SHA1 collisions proven possisble

Mon Feb 27 06:15:28 UTC 2017

Composed on a virtual keyboard, please forgive typos. 

On Feb 26, 2017, at 21:16, Matt Palmer <mpalmer at hezmatt.org> wrote:
>> On Sun, Feb 26, 2017 at 05:41:47PM -0600, Brett Frankenberger wrote:
>>> 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.
> 
> Even better: I want a CA cert.  I convince a CA to issue me a regular,
> end-entity cert for `example.com` (which I control) in such a way that I can
> generate another cert with the same SHA1 hash, but which has `CA:TRUE` for
> the Basic Constraints extension.
> 
> Wham!  I can now generate certs for *EVERYONE*.  At least until someone
> notices and takes away my shiny new toy...

Since I have said this somewhere on the order of half a dozen times, I will assume I am missing something obvious and all of you are doing it right. 

So let me ask you: The attack creates two docs. You do not know the hash before the attack starts. You cannot take an existing file with a known hash and create a second file which matches the known hash. You start with nothing, run the "attack", and get two NEW docs that have the same hash. A hash which is brand new. 

Now, please explain how you take a cert with one hash and somehow use this attack, which creates two new docs with a new hash, to do, well, anything?

In the example above, the CA knows the SHA-1 hash of the cert it issued. (We are assuming there is a CA which still does SHA-1.) How do you get that CA to believe the two OTHER certs with DIFFERENT hashes you have to create so you can have two docs with the same hash?

-- 
TTFN,
patrick