SHA1 collisions proven possisble

[rbf+nanog at panix.com (Brett Frankenberger)]

On Sun, Feb 26, 2017 at 12:18:48PM -0500, Patrick W. Gilmore wrote:
> 
> I repeat something I've said a couple times in this thread: If I can
> somehow create two docs with the same hash, and somehow con someone
> into using one of them, chances are there are bigger problems than a
> SHA1 hash collision.
> 
> If you assume I could somehow get Verisign to use a cert I created to
> match another cert with the same hash, why in the hell would that
> matter?  I HAVE THE ONE VERISIGN IS USING.  Game over.
> 
> Valdis came up with a possible use of such documents. While I do not
> think there is zero utility in those instances, they are pretty small
> vectors compared to, say, having a root cert at a major CA.

I want a google.com cert.  I ask a CA to sign my fake google.com
certificate.  They decline, because I can't prove I control google.com.

I create a cert for mydomain.com,that hashes to the same value as my
fake google.com cret.  I ask a CA to sign my mydomain.com cert.  They
do, because I can prove I control mydomain.com.

Now I effectively have a signed google.com cert.

Of course, SHA1 is already deprecated for this purpose, and the
currently demonstrated attack isn't flexible enough to have much chance
at getting a colliding certificate signed.  So, practically speaking,
this isn't a problem *today* (even if SHA1 were deprecated).  So this
is more of a "here's the sort of thing collision attacks can be used
for" point, rather than "here's what you can do with this attack right
now" point.

     -- Brett