Hacker News Reader

How does GPG-signing help, when the signature is over a SHA-1 hash?

Thanks, I forgot that git got that wrong :/

Edit: Actually, looking at the code (do_sign_commit), git appears to gpg sign the whole commit object.

I think it's in signed tags where git only signs the sha1 being tagged.

Yeah, I just emerged from the Git code myself ;-)

So you're correct that GPG-signing commits (but not tags) prevents collisions in commit objects. The problem though is that a commit ultimately contains a SHA-1 hash of a tree object, so now the concern is someone generating colliding tree objects.

Edit: fortunately, the format of tree objects looks pretty rigid. I feel somewhat reassured, but only somewhat.

http://joeyh.name/blog/entry/size_of_the_git_sha1_collision_... has some discussion of colliding tree objects.

GPG does not require SHA1 so you can enfore SHA-2 for example.

GPG can use SHA-2, but the way that GPG signing works in Git is that you essentially GPG-sign the SHA-1 hash of a Git object. So even if GPG is using SHA-2 for its signature, it's no protection against a collision in one of the underlying Git object IDs.

Edit: this is only true of tag signing - with commit signing you GPG-sign the whole commit object.

Reply on news.ycombinator.com