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Date: Mon, 14 Apr 2014 09:06:50 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: The best of the best, IMO

On Sun, Apr 13, 2014 at 9:11 PM, Steve Thomas <steve@...tu.com> wrote:

> > On April 12, 2014 at 8:33 PM Bill Cox <waywardgeek@...il.com> wrote:
> >
> > By Script inspired, I mean algorithms that bust out of cache and do a
> *lot*
> > of memory hashing, like Script.  To do that, an algorithm requires a
> > strategy for mitigating cache-miss penalties.  Some entries look to me
> like
> > simplified versions of Script, but those that dropped Script's ability to
> > efficiently bust out of cache I put in the Bcrypt-like category.  I think
> > it makes more sense to compare them against algorithms like Pufferfish
> and
> > Battcrypt than against large memory capable algorithms like Lyra,
> Yescript,
> > EARWORM, and TwoCats.
>
> Battcrypt should scale linear with memory usage. Since it encrypts blocks
> of 2 KiB at a time and the 4 KiB of s-boxes should stay in cache. The only
> problem is that Blowfish is slow compared to other algorithms. I think it's
> max speed is going to be around 185 MiB/s (3.5 GHz).
>
> Centrifuge would also be able to bust out of cache if they made "out"
> larger then once it's done shrink it down. Since this probably will have
> similar speeds to RC4 it will be 2.6 times faster than battcrypt because
> Blowfish takes 18 clocks/byte* vs 7 clocks/byte**. This is outdated since
> both of those were measured on a Pentium. Although Centrifuge doesn't swap
> for every byte of output, every block of "out" it swaps based on "t_cost"
> then encrypts "out" with AES. I'm guessing with AES-NI it might be a little
> slower than RC4.


I am surprised at how few entries have a strategy for running in larger
memories than what fits in cache, Centrifuge included.  Like several
others, simply making the hash length parameter larger, like >= 1KiB, would
allow it to run efficiently in external DRAM.  It's the same situation for
Catena and others.  There seems to be a strong correlation between authors
who feel strongly that the full un-shortened encryption algorithm be used
on all memory written, and authors who only seem to intend to run in cache.
 This is very confusing to me, but since so many brilliant authors are
following this pattern, there must be something to it.

I got some time this morning to go back and carefully read Centrifuge.
 Centrifuge will be speed limited by it's reliance on AES-cfb, rather than
it's swapping loop.  It uses the full 10-round CFB version, which on the
latest CPUs will take at least 60 cycles per 16 bytes if I'm not mistaken
(6 cycle latency per AESENC, 10 sequentially).  This agrees well with this
paper:

https://www.cosic.esat.kuleuven.be/ecrypt/AESday/slides/Use_of_the_AES_Instruction_Set.pdf

It's the CFB mode that causes the long compute time.  Also, contrary to a
statement in the paper, Centrifuge's swapping loop is somewhat
parallelizable.  We get 16 bytes in parallel coming from AESENC, and by
guessing that the swaps can be done independently, and backing off and
doing it sequentially when we're wrong, we can often perform all 16 swaps
at once.  This small state size (2048 bits) is easily built in registers
rather than RAM, so modifying them all at once is reasonable.  Optimized
logic could do dependency detection and some number of non-dependent swaps
simultaneously, leading to at least a few swaps per clock on average, much
faster than the AES-cfb loop.  I would advise switching to non-CFB mode,
but doing so would allow attackers to speed up the algorithm more than the
defender can.

As p_time increases, Centrifuge memory bandwidth to the M memory array goes
to 0, and with a default recommended value of 256, attackers will have no
trouble keeping up.  For ASIC attacks, with a tiny 256-byte S box and
essentially zero large memory bandwidth, Centrifuge seems highly
parallelizable to me.  Maybe the frequent small memory swaps in the S-box
will be a problem for GPUs, but ASICs will tear through Centrifuge hashes,
doing many in parallel per die, sharing a single cheap external DRAM for M
storage.

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.

Some more thoughts on Centrifuge: The author may not be aware that doing
256 random swaps on S does not perform a random shuffle.  It has a slight
bias in the result.  It's no biggie, however.  He only uses the S state as
an input to AES_CFB, and he could feed in all 0's and still get secure data
out.  All that matters is that an attacker has to do the work of updating
S, not that S hold cryptographically random data.  Centrifuge does memory
dependent addressing from the beginning, making it more Bcrypt-like than
Catena-like.  I prefer the Bcrypt-like algorithms, so this is just an
observation rather than criticism.  Another observation is that the entire
Seq array could be compressed to 256 bytes and get the same result, so in
case the updating of S somehow became compute intensive, we'd still need to
deal with keeping attackers from just compressing Seq.  In the first loop
for filling M, the output bytes fed into AES are never equal, so the input
to AES has a strong non-random-looking bias.  Again, this is not a big deal
because the AES output appears perfectly random even when encrypting all
0's.

Centrifuge, like several others, did not encode input lengths, and has
input collisions as a result.  In this case, I think appending 0 bytes to
either password or salt, up to a total of 16 bytes, has no effect on the
result.  I feel like we almost need a separate input hashing step where we
properly hash all inputs to derive a pseudo-random key, and that this key
is what we should pass to the various PHC entries, since so many of them
did not pay attention to this detail.  If I write a common API for the PHC
entries that is usable in real systems, I might do just this.  That would
also give me a place to clear the password before passing it to the PHS
call, and I could provide an init/update/final interface as well for people
with extra input data.

On the positive, Centrifuge appears secure to me, though it's time*memory
cost is low.  It's rapid small random reads might do well against GPU
attacks.  I'm not sure if it's 256 byte state is enough to thwart GPUs,
though.  Data it writes to memory is also cryptographically pseudo-random.
 It is a nice simple KISS algorithm.  Having the ability to increase beyond
256 bytes would harden this algorithm against GPUs and also provide a way
to bust into external memory efficiently.

Bill

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