Re: Migrating away from SHA-1?

> On Tue, Apr 12, 2016 at 07:15:34PM -0400, David Turner wrote:
> > 
> > If SHA-1 is broken (in certain ways), someone *can* replace an
> > arbitrary blob.  GPG does not help in this case, because the
> > signature
> > is over the commit object (which points to a tree, which eventually
> > points to the blob), and the commit hasn't changed.  So the GPG
> > signature will still verify.
> 
> The "in certain ways" is the critical bit.  The question is whether
> you are trying to replace an arbitrary blob, or a blob that was
> submitted under your control.
> 
> If you are trying to replace an arbitrary blob under the you need to
> carry a preimage attack.  That means that given a particular hash,
> you
> need to find another blob that has the same hash.  SHA-1 is currently
> resistant against preimage attack (that is, you need to use brute
> force, so the work factor is 2**159).  
> 
> If you are trying to replace an arbitrary blob which is under your
> control, then all you need is a collision attack, and this is where
> SHA-1 has been weakened.  It is now possible to find a collision with
> a work factor of 2**69, instead of the requisite 2**80.
> 
> It was a MD5 collision which was involved with the Flame attack.
> Someone (in probably the US or Isreali intelligence services)
> submitted a Certificate Signing Request (CSR) to the Microsoft
> Terminal Services Licensing server.  That CSR was under the control
> of
> the attacker, and it resulted in a certificate where parts of the
> certificate could be swapped out with the corresponding fields from
> another CSR (which was not submitted to the Certifiying Authority)
> which had the code signing bit set.
> 
> So in order to carry out this attack, not only did the (cough)
> "unknown" attackers had to have come up with a collision, but the two
> pieces of colliding blobs had to parsable a valid CSR's, one which
> had
> to pass inspection by the automated CA signing authority, and the
> other which had to contain the desired code signing bits set so the
> attacker could sabotage an Iranian nuclear centrifuge.
> 
> OK, so how does this map to git?  First of all, from a collision
> perspective, the two blobs have to map into valid C code, one of
> which
> has to be innocuous enough such that any humans who review the patch
> and/or git pull request don't notice anything wrong.