Re: SHA-1 chosen-prefix colission attack

From: Jeff King <peff@peff.net>
Date: Jan 8, 2020, 07:30 UTC
Message-ID: <20200108073042.GD1675456@coredump.intra.peff.net>
In-Reply-To: <20200107203147.r33c5plp5g7pmxmj@LykOS.localdomain>

On Tue, Jan 07, 2020 at 03:31:48PM -0500, Santiago Torres Arias wrote:

Show 10 quoted lines

> > > As a side result, this shows that it now costs less than 100k USD to
> > > break cryptography with a security level of 64 bits (i.e. to compute
> > > 264 operations of symmetric cryptography).
> 
> Just to clarify:
> 
>     As a stopgap measure, the collision-detection library of Stevens and Shumow [SS17]
>     can be used to detect attack attempts (it successfully detects our attack).
> 
> At the end of section 7.0,

And if anyone is curious, you can test your build of Git against their sample files by running:

  $ t/helper/test-tool sha1 <messageA
  fatal: SHA-1 appears to be part of a collision attack: 8ac60ba76f1999a1ab70223f225aefdc78d4ddc0

Unfortunately you can't test with actual Git objects, because their chosen-prefixes don't have object headers. They do estimate that a classical collision is down to ~11k USD to compute, so maybe we'll see one eventually. :)

-Peff