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Date: Mon, 20 Oct 2014 23:47:15 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] simplifying yescrypt

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On 10/20/2014 09:58 PM, Solar Designer wrote:
> Hi,
> 
> As I recognize that complexity is yescrypt's worst drawback, I've
> been thinking of how to simplify it.  There's not a lot that can be
> removed without making it substantially worse in some way.

A low-complexity memory-hard algorithm could be useful, and maybe some
such algorithm should be promoted by the PHC.  However, no entry that
has decent potential to replace Scrypt as a better memory-hard
solution is simple.  Most of the memory-hard entries don't even deal
with the cache-miss penalty problem.  That makes them simpler, but
kills their chance of supplanting Scrypt.

Of the three I feel succeeded, how well they succeeded is in
proportion to their complexity.  Yescrypt is the most complex, but as
anyone can see from my comparison chart, it is the best Scrypt
successor of the strong contenders.  Yescrypt wins or comes in a close
second whether the use case is authentication servers, tiny L1 cache
hashes, or several GiB memory hashes for FDE.  It wins or comes in a
close second in GPU, ASIC, and FPGA defense.  On top of that, it is
Scrypt upward compatible.

The other two entries I feel succeeded are my own TwoCats and Lyra2.
Not coincidentally, the Lyra2 team and I realized you had some great
advice on the forum, and acted on it.  I was somewhat rabid about it,
and succeeded in making TwoCats the second most complex entry, and in
my biased opinion, TwoCats comes in second on those same measures.

Lyra2 is simpler, but when it has multi-threading, some TMTO defense
for it, better compute-time hardening, and maybe improved GPU defense,
it will increase from it's moderate complexity to high complexity.
You simply can't compete in all those security dimensions at the same
time otherwise.

Let me put it this way.  You're not going to see a Lyra2-coin based on
the current code :-)  It's GPU defense isn't there.  That said, I have
high confidence the Lyra2 team can get there if needed.

Honestly, the Lyra2 team and I have been eating up your ideas you've
posted on this forum, and upgrading our entries.  However, IMO, we did
not succeed in beating your entry.  Our entries are less complex,
primarily because we haven't addressed as many different security needs.

> I think the same applies to scrypt, so I don't find scrypt exactly 
> over-engineered.  It's just complex - more so than we would have
> liked it to be.  The only likely-unjustified thing in scrypt is
> BlockMix's block shuffling.  Everything else is justified in some
> way - e.g., the use of PBKDF2-HMAC-SHA256 on the outer layer lets
> us say that scrypt's basic crypto properties rely solely on
> NIST-approved crypto, which may be crucial for adoption by some
> users.  (Of Open Source projects, at least glibc and Drupal have
> stipulated such requirements, although scrypt in particular would
> not be suitable for them for other reasons.)
> 
> As long as yescrypt supports computing not only its native, but
> also classic scrypt hashes, it obviously can't be made simpler than
> scrypt. However, I am planning on introducing yescrypt-lite
> supporting a subset of yescrypt's full functionality, and
> yescrypt-lite would not be a superset of the full scrypt either.
> Here's what I'd change in -lite:
> 
> - Drop support for (ye)scrypt's p parameter. (Only p=1 will be
> supported in -lite, and hence -lite will not include the different
> handling of p that now exists between classic scrypt and native
> yescrypt hashes.)

Sounds good.

> - Maybe: drop support for classic scrypt hashes. (Have only native
> yescrypt hashes supported in -lite.)

Yes, in Yescrypt-lite, please drop backwards compatibility with Scrypt.

> - Maybe: drop support for a ROM.

Yes, in Yescrypt-lite, please drop ROM support.  I feel this is
important for authentication servers, but not most other applications.
 Let authentication servers run the full Yescrypt, while having a
simpler version that regular coders like me can port to various languages.

> I'd appreciate arguments for/against the two "maybes" above,
> considering that I intend -lite to be usable in Unix crypt(3) and
> PHP crypt(), etc.
> 
> Overall, I don't expect this to make yescrypt-lite simpler than
> scrypt (it will have pwxform, which isn't in scrypt), but it'd
> certainly be simpler than full yescrypt.

I am tempted to suggest also aproviding an Yescrypt-anorexic.  In my
"SkinnyCat" algorithm, I deleted multiplication chain hardening,
multi-threading for taking advantage of all those CPU cores,
small-memory reads for GPU defense, the t_cost parameter (of which I
remain not much of a fan), variable lanes, long input parameters, the
init-update-finalize API, variable block sizes for cache tuning, and
the early memory overwriting TwoCats does to provide basic protection
in case memory gets written to swap or hibernated to disk.

The result, in my biased opinion, is simple, elegant, and fast.
However, there's no way I'd use it in an application like TrueCrypt!

However, there are *many* applications where I feel something that
simple is warranted.

> In PHC context, I am willing to offer replacing full yescrypt with 
> yescrypt-lite, and keeping the full yescrypt (a strict superset of 
> yescrypt-lite, re-adding full classic scrypt support and the rest
> of full yescrypt features) as a non-PHC extra.  That is, if the PHC
> panel prefers so, yescrypt-lite and not full yescrypt would be the
> PHC candidate, although of course it can/should be kept in
> consideration that there exists this sort of superset (which would
> remain a reason for yescrypt-lite to be defined the way it would
> be).  I don't actually recommend doing things in this way (I'd
> rather keep full yescrypt in PHC, with -lite being a subset of it,
> much like how TwoCats/SkinnyCat is currently in PHC), but this is a
> valid option.

