Date: Wed, 5 Mar 2014 04:44:04 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: escrypt 0.3.1

Hi,

I've attached a revision of escrypt.  It lacks some of the planned
features/properties, and in fact I (temporarily?) removed code for L2
cache accesses from it to make this revision more consistent.

Bill, I suggest that you use escrypt in scrypt compatibility mode as a
baseline for comparing the performance of your TigerKDF against.
escrypt's native modes are still a moving target, but its scrypt
compatibility is unlikely to change in performance much.  (This should
replace your use of Arch Linux's known suboptimal build of scrypt.)


	Testing, demos, benchmarks.

See the README and PERFORMANCE-* files in the attached tarball.


	Features and properties.

- Much optimized SIMD-enabled implementation of scrypt (including
use of SSE prefetch and AMD XOP intrinsics).  This establishes a
performance baseline for further incompatible changes.

- Additional APIs (see crypto_scrypt.h), including a tentative crypt(3)
encoding syntax for both the official scrypt and our revisions of it, as
well as for separation of global and thread-specific initialization from
per-password computation (as it would need to be done in many actual
deployments, and thus as relevant to our performance testing).

- Optional initialization and use of a large ROM (using the added API
to treat the ROM as global context).

- Optional time-memory tradeoff discouraging measures for individual
hash computation (ESCRYPT_RW flag) and mandatory for ROM initialization.

- Support for and demo of placing the ROM into SysV shared memory via a
separate program.

- Multi-threaded initialization of the ROM (112 GiB ROM initialization
shown in PERFORMANCE-scaling-up completed in just 42 seconds, while
providing decent time-memory tradeoff resistance).

- ROM-on-SSD support.  See PERFORMANCE-SSD for a usage example and some
performance figures.

- Compile-time tunable instruction-level and SIMD parallelism, (with
current default settings for total ILP*SIMD width of 512 bits.  The
smallest amount of parallelism is 64-bit (one lane).  Despite of reuse
of scrypt's Salsa20 for the lane mixing step, there is support for
amounts of parallelism greater than 512 bits as well.  So it's 64 bits
to "infinity".

- Rapid small random lookups in L1 cache, to achieve GPU resistance
comparable to bcrypt's even at unusually low memory cost settings.
(See below for limitations of what's currently achieved.)

- Multiply latency hardening: use of sequences of integer multiply
operations to tie up memory for a certain number of cycles, even in
ASIC, since this is one of few common CPU instructions that is not
leaving a lot of room for latency reduction.  Currently, 32x32->64
multiplies are used.
(See below for limitations of what's currently achieved.)

- Alternative way to implement thread-level parallelism, requested via
the optional ESCRYPT_PARALLEL_SMIX flag, which requires that for
efficient processing the attacker provides as much memory as the
defender had specified, even if the attacker does not intend to fully
use the parallelism on a given node.  See the description of
ESCRYPT_PARALLEL_SMIX in the revised crypto_scrypt.h for more detail.


	Issues and future work.

I am not happy about the complexity.  Clearly, when supporting classic
scrypt and more, complexity is higher than scrypt's.  Is this worth
keeping, or should we drop support for computing classic scrypt hashes
in order to let us simplify things somewhat?  I guess this would depend
on how much we're able to simplify the design and code while keeping an
scrypt compatibility mode vs. without scrypt compatibility.  This is yet
to be seen, but I suspect the difference is not very large.  We'd move
from Salsa20 to ChaCha20, which should in turn allow us to drop the SSE
shuffling, but compared to total complexity this is not much.  We'll
also fully drop scrypt's sub-block shuffling in BlockMix, but this is
also not a lot compared to total complexity.  The rest of scrypt's
complexity I'm afraid is well justified (meaning that simpler PHC
candidates will have serious shortcomings, I'm afraid), even though JP
had expressed (on his slides) unhappiness about scrypt being too
complex, and hope for something much simpler to come out of PHC (it may,
but I'm afraid it'll also be worse than scrypt in important ways).

