Date: Fri, 29 Aug 2014 01:37:08 +0400
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Re: A review per day - Yescript

On Thu, Aug 28, 2014 at 04:05:32PM -0400, Bill Cox wrote:
> As promised, here's where I finally say all the things I don't like
> about Yescrypt.

Thank you!

> Here's my list:
> 
> - - It's too complex.

It's definitely more complex than I would have liked it to be.  But I
have difficulty dropping anything from it to reduce complexity, and I
have yet more flags to add, including to address your wishes
(cache-timing resistant initial phase, tunable pwxform rounds).

First on my list to potentially drop is YESCRYPT_PARALLEL_SMIX (keep
support only for scrypt-style thread-level parallelism), but it sounds
like you wouldn't like me dropping that.  And Argon authors' analysis
suggests that such theoretically 1:1 TMTO opportunities are in fact bad
in practice (need to slow down attack by a factor greater than the
reduction in memory, if at all possible), especially at very high p.

> - - No cache-timing resistant initial phase has not yet been implemented.

Yes.  Planned for final revision.

> - - OPENMP support is required for current version to run in parallel

Oh, that's a mere implementation detail.  To me, OpenMP-using code is
cleaner and smaller than pthread-using code, but it's easy to switch it
to using pthreads in a fork of the tree, or to include both versions, if
you want to.  I just don't want to add the complexity of including both.

Why do you find it a drawback?  OpenMP has been around for years, and it
hides some of the complexity and removes room for some potential bugs.

Oh, here's where it can in fact be a drawback: if the app into which the
code is to be linked is already using some other abstraction layer for
threads.  But I can't possibly happen to match every possible
abstraction layer at once.

> - - Like Samuel Neves code, understanding what's going on can be tricky!

In case of yescrypt, is it caused by complexity of what's in yescrypt or
by the way the code is currently written, in your opinion?

If you have any suggestions on making yescrypt or/and its implementations
simpler, I'd be happy to hear those!

> - - 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.

Is it the "banana attack" you mention below?  I'll re-read it tomorrow.

> 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.

Unfortunately, yes.

As to cryptanalysis, though, it should be pretty easy to verify
yescrypt's entropy bypass paths for both password and salt, as well as
separation of password and salt, from looking at the wrapper functions
only.  It's analysis of susceptibility to TMTO, etc. that is
complicated.

> 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.

I did the math, as well as simulations (see extra/sim-normat*), and
optimal normalized area-time cost to attacker is achieved at yescrypt's
current behavior for t=0, with its SMix2 intact and corresponding to
1/4th of total iteration count (which is 4/3*N).  This is assuming no
efficiency-improving TMTO exists (one that would reduce the area-time
cost).  If such TMTO attack exists at t=0, then t=1 or higher would be
optimal, meaning even more iterations in SMix2.

In other words, the few iterations in SMix2 that are invoked at t=0 are
there primarily not because of TMTO concerns, but simply because they
increase the normalized area-time even assuming no TMTO.  Even more
iterations, beyond this optimal point, would be decreasing the
normalized area-time, unless an efficiency-improving TMTO exists.

Hence my choice of settings to use at t=0.  Nothing arbitrary there.

Assuming YESCRYPT_PARALLEL_SMIX is set for the below:

> 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?

Everything you describe in here is on purpose.  The random writes exist
for p>1 at all only to keep yescrypt's absolute TMTO resilience (against
whatever fixed TMTO attack) for p>1 at p times higher total memory no
worse than it is for p=1 at correspondingly less memory, with no
surprising gap.  It would be rather nasty if e.g. going from 1 GB p=1
to 2 GB p=2 or even to 32 GB p=32 would potentially allow for an
efficient attack in a smaller amount of memory than on the 1 GB p=1
hash.  This extra magic with division by p^2 removes that risk.  For
32 GB p=32, it feels irrelevant as it's unlikely you could find an
efficient attack at < 1 GB on a 32 GB hash, and as you point out this
random writes portion becomes almost negligible, but for e.g. 2 GB p=2
the risk of an efficient < 1 GB attack could be for real.

I first briefly mentioned this detail in here (although I was confused
by some other important things in that early posting on the subject):
http://www.openwall.com/lists/crypt-dev/2013/12/19/1
"When ESCRYPT_RW is set, there's some
additional magic here, which ensures there's no reduction in "absolute"
TMTO resistance when going from p=1 to p>1 and increasing N
proportionally."

This is definitely one of the design decisions that ideally I should
have explained in the specification document.

> 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'll re-read this tomorrow, but for now: I think SATA is 6 Gbps, not
6 GiB/s.  So you'll need ~10x more interfaces.

> 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 :-)

I don't think my mind is that special.  It's more like I find yescrypt's
complexity not so complex for me for the simple reason that I spent a
lot of time analyzing scrypt and its code, and then designing yescrypt,
step by step and removing/replacing parts, so I have a good feeling of
why every part is in there, what we'd miss without it, and how it
interacts with the rest.

As to having fewer modes, I am likely to create at least one cut-down
implementation of yescrypt with partial functionality.  I will likely
exclude support for p>1, YESCRYPT_PARALLEL_SMIX, and ROM from that one,
keeping only what's relevant for a crypt(3) with no extra inputs.
I might also exclude scrypt compatibility from it (without support for
p>1, it wouldn't be full scrypt anyway).

Alexander

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