GPG verification of Git repositories without TLS

Posted on July 23, 2021

For online Git repositories that use the Git Protocol for serving code, you can can use GPG to handle authentication, if you have the committer’s public key.

Here’s how I’d verify that I’ve cloned an authentic version of remembering:

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$ wget -qO- https://euandre.org/public.asc | gpg --import -
gpg: clef 81F90EC3CD356060 : « EuAndreh <eu@euandre.org> » n'est pas modifiée
gpg:       Quantité totale traitée : 1
gpg:                 non modifiées : 1
$ pushd `mktemp -d`
$ git clone git://euandreh.xyz/remembering .
$ git verify-commit HEAD
gpg: Signature faite le dim. 27 juin 2021 16:50:21 -03
gpg:                avec la clef RSA 5BDAE9B8B2F6C6BCBB0D6CE581F90EC3CD356060
gpg: Bonne signature de « EuAndreh <eu@euandre.org> » [ultime]

On the first line we import the public key (funnily enough, available via HTTPS), and after cloning the code via the insecure git:// protocol, we use git verify-commit to check the signature.

The verification is successful, and we can see that the public key from the signature matches the fingerprint of the imported one. However git verify-commit doesn’t have an option to check which public key you want to verify the commit against. Which means that if a MITM attack happens, the attacker could very easily serve a malicious repository with signed commits, and you’d have to verify the public key by yourself. That would need to happen for subsequent fetches, too.

Even though this is possible, it is not very convenient, and certainly very brittle. Despite the fact that the Git Protocol is much faster, it being harder to make secure is a big downside.