Subject: Re: Possible vulnerability to SHA-1 collisions

From: Andreas Ericsson <[email-masked]>
Date: Wed, 28 Nov 2012 10:35:05 +0100

On 11/28/2012 01:27 AM, Jeff King wrote:
> On Tue, Nov 27, 2012 at 06:30:17PM -0500, Aaron Schrab wrote:
> 
>> At 18:07 -0500 27 Nov 2012, Jeff King <[email-masked]> wrote:
>>> 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.
> 
> Agreed, it does not save us unconditionally. It just makes it harder to
> execute the attack. Would you take a patch from a stranger that had a
> kilobyte of binary garbage in a comment?
> 
> A more likely avenue would be a true binary file where nobody is
> expected to read the diff.
> 
>> 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.
> 
> That's not how birthday collision attacks usually work, though. You do
> not get to just mutate the malicious side and leave the innocuous side
> untouched. You are mutating both sides over and over and hoping to find
> a matching sha1 from the "good" and "evil" sides.
> 
> Of course, I have not been keeping up too closely with the efforts to
> break sha-1. Maybe there is something more nefarious about the current
> attacks. I am just going off my recollection of the md5 collision
> attacks.
> 

AFAIR, collision attacks can be executed with a 2^51 probability (with
a 2^80 claim, that's pretty bad), but preimage attacks are still stuck
very close to the claimed 2^160.

That means every attack involving SHA1 means Mr. Malicious creates
both the involved files or does exceptional research without sharing
it.

I think git's job is to make sure that write access to only one of
the repositories is insufficient to launch an attack. If the attacker
manages to change all repositories involved then the hash function
used is really quite irrelevant.

-- 
Andreas Ericsson                   [email-masked]
OP5 AB                             www.op5.se
Tel: +46 8-230225                  Fax: +46 8-230231

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