Subject: Re: About git and the use of SHA-1

From: Paolo Bonzini <[email-masked]>
Date: Tue, 29 Apr 2008 15:05:40 +0200

> I can think of one way to make git a lot more resilient to hash
> collisions, regardless of which hash is used, namely: Add the length
> of the hashed object to the hash.

Not really, because most attacks are about collisions, not second 
preimages.  They produce two 64-byte blocks (hence, same length) with 
the same hash value.

As such, they allow to change a blob that *the attacker* injected in the 
repository.  The way the more "spectacular" attacks are devised requires 
a "language" with conditional expressions -- for documents, for example, 
Postscript is used.  If you prepare a postscript file whose code is

    if (AAAA == BBBB)
      typeset document 1
    else
      typeset document 2

where AAAA and BBBB are collisions, and you change it to "if (BBBB == 
BBBB) the hash will be the same, but the outcome will be document 1 
instead of document 2.

The fact that this requires having the two "behaviors" in the blob is 
not a big deal for source code, going in the wrong branch of an "if" can 
be an attack.  On the other hand, it makes adding the length useless for 
collision attacks.  True, it wouldn't be useless for second preimage 
attacks, but SHA-1 is still secure with respect to those.

Paolo