Re: Git and SHA-1 security (again)

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Jul 19, 2016, 07:21 UTC
Message-ID: <alpine.DEB.2.20.1607190920020.3472@virtualbox>
In-Reply-To: <CAPp-Vra8c7eFegysKrM-hZKNmD6B21XqVkE8HB0QcG_m8hu+Ow@mail.gmail.com>

Hi Zsolt,

On Mon, 18 Jul 2016, Herczeg Zsolt wrote:

Show 15 quoted lines

> >> My point is not to throw out old hashes and break signatures. My point
> >> is to convert the data storage, and use mapping to resolve problems
> >> with those old hashes and signatures.
> >
> > If you convert the data storage, then the SHA-1s listed in the commit
> > objects will have to be rewritten, and then the GPG signature will not
> > match anymore.
> >
> > Call e.g. `git cat-file commit 44cc742a8ca17b9c279be4cc195a93a6ef7a320e`
> > to see the anatomy of a gpg-signed commit object.
> >
> 
> Yes and no. That's the reason you need the two-way lookup table. If
> you need to verify a commit which was signed as SHA-1, you must use
> the lookup table in reverse.

That pretends that it is both easy and trustworthy to know when (and how) to recreate the SHA-1-ified version of the commit object.

Neither is the case, though.

Ciao, Johannes