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Date: Wed, 27 Aug 2014 17:43:36 -0400
From: Bill Cox <>
Subject: Re: [PHC] A review per day, starting with Yarn

Hash: SHA1

Way back in April, we had a Yarn discussion:

Here's a quick dump of my updated "issues" list for Yarn.  Some
benchmark data is at the end, though we only have the reference
version.  I make my best guess how fast the optimized version will
run.  I also make a guess about the speedup I could get with an ASIC
in a similar process as the CPU.

Yarn issues:

- - Only runs fast compared to ASIC if AES instructions are available,
making it suitable only for authentication servers
- - Is unsuitable for large-memory hashes such as 1GiB, because it
read/writes 16 bytes at a time to pseudo-random locations throughout
memory, reducing his potential memory bandwidth to external DRAM more
than 10X.
- - Has constant time cache-timing attack requiring only enough blake2b
computations to fill up the state array (272 bytes by default).
- - There is no optimized version offered, making it far hard to analyze
- - The reference version is unacceptably slow.
- - Has input collisions on password, and salt - adding a few zeros to
the end does nothing.
- - 10 rounds of sequentially dependent AES for filling memory is slow.
 On Ivy Bridge, there is a 6 clock latency between each AESENC
instruction.  That's only a about 1GiB/s.  Other entries max out my
DDR3 memory bandwidth at 12GiB/s.  That would require 12-way
parallelism, but the default parallelism is 6.  Setting par to 12
would allow two threads to do the job, but there is no option for
multiple threads.
- - He does not want to support multiple threads.  Running threads
externally and combining results gives an attacker a free TMTO.  Also,
few applications are going to provide multi-threading externally.
- - His default choice of 72 for m_step is way too high for a 1GiB hash
that busts out of cache, and way too low for a 16KiB hash that fits in
L1 cache.
- - Contents of memory while filling are easily determined to be Yarn
data, rather than truely random, since the AES keys are known.  Simply
encrypt a 16 byte block of memory using the Yarn key schedule with 10
AES rounds, and see if your result matches the data in memory par*16
bytes later.  If 10 AES rounds are going to be used, it should be
indistinguishable from true-random.
- - Unless t_cost is around m_step*2^m_cost or larger, a lot of memory
will not be overwritten, leading to a TMTO option like Script.
- - For parallelism setting of 1, he calls aesenc using state[0] as both
the data and key.  Is this is reversible?  We may want to require
parallelism > 1.  I probably would use a minimum of 4 for my Intel
processors that support AES instructions.
- - The reference code difficult to use as a starting point for creating
an optimized version.  The parallelism is not explicit enough.

Here are benchmarks for 16MiB and 1GiB hashes using the inefficient
reference implementation.  This might give us a feel for what happens
on machines without SIMD and AES units, but it in no way reflects how
fast an optimized version of Yarn will run.

16GiB hash benchmark:

We need t_cost of about m_cost*m_step = 2^20*72 = 75.5M

PHC> time ./phs-yarn 75500000 20

9a 5a ec 93 6a 91 d0 d5
f0 d3 da af cb ee de 1f
95 b7 42 67 2b 6d dd fe
1e b8 3b e0 b6 04 e4 1d      32 (octets)

real	0m4.565s
user	0m4.563s
sys	0m0.004s

1GiB hash benchmark:

We need t_cost of about m_cost*m_step = 2^26*72 = 4.8B.  However, it's
a 32-bit unsigned int in the PHS API, so 4.2B is the max.

PHC> time ./phs-yarn 4200000000 26

61 44 c9 1f be a9 8a 90
43 47 99 f3 eb b2 8f 6d
0e 8f dd 59 a6 08 61 7b
58 8d 0f a4 56 a5 de 83      32 (octets)

real	4m18.725s
user	4m18.644s
sys	0m0.161s

Optimized estimate:

I wrote a simple cache test program that determines how fast I can
read then write 128 bit values to random locations.  The 32K L1 cache
was 1.5ns, the 256K L2 cache was 2.0ns, and the 8MiB L3 cache was
12ns.  External memory for 1GiB random read/writes was 18.2ns.

Optimized runtime estimate for 16MiB, with m_cost = 20, and t_cost =

Filling memory, one thread, max of 6GiB/s: 2.6ms
Performing m_cost pseudo-random 16 byte read/writes to L3 cache: 12.6ms
Performing 71*m_cost AES operations, 1 per cycle: 21.9ms
Total: 21.9ms + 2.6ms = 24.5ns

Note: This is probably optimistic.  It represents the highest possible
speed assuming everything works out perfectly.

Optimized runtime estimate for 1GiB, with m_cost = 26, and t_cost = 4.2B:

Filling memory, one thread, max of 6GiB/s: 0.17s
Performing m_cost pseudo-random 16 byte read/writes to DRAM: 1.22s
Performing 71*m_cost AES operations, 1 per cycle: 1.4s
Total: 1.4s + 0.17s = 1.57s

For comparison, TwoCat's does a 1GiB hash in 0.171s using two threads,
and 16MiB in 7ms with one thread.

ASIC resistance:

For cache-bound 16MiB hashing, an ASIC with 16 banks of 1GiB GDDR5
each should provide 16 channels at about the same L3 cache speed as
the CPU.  When doing 16MiB hashing, it is simply 16X faster.  For 1GiB
hashing, it benefits a bit from the reduced latency vs my DDR3 DRAM,
which is about 50% faster, so I expect to run up to 24X faster in that

This is actually excellent ASIC resistance.  The ASIC as 16 times more
memory than the hash uses, naturally resulting in a 16X speedup with a
16-guess at a time attack.  No password hashing scheme can defeat an
attacker's ability to make guesses in parallel, and this very high-end
ASIC can only do 16.  The 1.57s runtime is about 8X slower than
TwoCats, but it's pretty close to Scrypt, with better ASIC defense.

Overall, I do not consider Yarn to be a worthy replacement for Scrypt.
 The optimistic speed estimated above is too similar to
single-threaded Scrypt, but it requires specialize AES hardware to
achieve this.  Scrypt has multi-threading built in, while Yarn does
not, Scrypt has better cache-timing defense, and we do not have any
optimized version to test.  Script is already a standard.  To replace
it, I would expect to see more significant improvements.  However the
improved ASIC resistance relative to Scrypt is cool.
Version: GnuPG v1


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