Re: Suggestion on hashing

From: Jeff King <peff@peff.net>
Date: Dec 2, 2011, 17:54 UTC
Message-ID: <20111202175444.GB24093@sigill.intra.peff.net>
In-Reply-To: <1322813319.4340.109.camel@yos>

On Fri, Dec 02, 2011 at 12:08:39AM -0800, Bill Zaumen wrote:

Show 9 quoted lines

> At one point Nguyen said that "What I'm thinking is whether it's
> possible to decouple two sha-1 roles in git, as object identifier
> and digest, separately. Each sha-1 identifies an object and an extra
> set of digests on the "same" object."
> 
> My code pretty much does that (it just uses a CRC instead of a real
> digest, but I can easily change that).   So the question is whether
> using SHA-1 as an ID and SHA-256(?) as a digest is a better long term
> solution than simply replacing SHA-1.

I think your code is solving the wrong problem (or solving the right problem in a half-way manner). The only things that make sense to me are:

  1. Do nothing. SHA-1 is probably not broken yet, even by the NSA, and
     even if it is, an attack is extremely expensive to mount. This may
     change in the future, of course, but it will probably stay
     expensive for a while.

  2. Decouple the object identifier and digest roles, but insert the
     digest into newly created objects, so it can be part of the
     signature chain. I described such a scheme in one of my replies to
     you. It has some complexities, but has the bonus that we can build
     directly on older history, preserving its sha1s.

  3. Replace SHA-1 with a more secure algorithm.

I'm probably in favor of (1) at this point. Whether to do (2) or (3) will depend on where we are when SHA-1 gets feasibly broken. It may be many years away, at which point we may be considering a git 2.0 that breaks repository compatibility, anyway. That would be a natural time to consider changing the algorithm.

> Replacing SHA-1 with something like SHA-256 sounds easier to implement,
> but the problem is all the existing repositories.

Right. I don't think anyone is denying that it would be a giant pain.

-Peff