Re: Migrating away from SHA-1?

From: Jeff King <peff@peff.net>
Date: Apr 15, 2016, 02:22 UTC
Message-ID: <20160415022233.GE22112@sigill.intra.peff.net>
In-Reply-To: <xmqq37qnehrm.fsf@gitster.mtv.corp.google.com>

On Thu, Apr 14, 2016 at 07:18:53PM -0700, Junio C Hamano wrote:

Show 9 quoted lines

> Jeff King <peff@peff.net> writes:
> 
> > [2] Somewhere in the list archive is my patch to find partial
> >     collisions like "git commit --sha1=31337", and I did in fact use
> >     that micro-optimization. That, along with multi-threading, made it
> >     feasible to do 6-8 character prefixes, as I recall.
> 
> In our testsuite, we have a test that uses many objects, all of
> which have object names that begin with 10 '0' characters.

Can you give more details on which test? 10 zeroes is 40 bits, which means that by random chance, only about one in a trillion objects would match that. We certainly didn't hit that randomly, and it seems like it would be computationally expensive to have come up with the input for even one such object, let alone "many".

-Peff