phc-discussions - Re: [PHC] omegacrypt and timing

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Message-ID: <541ADA56.3030900@ciphershed.org>
Date: Thu, 18 Sep 2014 09:12:54 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] omegacrypt and timing

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On 09/18/2014 05:07 AM, epixoip wrote:
> Peregrine:
> 
> Please quote the text you are replying to, as I am now doing.
> 
> On 9/17/2014 6:58 PM, Peregrine wrote:
>> If we had a widely used password hash function that was extremely
>> secure against offline attacks but leaked enough timing 
>> information for attackers to find the password, then offline 
>> attacks would severely decline and timing attacks would rise.
> 
> descrypt, bcrypt, and scrypt all have theoretical timing attacks, 
> yet offline attacks for these algorithms have certainly not 
> declined.
> 
> I believe a timing attack that actually reveals the password is 
> pretty far-fetched. The best you can probably hope for is leaking 
> some information that enables early-reject for offline attacks,
> and that's provided you have the hash+salt in addition to the side 
> channel. And if an attacker only has the side channel and doesn't 
> have the salt, the side channel attack is largely irrelevant.

I tend to agree the attack is still mostly theoretical.  However, I
think it is worth adding a bit of complexity to defend against timing
attacks, so long as it does not significantly weaken the main defense.
 I added an additional loop to TwoCats, where the first is
timing-attack resistant, but the second loop is unpredictable. Scrypt
also has a timing-resistant first loop, but with it's low memory
requirement, a cache timing attack is pretty strong.

It does make the algorithm more complex.  It's not an easy call.  I
consider several entries that make no attempt at timing defense of any
kind as still strong candidates.  I would rather have one of those
defending my passwords than a pure timing-attack resistant algorithm.
 Unpredictability puts extra hurt on brute-force attackers.

Bill
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