phc-discussions - Re: [PHC] A review per day

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Date: Mon, 1 Sep 2014 20:54:44 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] A review per day - PufferFish

On Mon, Sep 01, 2014 at 11:58:15AM -0400, Bill Cox wrote:
> I read through the code.  I did not properly analyze it for security
> vs the original bcrypt.  I just read the code.  Others more familiar
> with bcrypt will have to do a proper cryptanalysis.

I think we can proceed with that if Pufferfish becomes a finalist.
I don't expect any issues there that couldn't be easily tweaked.

However:

Bill, you were going to do pre-final-hashing randomness tests on all PHC
candidates.  What's happened to that plan?

> In short, PufferFish is very similar to bcrypt, except that the 4
> S-boxes can be bigger, controlled by m_cost, and the iteration count
> is controlled by t_cost.  It uses 64-bit arithmetic, and just in case
> it's this modified version of BlowFish is not very secure, it hashes
> the inputs first with SHA512-HMAC.  The author got the details of the
> password and salt hashing right, working around the input collisions
> of HMAC.  Very nice!

I guess the pre-hashing is needed to avoid a bcrypt-like password length
limit.  For bcrypt it's 72; while 144 would be much better and would
satisfy PHC requirements as-is (128+), it would still be a drawback.

> Overall, for an algorithm that simply wants to extend the life of
> bcrypt by using more than 4KiB of L1 cache and using 64-bit data
> types, PufferFish looks great.  It's nice to have an easy review for
> Labor Day :-)

Great.

A drawback of going 64-bit, yet keeping the 4x lookups parallelism, is
that Pufferfish may be 2x weaker than bcrypt on old or low-end 32-bit
systems (e.g. some ARM), which use little instruction-level parallelism.
On newer and high-end 32-bit systems, I think the extra parallelism is
fine, as seen via bcrypt attack vs. defense speedup on CPUs, which is up
to 2x on many x86-64 CPUs.  In fact, Pufferfish might still lack enough
parallelism to fully use modern x86-64 CPUs in 64-bit mode, with 1
instance of it per core, so CPU-based attacks may achieve some speedup
by interleaving (like we do for attacks on bcrypt).  This may be a fine
tradeoff, though, since like I said adding more parallelism "hurts"
defensive use on smaller systems (or more precisely, it provides speedup
to attacks on some kinds of hardware without a matching speedup for
defensive use on those smaller systems).  Making instruction-level
parallelism configurable would add complexity.

Many other PHC entries, including yescrypt at default settings, share
this sort of drawback too.  So Pufferfish isn't actually worse than them
when run on too-old or too-small CPUs.  It's just that many PHC entries
are (in a way) worse than bcrypt on too-old and too-small CPUs.  I'm
considering ways of dealing with that in yescrypt (non-default settings
for such uses; yescrypt is very generic and complicated anyway).

Alexander