I prefer the full Yescrypt as the main entry with a Yescript-lite
option for some users.  Even better - a Yescrypt-anorexic.

I am not sure I would use a middle complexity Yescrypt-lite.  When I
tried coding something half way between the core idea of the TwoCats
algorithm and the full implementation, I failed to come up with
anything satisfying.  Cutting complexity in half while making the
algorithm worse for defense overall did not sit well with me.  I did
not feel I had succeeded until SkinnyCat was as bare-bones as Bcrypt,
yet still provided it's basic features: hogging bandwidth and filling
memory fast, with a hybrid cache-resistant/unpredictable architecture.
 Those are the things I enjoy most about TwoCats, and I was able to
highlight them in SkinnyCat, at least to me when I read it.

I think my need to create SkinnyCat, which is *not* a worthy Scrypt
replacement, comes from my desire to port it everywhere.  Good
algorithms should be so appealing to geeks that they can't help but
write an implementation of their own.  They need to be so simple, that
most of these versions actually work.  A decent coder should get it
done in an afternoon.  Porting the full TwoCats or Yescrypt to Go, for
example, sounds like tedious work.  I'd never do it just for fun, but
it'd be awesome as an actual project at work.  However, porting
SkinnyCat would be fun for me.

I am sure that this principle can be applied to Yescrypt.  All you
have to do is eliminate everything but the part you love so much about
the algorithm, you'd be sad to see it go.  What would you keep if you
had to keep it under 200 lines of code?  I personally would like to
see the parallel-wide-transform in a simple piece of code that
highlights it.  That t_cost == 0 behavior is what I'd personally keep.
 The hybrid architecture with the partial second loop is cool.  I'd
also keep the PBKDF2-SHA256 fix you invented.  It might bust a bit
beyond 200 lines :-)

> Regardless of what exactly goes into yescrypt-lite (and what's
> excluded from it), and regardless of whether/which yescrypt remains
> in PHC, here are some other changes I am considering to simplify
> yescrypt (the full thing, not only -lite):
> 
> - Drop support for ROM access frequency mask.  In practice, this
> will mean no ROM-on-SSD support (only ROM-in-RAM will be
> supported).  The reason why I am considering dropping this is that
> in most cases where I would have recommended use of ROM-on-SSD I
> would _also_ recommend simultaneous use of ROM-in-RAM, and then it
> would need to be two ROMs, with different access frequency,
> resulting in even greater complexity. Perhaps just one mask (only
> for the ROM-on-SSD) would be enough even in that two-ROMs case,
> though.

ROM in memory seems like the way I'd want to use Yescrypt, if I were
doing an authentication server.  ROM on SSD is cool, but I think you
can drop the frequency mask.

> - Merge the various yescrypt flags into one - to choose native
> yescrypt mode or classic scrypt mode.  This doesn't simplify the
> reference code much, but it will simplify documentation, analysis,
> testing, benchmarking, and it may simplify optimized
> implementations.  A drawback is that there will no longer be any
> deliberately-TMTO-friendly native yescrypt mode (for which I think
> some use cases exist), requiring that people use the classic scrypt
> mode if they require that.

I think providing a simple "classic" API, and a simple Yescrypt basic
API would be good.  I saw Catena had a simple API (the one I'd most
likely use in real life), and called it the "naive" API.  It is naive,
but we can't expect our users to be knowledgeable about all the issues
involved in password hashing.  Even providing both a t_cost and m_cost
parameter is asking too much from users.  We need a "don't make me
think too hard" API.

At the same time, is there any reason not to reveal more interesting
parameters in an "extended" API?

> (On the other hand, I need to add a new flag for enabling cache
> timing resistance.  So get rid of separation between the existing
> flags, and add that new flag.)

If you are adding a cache timing resistant loop, I'd like to see it in
the first loop (or some portion of it) in Yescrypt mode by default.
If it weakens password security when an attacker does not have
cache-timing data by too much, then I'd say just drop the feature
entirely.  There is a reason we do unpredictable password derived
addressing.  It makes password hashes more secure.  How much
complexity to add and how much weakening vs traditional GPU farm
attacks you can tolerate is a tough trade-off.

> - Without scrypt compatibility (as in -lite) and when in native
> mode, I can replace Salsa with ChaCha.  If a given build lacks
> scrypt compatibility (would be true for -lite), this would probably
> make it slightly simpler by eliminating the need for 32-bit word
> shuffling associated with interfacing with Salsa.  The drawback is
> that full yescrypt would become more complicated, requiring both
> Salsa (for scrypt compatibility) and ChaCha.  This is why I stayed
> with Salsa only so far.
> 
> Any comments?
> 
> Thanks,
> 
> Alexander

Doesn't the PWX loop dominate runtime?  Assuming this is the case, I'd
say keep Salsa.  I do wish that could be in a library, the way Bcrypt
calls Blowfish.  There's no point complicating a memory-hard password
hashing algorithm by dumping code for a cryptographic hashing
primitive into the heart of it.  Was it the 8-rounds that required a
custom implementation?

Bill
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