Re: Which hash function to use, was Re: RFC: Another proposed hash function transition plan

[Mike Hommey <mh@glandium.org>]

On Thu, Jun 15, 2017 at 09:01:45AM -0400, Jeff King wrote:
> On Thu, Jun 15, 2017 at 08:05:18PM +0900, Mike Hommey wrote:
> 
> > On Thu, Jun 15, 2017 at 12:30:46PM +0200, Johannes Schindelin wrote:
> > > Footnote *1*: SHA-256, as all hash functions whose output is essentially
> > > the entire internal state, are susceptible to a so-called "length
> > > extension attack", where the hash of a secret+message can be used to
> > > generate the hash of secret+message+piggyback without knowing the secret.
> > > This is not the case for Git: only visible data are hashed. The type of
> > > attacks Git has to worry about is very different from the length extension
> > > attacks, and it is highly unlikely that that weakness of SHA-256 leads to,
> > > say, a collision attack.
> > 
> > What do the experts think or SHA512/256, which completely removes the
> > concerns over length extension attack? (which I'd argue is better than
> > sweeping them under the carpet)
> 
> I don't think it's sweeping them under the carpet. Git does not use the
> hash as a MAC, so length extension attacks aren't a thing (and even if
> we later wanted to use the same algorithm as a MAC, the HMAC
> construction is a well-studied technique for dealing with it).

AIUI, length extension does make brute force collision attacks (which,
really Shattered was) cheaper by allowing one to create the collision
with a small message and extend it later.

This might not be a credible thread against git, but if we go by that
standard, post-shattered Sha-1 is still fine for git. As a matter of
fact, MD5 would also be fine: there is still, to this day, no preimage
attack against them.

Mike