Re: Possible vulnerability to SHA-1 collisions

From: Aaron Schrab <aaron@schrab.com>
Date: Nov 27, 2012, 23:30 UTC
Message-ID: <20121127233016.GC3937@pug.qqx.org>
In-Reply-To: <20121127230753.GA22730@sigill.intra.peff.net>

At 18:07 -0500 27 Nov 2012, Jeff King <peff@peff.net> wrote:

Show 8 quoted lines

>PS I also think the OP's "sockpuppet creates innocuous bugfix" above is
>   easier said than done. We do not have SHA-1 collisions yet, but if
>   the md5 attacks are any indication, the innocuous file will not be
>   completely clean; it will need to have some embedded binary goo that
>   is mutated randomly during the collision process (which is why the
>   md5 attacks were demonstrated with postscript files which _rendered_
>   to look good, but contained a chunk of random bytes in a spot ignored
>   by the postscript interpreter).

I don't think that really saves us though. Many formats have parts of the file which will be ignored, such as comments in source code. With the suggested type of attack, there isn't a requirement about which version of the file is modified. So the attacker should be able to generate a version of a file with an innocuous change, get the SHA-1 for that, then add garbage comments to their malicious version of the file to try to get the same SHA-1.