phc-discussions - Re: [PHC] multiply-hardening (Re: NoelKDF ready for submission)

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Message-ID: <20140213154727.GA6161@openwall.com>
Date: Thu, 13 Feb 2014 19:47:28 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] multiply-hardening (Re: NoelKDF ready for submission)

On Thu, Feb 13, 2014 at 09:44:13AM -0500, Bill Cox wrote:
> On Wed, Feb 12, 2014 at 11:16 AM, Solar Designer <solar@...nwall.com> wrote:
> > BTW, you might not need the "| 3" when you do 32x32->64.  Your rationale
> > for the "| 3" was to preserve entropy, but 32x32->64 is lossless (in
> > fact, it's reversible, which might allow for attacks - you might need
> > something else instead of the "| 3", perhaps after the multiply, to make
> > it one-way).
> >
> > Alexander
> 
> In the version of this I tested, I make "value" (horrible name...)
> 64-bit, so the cast to 32-bit is not reversible, so there's no problem
> there.  However, without the "| 3", I see that most of the LSBs after
> 1B iterations of the hash loop are 0.

Oh, I think you need to be passing "value" (or whatever other small
state you have) through a crypto hash once in a while.  Letting a
billion iterations of non-crypto hashing accumulate is unjustified risk.
There's little reduction in speed if you invoke a crypto hash before
and/or after every block processed (IIRC, you call it a "page").

In fact, once we add low-level parallelism we also need something very
much like a crypto hash to be applied at least once or twice per block
anyway, for data mixing between the parallel lanes - and we can just go
ahead and use a crypto hash there (as long as the block is large enough
for this to be affordable).  (I am experimenting with this for a
revision of escrypt.)

Alexander