configure Git to use your key:

Configure Git to use SSH for commit signing:
```
git config --global gpg.format ssh
```
Specify which public SSH key to use as the signing key and change the filename (~/.ssh/examplekey.pub) to the location of your key. The filename might differ, depending on how you generated your key:
```
git config --global user.signingkey ~/.ssh/examplekey.pub
```

Sign commits with your SSH key

Prerequisites:

You've created an SSH key.
You've added the key to your GitLab account.
You've configured Git to sign commits with your SSH key.

To sign a commit:

Use the -S flag when signing your commits:
```
git commit -S -m "My commit msg"
```
Optional. If you don't want to type the -S flag every time you commit, tell Git to sign your commits automatically:
```
git config --global commit.gpgsign true
```
If your SSH key is protected, Git prompts you to enter your passphrase.
Push to GitLab.
Check that your commits are verified. Signature verification uses the allowed_signers file to associate emails and SSH keys. For help configuring this file, read Verify commits locally.

Verify commits

You can verify all types of signed commits in the GitLab UI. Commits signed with an SSH key can also be verified locally.

Verify commits locally

To verify commits locally, create an allowed signers file for Git to associate SSH public keys with users:

Create an allowed signers file:
```
touch allowed_signers
```

Configure the allowed_signers file in Git:

git config gpg.ssh.allowedSignersFile "$(pwd)/allowed_signers"

Add your entry to the allowed signers file. Use this command to add your email address and public SSH key to the allowed_signers file. Replace <MY_KEY> with the name of your key, and ~/.ssh/allowed_signers with the location of your project's allowed_signers file:
```
# Modify this line to meet your needs.
# Declaring the `git` namespace helps prevent cross-protocol attacks.
echo "$(git config --get user.email) namespaces=\"git\" $(cat ~/.ssh/<MY_KEY>.pub)" >> ~/.ssh/allowed_signers
```
The resulting entry in the allowed_signers file contains your email address, key type, and key contents, like this:
```
example@gitlab.com namespaces="git" ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAmaTS47vRmsKyLyK1jlIFJn/i8wdGQ3J49LYyIYJ2hv
```
Repeat the previous step for each user who you want to verify signatures for. Consider checking this file in to your Git repository if you want to locally verify signatures for many different contributors.

Use git log --show-signature to view the signature status for the commits:

$ git log --show-signature

commit e2406b6cd8ebe146835ceab67ff4a5a116e09154 (HEAD -> main, origin/main, origin/HEAD)
Good "git" signature for johndoe@example.com with ED25519 key SHA256:Ar44iySGgxic+U6Dph4Z9Rp+KDaix5SFGFawovZLAcc
Author: John Doe <johndoe@example.com>
Date:   Tue Nov 29 06:54:15 2022 -0600

    SSH signed commit

Revoke an SSH key for signing commits

Introduced in GitLab 15.9.

If an SSH key becomes compromised, revoke it. Revoking a key changes both future and past commits:

Past commits signed by this key are marked as unverified.
Future commits signed by this key are marked as unverified.

Revoking an SSH key removes it from your account. SSH keys that are only used for authentication do not have the option to be revoked.

To revoke an SSH key:

On the left sidebar, select your avatar.
Select Edit profile.
On the left sidebar, select SSH Keys ({key}).
Next to the SSH key you want to revoke, select Revoke.
Optional. Delete the SSH key.