Clincher: checking your signed git commits

Oct. 26, 2018

git 

Originally posted at https://tech.labs.oliverwyman.com/blog/2018/10/26/clincher-checking-your-signed-git-commits/

Recently for a project with tight regulatory requirements we decided that git signing throughout the project was a good idea. There’s a debate about it’s level of effectiveness, given that all it tells you is that a particular commit was made from a particular developers machine, and if they’re not careful, they can end up committing code they didn’t actually write (especially in larger commits). Most of the time this other code is cruft from build systems, but it could potentially also include malicious items. On the other hand, this does mean you can reduce the ability of external actors to add commits if they compromise the revision control system, and at least conclude that all the affected code came from a specific developer X if you have a single malicious actor.

These are useful features from a defense-in-depth approach to this problem, especially given that the standard git author/email stuff is trivially spoofable, but we all tend to trust it by default. There’s some existing work out there regarding signature verification e.g. just “git –verify-commit“, and Github now supports signature verification inline, but really we need to automatically check your entire git tree and fail CI if someone’s forgotten to sign things.

We’re releasing Clincher which is a tool we built to do this. Mostly it’s a wrapper around “git –verify-commit”, but it also does a number of other tasks:

At the moment it assumes that the git repo is itself a good source of keys (which leads to the interesting pattern of a new developers first commit often being them committing their own key), and expanding this to a web of trust model, either with external keyservers or by enforcing some sort of history mechanism where early keys are implicitly trusted somehow and then later keys must be committed by existingly trusted committers to be trusted is a possible area of future expansion.

For the moment though, it’s a pretty good mechanism for checking signing, and certainly an improvement over repeatedly eyeballing “git log –show-signature”

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