Set up a GPG key for signing Git commits on MacOS (M1)

Based on this blogpost.

To sign Git commits, you need a gpg key. GPG stands for GNU Privacy Guard and is the de facto implementation of the OpenPGP message format. PGP stands for ‘Pretty Good Privacy’ and is a standard to sign and encrypt messages.

Setting up

Install with Homebrew:

$ brew install gpg

Create config files for gpg and the gpg-agent. The agent will make sure you don’t have to type in your GPG passphrase for every commit.

$ mkdir ~/.gnupg
$ touch ~/.gnupg/gpg.conf ~/.gnupg/gpg-agent.conf

Open the gpg.conf file and add:

use-agent

In gpg-agent.conf, add the following lines to make sure your credentials are ‘kept alive’ (source):

default-cache-ttl 34560000
max-cache-ttl 34560000

Optionally, you can install a GUI for entering your passphrase. You don’t need to, but the default is a CLI program and might not provide a nice user experience. With pinentry-mac you can choose to save your passphrase in your MacOS keychain. That’s up to your personal preference.

$ brew install pinentry-mac

If you installed pinentry-mac, make sure to configure the agent. Open the gpg-agent.conf file and add this line:

pinentry-program /opt/homebrew/bin/pinentry-mac

Note: if you’re on Intel, /opt/homebrew should be /usr/local.

Add the following lines to ~/.zshrc (the GPG_TTY environment variable is a requirement for GPG; the second line launches the gpg-agent when you open a new shell):

export GPG_TTY=$(tty)
gpgconf --launch gpg-agent

To effectuate the changes to .zshrc, type:

$ source ~/.zshrc

Create GPG keypair

Now that your environment is properly set up, we need to generate a public/private GPG keypair.

$ gpg --full-gen-key

A wizard is printed to your terminal. You should configure as follows:

Kind of key: 4 (RSA, sign only)
Keysize: 4096
Expiration: 2y (your key will expire after 2 years; you should set a reminder somewhere)
Real name: <your github username>
Email address: <your email address>

Note: I heartily recommend setting your email address to your 'noreply' GitHub address: [email protected]. You can find your email address on the GitHub Email settings page. Note that if you created a GitHub account after July 2017, your address will also have an ID prefixed to your username; read more here.

The final step in setting up the GPG keypair is typing a passphrase. Make sure it is strong and you have it safely stored in your password vault (I recommend Bitwarden). Whoever has your passphrase can sign your commits and there is no way to prove it wasn’t you.

After creating the keypair, output similar to the following is printed to your terminal:

pub   rsa4096 2021-11-12 [SC] [expires: 2023-11-12]
      AAABBBCCCDDDEEEFFF1112223334445556667778
uid                      username <[email protected]>

The string of characters is your key ID. To confirm you can sign messages with your newly created key, enter in your terminal:

$ echo 'it works' | gpg --clearsign

A message similar to this should appear:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

it works
-----BEGIN PGP SIGNATURE-----
<many characters>
-----END PGP SIGNATURE-----

Adding to Git

We need to add your key to your git config, and to GitHub. First, you need to find the key ID. The (short) ID uses the last 8 characters of the key that was printed to the terminal before. You can retrieve it:

$ gpg --list-secret-keys --keyid-format SHORT

Outputs:

/Users/username/.gnupg/pubring.kbx
----------------------------------
sec   rsa4096/56667778 2021-11-12 [SC] [expires: 2023-11-12]
      AAABBBCCCDDDEEEFFF1112223334445556667778
uid         [ultimate] username <[email protected]>

The 56667778 bit after rsa4096/ is your short key ID. We need it to configure Git to sign commits and tags. Replace the user.signingkey value below with your own key ID:

$ git config --global user.signingkey 56667778
$ git config --global commit.gpgSign true
$ git config --global tag.gpgSign true

Git needs to know your email, and it needs to be the same as the one for your GPG key. This email address needs to be verified on GitHub as well. If you use your ‘private’ GitHub email, that’s already the case.

$ git config --global user.email [email protected]

Finally, you need to add your public GPG key to GitHub. Again, make sure to replace the ID with your own ID:

$ gpg --armor --export 56667778

Outputs:

-----BEGIN PGP PUBLIC KEY BLOCK-----

<many characters>
-----END PGP PUBLIC KEY BLOCK-----

You need to copy the whole block and add it to GitHub. If you’re not sure what to copy, use this command:

$ gpg --armor --export 56667778 | pbcopy

The | pbcopy part will pipe the output of the first part directly to your copy-paste memory.

Go to the GitHub SSH and GPG keys section, click [New GPG key] and paste into the box. Click [Add GPG key], and you’re done!

After getting this done, and after having made your first signed commit, you can see the ‘Verified’ badge on GitHub for that commit (see an example here). Your GPG key ID will be shown when the badge is clicked.

Visual Studio Code

If you use Visual Studio Code, you can turn on signing by changing a setting.

Open VSCode, go to Preferences > Settings, and search for git.enableCommitSigning. Turn this setting on, and you’re good to go.

Troubleshooting

1. Kill the agent

If for some reason you can’t sign, simply kill the agent. It will restart when needed:

$ gpgconf --kill gpg-agent

2. Older/remote shells

On older MacOS versions or certain (remote) shells, you might encounter the error inappropriate ioctl for device. (This error might also turn up if you haven’t configured the GPG_TTY environment variable correctly, see above for instructions.) More context here. You can fix this by using the so called ‘loopback’ option to enter your passphrase directly on the CLI.

Edit gpg.conf and add:

pinentry-mode loopback

Edit gpg-agent.conf and add:

allow-loopback-pinentry

Now, when the agent wants your passphrase it will simply render a basic password input on the CLI:

$ echo 'it works' | gpg --clearsign
Enter passphrase:

3. Using SourceTree

If you use SourceTree, you should point it to the right binary. A solution is posted on Stack Overflow, make sure to also follow this comment.

4. X.509 Certificate

If the commit still can’t be signed, it could be that you use an X.509 certificate to sign your commits (this might be the case in a corporate environment, for example). Tell Git about it:

$ git config --global gpg.x509.program gpg

If that doesn’t cut it, install smimesign, an S/MIME signing utility for use with Git with Homebrew:

$ brew install smimesign

Then, configure git to use an X.509 certificate and smimesign as the gpg program.

$ git config --global gpg.x509.program smimesign
$ git config --global gpg.format x509

H/t to @benhickson and this excellent GitLab guide.

5. I don’t know what `.zshrc` is or how to edit it

The .zshrc file is a configuration file for your zsh shell (rc stands for runcom) that might not exist yet on your system. You can easily create it like this:

$ cd ~
$ touch .zshrc

These kinds of 'dot files' are usually not edited with standard MacOS apps like TextEdit. You can use vi or nano from the command line instead. Find some useful instructions here.

H/t to @intricateavocado

NandoMB/signing-git-commits.md