Both scrypt's high-level and my SMix-level approaches to thread-level
parallelization are implemented, selectable via a flag.  This has some
added code complexity (luckily not a lot), and unfortunately it may also
be confusing (when to choose which approach?)  It will probably stay
this way for the PHC submission, but we might drop one or the other
later _if_ the complexity+confusion is deemed inadequate.  (Of course,
dropping scrypt's may only be done along with dropping or limiting
support for the classic scrypt mode.)

An advantage of having an scrypt compatible mode is that we reduce
complexity and size of crypto libraries that would otherwise include
separate code for scrypt.  Another advantage is that it might ease
adoption of the new hash type(s): a crypto library might include our
code (or another implementation) initially just for its scrypt
functionality, but it'd also happen to provide support for our new hash
type(s), which share much design and code with scrypt.  Yet another
advantage is that when our code (or another implementation) is included
in a library, a programming language runtime, a SQL server, or whatever
for its new functionality, it also happens to provide classic scrypt,
which might be handy to another user.

An issue, though, is that inclusion of any tunable-cost KDF or password
hashing scheme is risky, e.g. in the SQL server example.  I have some
thoughts on additional API to let such uses mitigate these risks.

In this code revision, crypto_scrypt-sse.c has suffered from premature
optimization in its smix1() and smix2().  I happened to move if's out of
the loops, duplicating the specialized loops instead, which worked great
when the functionality was more limited.  I then proceeded to add more
functionality, which resulted in some if's popping up inside the loops
again.  I intend to de-optimize this, considering that these loops are
not very performance critical except when r is very low, and considering
that probably keeping blockmix*() last sub-block's X0/X1/X2/X3 in
registers across calls to blockmix*() makes more of a difference than a
few (correctly predicted) conditional branches do.  So I might implement
the latter optimization instead (maybe the compiler does it already when
blockmix*() are inlined).

The tentative 64-bit lane round function:

	x += ((uint64_t)xl * s0l + s1) ^
	    ((uint64_t)xl * s1l + s0);

(where the 64-bit s0 and s1 come from S-box lookups, and s0l and s1l are
their least significant 32 bits) has some significant biases, as can be
seen with the attached analyze.c program on a local->aligned memory dump:

r=6 N=2^12 NROM=2^22
Will use 3145728.00 KiB ROM
         3072.00 KiB RAM

min = 10801 max = 71264 (min+max)/2 = 41032.5 (expected 12329.0)
diff = 788534 (expected 788983.5)
diff_lo = 65515 diff_hi = 65536 (expected 65535.6)

(where min and max are the number of occurrences of the least common and
most common byte value, diff is the number of different 32-bit values,
and diff_lo and diff_hi are of different 16-bit halves in each 32-bit
value).  As you can see, (min+max)/2 is way off, and diff and diff_lo
are non-perfect.  This can be much improved by simply adding:

	x += (uint64_t)xl * xl;

right before the expression given above, but after "x" has been used to
derive S-box lookup indices.  (This added multiplication occurs in
parallel with the two S-box lookups, so there's almost no slowdown from
it.  In fact, I've been experimenting with repeating that line (or
rather its SSE2 equivalent) a few times.  With one or more of these
added lines (_not_ included in the escrypt 0.3.1 code attached here),
"analyze" output improves to:

min = 12018 max = 12638 (min+max)/2 = 12328.0 (expected 12329.0)
diff = 788789 (expected 788983.5)
diff_lo = 65535 diff_hi = 65536 (expected 65535.6)

(diff is still slightly lower than perfect, but the rest is great).

For SSE2 experiments, the line to add is:

	X = _mm_add_epi64(_mm_mul_epu32(X, X), X); \

right after the existing lines:

#define SBOX_SIMD_1(X, x, s0, s1) \
	x = EXTRACT64(X) & 0x0ff000000ff0ULL; \

To analyze the "x += x * x" thing (in its different variations), or
"x = x * (x + 1)" (which is the same or almost the same, depending on
integer widths used), I used the attached sim-mul.c program.  Curiously,
aside from the obvious aspects (the result being always even, and there
being some fixed points), this trivial operation behaves reasonably
well.  The loss of 1 bit of entropy may be compensated by also using the
value of x prior to this operation.  (In the above case, for S-box
lookups.)  Is squaring much simpler or/and quicker in ASIC than
arbitrary multiplication?

