phc-discussions - Re: [PHC] Should we care about "parameter influence" attacks against PBKDF2?

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Message-ID: <20140226094731.GA6365@openwall.com>
Date: Wed, 26 Feb 2014 13:47:31 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Should we care about "parameter influence" attacks against PBKDF2?

On Wed, Feb 26, 2014 at 01:14:13PM +0400, Solar Designer wrote:
> As you probably realize, hashing of all the input parameters is not the
> only way to prevent this attack.  A final step may (or may not,
> depending on how it's chosen) also prevent the attack.  (Arguably, this
> translates into hashing of the iteration count anyway, via the iteration
> count indirectly affecting the input to this final crypto hash step.)

Here's an example to consider: given two scrypt hashes for the same
password, salt, N, and r, but for different p, can we test candidate
passwords faster than we would against the smaller-p one of these two
hashes?  I think the answer is "no", but I think it is a "no" only
because of what happens inside the final PBKDF2.  Luckily, PBKDF2
appears to be OK as it relates to this sort of attacks for differing
salts, even if the longer salt differs only by addition of a suffix to
the shorter one.  If scrypt used something other than PBKDF2 for this
final step, the answer could as well have been "yes".

Alexander