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

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Message-ID: <CAG+Gt9bugzYFdWwj2YXpi4xeNYq-4=z8EBhxEJDJ0n3tnhWRAQ@mail.gmail.com>
Date: Mon, 22 Sep 2014 07:50:26 +1000
From: Rade Vuckovac <rade.vuckovac@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Schvrch is broken

"I think you have an operator precedence error on the line that adds
memstate to mixer. In C, that will be interpreted as (carry % m_cost)
* statelen. I think you meant carry % (m_cost * statelen)."

Yes carry % (m_cost * statelen) is correct.

Rade

On Fri, Sep 19, 2014 at 10:24 PM, Bill Cox <waywardgeek@...hershed.org>
wrote:

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> On 09/17/2014 11:41 PM, Rade Vuckovac wrote:
> > Hi Bill
> >
> >
> >
> > Please find schvrch2prelim.h attached.
> >
> >
> >
> > Memory cost attempt:
> >
> > - write to the memstate is done by state stir stream
> >
> > - read is done by getting index of current carry modulo memstate
> > array size and reading memstate element with that index
> >
> > - read result modifies mixer
> >
> > - until next read, stir is run to produce a new current carry (it
> > runs statelen times but that can be fine tuned)
> >
> > - hash is just a partial final state (every 8th word provides one
> > byte =
>
> This is better.  This code stops the strong TMTO attack, as well as
> the attacks on evolve and revolve, and it's even simpler.  Very nice!
>
> The t_cost loop is still not right, but applying t_cost securely isn't
> how Schvrch will likely be judged, so let's not worry about that for now.
>
> I think you have an operator precedence error on the line that adds
> memstate to mixer.  In C, that will be interpreted as (carry % m_cost)
> * statelen.  I think you meant carry % (m_cost * statelen).
>
> While I think this is secure hashing, experts (meaning not me) in
> hashing will have to help here.  I would have thought Keccak is secure
> without the LFSR, and according to Wikipedia, it "breaks symmetry".  I
> don't know what that means, but without the LFSR, Keccak is using one
> of Wolfram's rules for it's nonlinear mixing.  I think the addition
> operation with the mixer should be enough, though, for secure hashing,
> in my poorly-informed opinion.
>
> Bill
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