SHA1 collisions proven possisble

Mon Feb 27 06:25:20 UTC 2017

On 26 February 2017 at 22:15, Patrick W. Gilmore <patrick at ianai.net> wrote:
> 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?

1. Create a certificate C[ert] for a single domain you control with hash h(c).
2. Create a second certificate A[ttack] marked as a certificate
authority such that h(C) = h(A).
3. Have a certificate authority sign cert C
4. Present the signature for A along with A for whatever nefarious
purpose you want.

See a similar version of this attack here using MD5 chosen-prefix
collision attack: https://www.win.tue.nl/hashclash/rogue-ca/

-- 
Eitan Adler