How to sign your commits

GitHub has a full guide to sign commits.

With a few command lines, you can configure Git to use your custom GPG or SSH key automatically, every time you add a new commit.

I prefer signing commits with an SSH key. Let’s see why.

Why do we need to sign commits?

The answer is relatively simple:

git config user.email "developer@yourcompany.org"
git commit -m "This modification looks legitimate"

Unless you sign your commits, there’s no way to ensure the code was added by a legitimate member of the team.

That’s why most Git platforms provide features to verify identities with cryptographic keys.

GPG keys in Git

GPG encryption consists of generating a pair of keys. The process allows signing various kinds of messages, including your Git commits.

If you want to dig further, read gnupg.org.

Git has a long history with GPG, that’s why most tutorials and commands on the topic will include it. However, SSH is more straightforward, to me.

How to use SSH keys

SSH is usually available by default on your system. If you don’t have it, it’s easy to install.

Then:

touch ~/.ssh/allowed_signers
echo "{EMAIL} ssh-rsa {KEY}" > ~/.ssh/allowed_signers
git config --global gpg.ssh.allowedSignersFile ~/.ssh/allowed_signers
git config --global commit.gpgsign true
git config --global gpg.format ssh
git config --global user.signingkey {/path/to/.ssh/key.pub}

Don’t forget to replace data wrapped in {} with your own credentials.

How to check the signature

git show --show-signature

If everything is ok, you’ll get something like the following:

Good “git” signature for {EMAIL} with ED25519 key {KEY}

Here, Git says “ED25519” because that’s the algorithm I use to generate my keys.

N.B.: Don’t forget to upload your key in the desired platform. For example, GitHub lets you define an SSH key as a signing key (and not the classic authentication key).

Is it really better?

The main caveat of using SSH to sign commits is that old versions of OpenSSH are probably not compatible, but that should not be a major issue.

Also, it’s essential to bear in mind that cryptographic keys should be used with caution. In other words, don’t use the same pairs for multiple machines and purposes.