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Date: Mon, 7 Apr 2014 05:38:19 -0400
From: Bill Cox <>
Subject: Re: [PHC] Re: Yarn issues

Because of the bit of debugging I had to do in Yarn, I've taken
another look and have some more thoughts:

Yarn inherits the same GPU-friendly parallel cache-timing attack as
Script.  See the end of the Catena paper for details, but basically,
the first loop for filling memory is cache-timing resistant, but once
it starts reading/writing memory in an unpredictable way based on data
derived from the password, an attacker can in theory abort a bad guess
if he is in possession of cache timing information.  If the first loop
did predictable reads from memory in addition to writes, it would
defeat this parallel attack.

Yarn is tightly integrated with Blake2d in the reference.  It would
not be trivial to switch from Blake2d to something else, like SHA512
for example, without changing a lot of code, and this would be
error-prone for most coders.  Could Yarn be "tweaked" to have a
simpler interface to it's cryptographic hash function?  This wouldn't
have to change functionality significantly.

The design of Yarn seems to be intended to run in cache, rather than
external DRAM (up to maybe 16MiB or so).  I infer this from the lack
of a strategy to mitigate cache miss penalties.  There are several PHC
entries that also seem cache-bound, so clearly there is interest in
such algorithms.

I think the default value of 72 for m_step is too high.  The README
says that m_step should be chosen to maximize both memory bandwidth
and AES encryption speed at the same time.  If 6 AESENC instructions
can execute in parallel, say at 3.4GHz (the speed of my testing
machine), that's only about 1.5GiB of total memory bandwidth (16
bytes, 1 read, 1 write, every 72 cycles).  EARWORM on one thread reads
12GiB/s from external DRAM, and uses AESENC to hash all of that data.
For a cache bound algorithm, I think one thread can sustain over
100GiB/s to L1 cache.  I suspect that the right value for m_step is 1.
 However, this might lead to weaker hashes being written to memory,
which likely could be detected as Yarn output rather than random data.
 This is only an issue if memory is somehow leaked to an attacker.  My
personal feeling is that this is not a big deal, but most entries seem
to be careful to only write the output of well respected cryptographic
primitives to memory.  It seems to me that Yarn can have either high
performance, or write only cryptographically pseudorandom data to
memory, but not both at the same time.

There is a mismatch between the code and the README spec in how AESENC
is used to update state[0].  The code uses state[1] as the key 71
times followed by addr one time, while the README uses block each


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