Re: t2u in the archive

[Russ Allbery <rra@debian.org>]

Simon Josefsson <simon@josefsson.org> writes:

> You can mitigate this by re-validating all commit hashes using a SHA1CD
> git implementation before trusting a git repository.  I have not seen
> confirmation that 'git fsck' actually do that.

I convinced myself that it does.  One of the things git fsck does is
recalculate the hash of every object in the repository and ensure that it
matches (this is, to a large extent, the entire point; the other checks
are sort of an addition), and since Git uses SHA1CD now, git fsck will
instantly detect this attack as soon as it does that.

I admittedly did not go so far as to track down test objects with the same
SHA-1 hash and construct an experiment.  But I couldn't see any way where
git fsck could *not* detect this problem unless I'm wrong that it
recalculates all the hashes, and I'm fairly sure I'm not wrong about that.

> If some new attack implementation on SHA1 appears, that isn't detected
> by your SHA1CD variant, your validation can be by-passed.

This is true.

-- 
Russ Allbery (rra@debian.org)              <https://www.eyrie.org/~eagle/>