phc-discussions - Re: [PHC] PolyPassHash is broken

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Date: Sun, 14 Sep 2014 17:11:13 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] PolyPassHash is broken

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On 09/14/2014 03:45 PM, Justin Cappos wrote:
> We have some discussion about the points list in our paper
> (Sections 5.2.3 and 6 for example). However, I'll be more explicit
> and direct here.
> 
> Method 1
>> * get partial hash from DB * find partial hash collision *
>> crash/restart server * login
>> 
> 
> Sure you can do this.  Once a threshold is reached, the attack will
> be detected because the full password hashes will not match.  That
> is, unless the attacker is fortunate enough so that the partial
> hash collision is the actual full hash / password.

I have to agree with Steve that the Partial Validation feature needs
work.  I was convinced the scheme proposed in the paper was secure
until I read that section, which puts security in doubt.

Revealing even two bytes of the password hash is dangerous.  For
example, if there are 4 administrators, and two of them have passwords
with only 32 bits of entropy, meaning that they exist in my 4-billion
entry dictionary, and if only 2 shares are needed, then without the
Partial Verification feature, I still have to try up to 2^64 guesses
to find the master key, once I've chosen the right two admin accounts.
However, with 16 bits of the hash from two password hashes, in 4*2^32
hashes I can easily build dictionaries for all of them containing only
matching hashes, which would be about 2^16 each.  I then only need to
try 2^32 combinations for each of the 4 possible pairs to find the
key.  Essentially, what this does is make it just about as easy to
attack the database as if PolyPassHash were not in use.

In general, I think most admins always want to be able to log in.
This is a significant problem for wide-spread adoption of
PolyPassHash, without the Partial Verification feature.  However, this
seems to need work.

Bill
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