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Date: Wed, 16 Apr 2014 18:59:04 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: The best of the best, IMO

On Wed, Apr 16, 2014 at 6:32 PM, Andy Lutomirski <luto@...capital.net>wrote:

> On Mon, Apr 14, 2014 at 6:06 AM, Bill Cox <waywardgeek@...il.com> wrote:
> >
> > I have a similar problem with my t_cost parameter in TwoCats.  I use it
> to
> > balance external memory bandwidth and internal cache bandwidth, ideally
> > maxing out both at the same time to provide two levels of defense.
>  However,
> > if used to run a long time hashing only a small amount of memory, it will
> > hash the same two blocks together many times before writing the result
> > block, lowering external memory bandwidth to nearly 0.  I could have made
> > t_cost repeat the entire memory hash operation like several entries do,
> but
> > then I could not use it to balance cache and external memory bandwidth at
> > the same time.  Also, some users may prefer to have TwoCats avoid maxing
> out
> > external memory bandwidth, and this gives them a knob to do that.  Rather
> > than confuse users with two separate time cost parameters, I chose to
> keep
> > only the one I find of higher value.  As a work-around, a user could just
> > call TwoCats repeatedly, providing his own outer loop.  The same can be
> done
> > with Centrifuge, with t_cost set to 0, substantially increasing it's
> memory
> > bandwidth.  However, in CFB mode, and writing only to on-chip cache,
> > Centrifuge cannot approach maxing out cache memory bandwidth, regardless
> of
> > settings.
> >
>
> I understand that this is needed to fit within the PHC framework, but
> this sounds like it'll cause screwups if you win and it stays like
> this (or if anyone else with the same issue wins).  For example, what
> if I want a really strong FDE hash, but I only have 4 GB of RAM?
>
> Would it make sense to adjust the real API (e.g.
> TwoCats_HashPasswordFull) to accept parameters for the total time to
> use (in arbitrary units), total memory to use (in bytes or some real
> unit) and something to control the cache/RAM bandwidth ratio?  It
> would be okay if not all combinations or arguments are valid (e.g. if
> you ask for very large memory and very short time).
>
> This way I could say "I'm willing to hash for 2 seconds and I can use
> 3GB of RAM, and tune for average systems in 2014" and get reasonable
> behavior?
>
> --Andy
>

Yes, that would be fine.  I wrote a simple wrapper to find these parameters
automatically:

// Find parameter settings on this machine for a given desired runtime and
maximum memory
// usage.  maxMem is in KiB.  Runtime with be typically +/- 50% and memory
will be <= maxMem.
void TwoCats_FindCostParameters(TwoCats_HashType hashType, uint32_t
milliSeconds, uint32_t maxMem,
    uint8_t *memCost, uint8_t *timeCost, uint8_t *multplies, uint8_t
*lanes);

This function first finds a memory cost setting to take up the allotted
time, up to maxMem, and if successful, it increases multiplies and timeCost
to the point that the start slowing  hashing down.  This gives close to the
max memory achievable in the given time, while compute time hardening with
multiplication chains, internal cache bandwidth, and external DRAM
bandwidth.

I've got a problem when maxMem is too low for a given runtime.  In that
case, It's probably best to iterate the whole algorithm a number of times
to fill up the desired runtime at maxMem.  I could add this as an external
loop around the algorithm.

Bill

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