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Date: Mon, 24 Mar 2014 20:13:30 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] pufferfish

On Mon, Mar 24, 2014 at 10:11:20AM +0100, beloumi wrote:
> But I agree to Bill, that increasing the memory cost of Pufferfish would
> achieve more confidence, even if it is much less than other memory-hard
> schemes. The authors of bcrypt thought (1999) of 14 KB as a ?moderate?

(You meant 4 KB.)

> memory amount. Maybe in this time ?moderate? means something like 1 MB.
> I like Bcrypt and I would be glad if we could extend this algorithm for
> the current requirements. Good luck, Jeremi!

Note that L1 cache sizes haven't changed much in the last 25 years -
that's more than bcrypt's age!

In the x86 world, Intel 486 released in 1989 had 8 KB of on-chip L1
cache (unified).  Before it, 386 systems typically had 64 KB or 128 KB
(off-chip).  Shortly after it, roughly when bcrypt was introduced, L1
cache sizes of then-relevant x86 systems varied between 8 KB unified
(Intel 486), 16 KB unified (some AMD 486, AMD/Cyrix "586 class CPUs"),
8 KB data (original Pentium), or 16 KB data (Pentium MMX).

On current x86 CPUs, we still only have 16 KB of L1 data cache per
thread.  On Bulldozer, it's separate 16 KB caches.  On Intel CPUs, it's
a 32 KB cache shared between two hardware threads.

Beyond x86, it's similar, with the major exception of PA-RISC - but it's
been this sort of exception for 20+ years, thus including the time when
bcrypt was introduced.  PA-RISC systems tend to have larger, off-chip L1
caches.  They may also have on-chip buffers, but the first level of
off-chip cache is an L1 cache, including in terms of its friendliness to
bcrypt's access pattern.  e.g. bcrypt ran faster per-MHz on my 712/80
with its off-chip 128 KB L1 data cache (only 4 KB of which is used for
the Blowfish S-boxes, obviously) than it did on a Pentium:
http://www.openpa.net/systems/hp-9000_712.html
This says "1KB on-chip L1 and 256KB off-chip L1 cache" and "The 1KB
on-chip L1 cache is not really a true cache", which is about right.
IIRC, the off-chip 256 KB is split into 128 KB instruction + 128 KB data.
Other PA-RISC machines generally have even larger L1 caches, into the
megabytes.  This page: http://www.openpa.net/pa-risc_processors.html
lists some impressive "on-chip" L1 cache sizes, up to 1.5 MB.  But
that's just PA-RISC - it was already an exception in the 1990s.

Can we really go much higher than 4 KB while retaining bcrypt's access
pattern (frequent, tiny accesses)?  Should we?  Why didn't we in 1990s -
did no one think of it, were we trying to save RAM (yet UFC-crypt in
glibc wasted 128 KB on expanded and doubled DES S-boxes, IIRC even
before bcrypt was introduced), was it simply because Blowfish was
designed that way and bcrypt built upon it, or did we not want to incur
the slowdown and waste bandwidth(*) by exceeding L1 cache size while
doing those tiny accesses?  I think it's some mix of these factors, and
the last one is still relevant and very important now.

(*) Yes, when we exceed L1 cache size in a bcrypt-like algorithm,
without increasing the size of individual accesses a lot, we incur not
only a slowdown, but we also _waste_ bandwidth (from higher level caches
or from memory), because entire cache lines (64 bytes on current x86, up
to 128 bytes on IBM POWER) are fetched on most accesses (except for L1
cache hits).  Do we really want to literally waste defender's resources
(waste, not use them for greater defense - this differs from stretching,
etc.), hoping that attackers will incur the same wastage?  Even if they
do, through them using similar systems with caches, do we want to
compete with those attackers in terms of L2+ cache and RAM bandwidth?
Don't we know that GPUs have higher memory bandwidth than server systems
do?  While we're in L2 or L3 cache, we might still be competitive with
GPUs' memory bandwidth, but when we exceed L3 cache, we'd give GPUs an
advantage - we'd be worse than the original bcrypt with its 4 KiB in
terms of GPU resistance, and that's while wasting memory and bandwidth.

Oh, and PA-RISC systems will be rather attractive to attackers. ;-)
Luckily, they might be behind the curve in terms of clock rates, and are
pricey, but HP might appreciate the free advertising from pufferfish
cracker benchmarks anyway (they might be cited in other contexts). ;-)

I am currently considering the range of 8 KB to 16 KB for bcrypt-like
accesses from escrypt's BlockMix.  In fact, I'll probably choose 8 KB or
12 KB, since there's a ~20% performance hit at 16 KB given escrypt's
other memory accesses.

Alexander

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