phc-discussions - Re: [PHC] Different cost settings and optional input

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Date: Sat, 16 Feb 2013 12:43:36 +0100
From: Jens Christian Hillerup <jens@...lerup.net>
To: Jens Steube <jens.steube@...il.com>
Cc: discussions@...sword-hashing.net
Subject: Re: [PHC] Different cost settings and optional input

On Sat, Feb 16, 2013 at 12:22 PM, Jens Steube <jens.steube@...il.com> wrote:
> Of course, jchillerup is right. I think we should update the CFS to
> make that clear. In a world full of wrong or partially wrong
> information troublemaker can simply abuse this lack of clarity to
> create an Illusion of a weakness in the PHC hash. Such an Illusion can
> easily cost its credibility.
>
>> 09:45:02 gat3way | Is it allowed to rely on a secret parameter other than the password then?
>> 09:45:36 gat3way | otherwise I am afraid such requirement would very likely inherently weaken security
>
> I am not sure what his concern is about. I think it is the following:
> We allow the use of a "optional input":

(I'm jchillerup from the previous IRC discussion - hi)

I also had some doubts when I read that. Given two hashes, it'd be
trivial to determine if they share a preimage; just, in parallel,
increase the work for both of them by 1 until you see a hash you've
seen before in the chain (hit), or you've increased-by-one for too
long (miss).

However, this attack is not that important I'd argue, because if two
equal passwords *exist* it is likely that that password is in some
dictionary anyway, or that they originate from the same person. For
the former case the attacker could perform a dictionary attack on the
variant that requires lesser work. But hopefully the final PHC
candidate will require too much work for dictionary attacks, even at
its lightest setting. For the latter case the attacker would know that
the two passwords are equal, and not a lot more than that.

JC