I am also going to consider other variations of the round function.  In
fact, I get better results when fully replacing the value of x with some
computation based on values looked up from the S-boxes, but I dislike
this because it may result in fatal entropy loss if the (variable) S-box
contents happen to be degenerate.  Perhaps I am overly paranoid here,
since currently the initial S-boxes are generated via Salsa20 only, but:

Currently, the L1 cache fitting S-boxes stay read-only after they're
initialized using an smix1() call (thus, are based on both the password
and the salt).  This will need to change.  In fact, I had a code
revision where the S-boxes were instead part of V, and their use would
only start when a V fill threshold is reached.  However, this was tricky
for optimized implementations, and it was conflicting with support for
ESCRYPT_RW in smix2().  In particular, I (and the C compiler) would no
longer be able to assume non-overlap of S-boxes with Bin2 being updated
by the current blockmix_*_xor_save(), even though in practice such
overlap would be rare (not enough to cause much slowdown to attackers,
but a pain for everyone to optimize the code more fully).  So another
approach at making the S-boxes read-write may be added, or ESCRYPT_RW
mode in smix2() may be dropped (and then the S-boxes in V approach may
be reintroduced).

The frequency of S-box lookups is not high enough, and their results are
not required soon enough, as compared with bcrypt, especially on older
machines.  This will need to be improved.  Some of the added parallelism
is compensated for by the S-boxes being slightly larger, though.

The instruction-level and SIMD parallelism is currently compile-time
tunable (which obviously changes the hash values being computed).  In
-ref and -nosse, these things are on #define's.  In -sse, they're also
partially hardcoded.  We may want to make them runtime tunable instead,
and include some specialized code for common combinations of settings.
I think this may fit in a range of bits in the "flags" parameter, to
avoid introducing lots of separate parameters.

The current settings are for total compile-time instruction-level * SIMD
parallelism of 512 bits, with additional 2x instruction-level
parallelism available during S-box lookups and multiplies (since there
are two S-boxes).  This is excessive for Sandy/Ivy Bridge, contributing
to the "S-box lookups [...] results are not required soon enough"
problem (to compete with bcrypt at low memory settings).  A better
default choice may need to be made, and the implicit extra parallelism
from parallel S-box lookups may need to be removed.  (bcrypt has 4
S-boxes, so it does have this kind of parallelism too, but if we add
explicit parallelism via multiple lanes, we may want not to also have
parallelism within each lane, or we lose to bcrypt.)

The L2 cache stuff should preferably be re-added, after the L1 cache
stuff is "finalized".  (BTW, when I temporarily had it, there was almost
no performance impact on Sandy Bridge and newer despite of the much
increased total number of memory+cache accesses, but there was
significant impact on Bulldozer.)

For ROM-on-SSD, support for read-ahead may need to be added, although
we've achieved reasonable results even without it (with many hardware
threads sharing one SSD).

Support for simultaneous use of multiple ROMs (with different access
frequencies) may need to be added, so that when using ROM-on-SSD it is
possible to also use a ROM-in-RAM.  (Alternatively, the APIs may be
simplified assuming that such support would never be added.)

Alternatively, ROM-on-SSD may be considered too exotic, and
simplifications may be made by excluding support for adjusting ROM
access frequency.

(In general, I assume that removal of optional features will be
considered as acceptable "tweak" under the PHC.)

Many other things are not in here yet, including postponing of cache
timing unsafe lookups, and the (relatively straightforward) wrapper code
to support hash upgrades, server relief, and hash (re-)encryption.
There's also no zeroization yet.

Finally, we need better names for escrypt itself (turns out there's a
company under that name...) and for the S-box using sub-block processing
algorithm (and thus for the corresponding BlockMix_* as well), even
though that algorithm is still subject to change.  (Currently, it lives
under the dull name of Sbox.)

Any feedback is welcome.

Alexander

Download attachment "escrypt-0.3.1.tar.gz" of type "application/x-gzip" (40626 bytes)

View attachment "analyze.c" of type "text/x-c" (2048 bytes)

View attachment "sim-mul.c" of type "text/x-c" (509 bytes)

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