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Date: Thu, 28 Aug 2014 16:05:32 -0400
From: Bill Cox <waywardgeek@...hershed.org>
To: discussions@...sword-hashing.net
Subject: Re: A review per day - Yescript

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As promised, here's where I finally say all the things I don't like
about Yescrypt.

Here's my list:

- - It's too complex.
- - No cache-timing resistant initial phase has not yet been implemented.
- - OPENMP support is required for current version to run in parallel
- - Like Samuel Neves code, understanding what's going on can be tricky!
- - For t == 0, most of hashing could be done serially, rather than in
parallel, though the results need to be loaded in parallel for the
final pass.

The complexity of Yescrypt will make it difficult for other authors to
create their own compatible versions.  It also makes cryptanalysis
harder.  I certainly have a difficult time reading it and convincing
myself it is bug-free.

However, I gave it a try.  I may have found a bug, but being Solar
Designer's code, I have to admit it is more likely an error in my
reading of his code.  The potential bug is on line 433 in
yescrypt-ref.c.  For t == 0, with YESCRYPT_RW enabled and parallelism
> 1, I think that randomly read/writing 1/9th of memory followed by
randomly reading 2/9ths of memory sounds like it would provide enough
TMTO defense.  Honestly, I'm not sure smix2 is needed at all for good
security when YESCRYPT_RW is set.  However, it looks like Nloop_rw is
set to N/(3*p^2).  It looks like it only does N/(3*p) total random
read/writes, followed by N/3 - N/(3*p)) random reads.  Is it supposed
to be this way?  This seems OK for p == 2 or p == 3, but what about p
== 32?  Why would I want fewer random writes as a total fraction of
memory as I increase parallelism?

The complaint I have about not forcing parallel computation is
basically a banana attack.  If an attacker can instantly flush his
memory to disk, and later instantly read it back, then he can do a
TMTO, running threads in series rather than in parallel, because no
data is exchanged between the threads.  They are all independent.  In
the last pass, all memory has to be loaded so that the random read
phase can complete in a reasonable time.  The problem with this attack
is that the attacker needs the bandwidth from memory to disk to be as
high as from the CPU to memory.  On my test machine, I'm able to get
about 12GiB/s bandwidth from the CPU to memory.  In theory, my Sata
interface is 6GiB/s.  I seriously doubt that having 2 Sata disks in my
system would let me do 12GiB/s read/write to them.  However, if I were
determined to make this TMTO attack, I might be able to do so with
only a 2X increase in time*memory cost for high parallelism values,
but I still would need to load it all in memory in the end.  That
compares to Scrypt's *reduction* in time*memory cost by 4X, so it's
hard to say this is a real concern.  It's more like me beating Solar
Designer with a banana :-)

I think Solar Designer errs on the side of over-defending passwords at
the expense of too much additional complexity.  I think his mind deals
with complexity like this more easily than most of the rest of us,
which leads him to make this mistake.  No other entry has gone to such
lengths to increase an attacker's power bill slightly when possible.
There are more modes and tunables than any other entry, to help defend
better in various situations.  He has the VROM mode for authentication
servers, and small random-read-writes for Bcrypt-like security.  He
has Scrypt compatibility mode, and code to detect any non-Scrypt
compatible options, in which case he fixes the PBKDF2-HMAC input
collisions.  I hate modes.  They confuse me :-)

However, even with these gripes, I believe Yescrypt provides the best
password defense of any entry the PHC competition.  It's probably the
strongest password hash ever written for defending against brute-force
guessing attacks.

It is also the only memory-hard algorithm in the competition that I am
willing to admit is stronger than TwoCats.

I was hoping to trash Yescrypt a bit more, but I keep re-reading the
code, and just can't.  It's like trying to track Samuel Neves Blake2
code.  Good luck with that :-)

Bill
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