Git and their various hosting platforms support commit signing as an additional step of verification. There seems to be an active debate on whether it should be used regularly, though I’ll describe it on here in case you want to set it up.

You’ll need to have a GPG key already created.

First locate the key you want to sign with

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

This will output something like

/home/user/.gnupg/pubring.kbx
------------------------------
sec   rsa4096/8294756F 2020-04-11 [SC] [expires: 2021-04-11]
      KDIAUBEUX837DIU79YHDKAPOEMNCD7123FDAPOI
uid         [ultimate] Brandon Rozek (Git)
ssb   rsa4096/9582109R 2020-04-11 [E] [expires: 2021-04-11]

If you want to sign your commits with your main private key, then you can use the main key’s fingerprint. In the example above, that’s the part that starts with KDI.

(Optional) Creating a signing subkey

Alternatively, we can create a subkey specifically for signing commits. To do that, first we need to enter the edit mode for that key.

gpg --edit-key $FINGERPRINT

where $FINGERPRINT is the same fingerprint above.

You’ll see something like the following

sec  rsa3072/3E40C8DB05FCCFAD
     created: 2022-12-18  expires: 2023-12-18       usage: SC  
     trust: ultimate      validity: ultimate
ssb  rsa3072/50CC6B37C26F7882
     created: 2022-12-18  expires: 2023-12-18       usage: E   
[ultimate] (1). Brandon Rozek

gpg>

From there you type addkey, which will then present you with some options

Please select what kind of key you want:
   (3) DSA (sign only)
   (4) RSA (sign only)
   (5) Elgamal (encrypt only)
   (6) RSA (encrypt only)
  (14) Existing key from card
Your selection? 

As before, I recommend going with the default signing key option. In this case it’s (3) DSA (sign only).

DSA keys may be between 1024 and 3072 bits long.
What keysize do you want? (2048) 

As before, either stick with the default or tweak based on your personal assesment of risk.

Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n months
      <n>y = key expires in n years

Same advice as before in terms of key expiration. I generally stick with 1y. Then, after confirming the sanity checks you should see the key created.

sec  rsa3072/3E40C8DB05FCCFAD
     created: 2022-12-18  expires: 2023-12-18   usage: SC  
     trust: ultimate      validity: ultimate
ssb  rsa3072/50CC6B37C26F7882
     created: 2022-12-18  expires: 2023-12-18   usage: E 
ssb  dsa2048/5C1B6FCA0DABB046
     created: 2022-12-18  expires: 2023-12-18   usage: S   
[ultimate] (1). TestKey

The signing key is denoted by the label usage: S. From there we can take its fingerprint, which for the example above starts with 5C1B and proceed with the next step.

Configuring Git

From here, we need to tell git the key we want to sign with

git config --global user.signingkey $FINGERPRINT

To sign a commit, add a -S flag

git commit -S -m "Initial Commit"

To always sign your commits

git config --global commit.gpgsign true

Remember to add your public key to Github, Gitlab, etc. You can get it by

gpg --armor --export $FINGERPRINT