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Date: Thu, 1 Oct 2015 22:22:44 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Multithreading in Argon2

Hi Dmitry,

Thank you for bringing this up.

On Thu, Oct 01, 2015 at 12:25:07PM +0200, Dmitry Khovratovich wrote:
> Currently Argon2 has the parameter p of the number of internal lanes. For
> given p, Argon2 can handle up to p parallel threads, which are regularly
> synchronized. Currently the number of threads is explicitly set equal to
> the number of lanes, but it is not a strong requirement.
> 
> We can imagine that an application may want to choose the number of threads
> in run-time if it feels that the password hashing process initiates too
> many threads in total. Thus we suggest an extension to API (not to the
> specification) that takes distinct inputs for thread number and lane number.

Yes, this is how it should be.

With OpenMP, like my optimized implementations of yescrypt use, this
happens automatically: the thread count is either detected automatically
(usually is limited to the number of logical CPUs in the system) or is
specified via the OMP_NUM_THREADS env var or is further overridden with
omp_set_num_threads(), which the calling program might use.  OpenMP also
hides the complexity of allocating (what you call) lanes to threads -
it's up to the OpenMP runtime library to do that in whatever way it
deems optimal, which may also be adjusted with some env vars and such.

BTW, it's interesting whether and to what extent OpenMP env vars may be
used to attack a program executed over a privilege boundary (SUID/SGID
or otherwise).  So far, I am assuming that multi-threaded builds are not
for use in such programs, but this may need to be made explicit.

If you use pthreads, which has pros and cons as compared to OpenMP (so I
don't have strong feelings one way or the other), you need to introduce
your own way to control the thread count, and your own explicit
(re)allocation of lanes to threads.  (Yes, it may sometimes be
preferable to reallocate; in OpenMP, this is called dynamic scheduling.
But you probably shouldn't complicate your code with that.)

> We however advise that during the first password hash generation the number
> of threads be equal to the number of lanes as otherwise this would give the
> cracker an extra advantage. After the hash is stored, the subsequent
> verifications can be done with any number of threads.

I think this is not exactly right.

For KDF use, I think the number of lanes should be chosen to match the
expected number of threads for subsequent use.  So e.g. if the key is to
be re-derived on an 8 hardware threads CPU, then 8 lanes should be used
even if the machine used initially has a different number of hardware
threads (regardless of if it's smaller or larger than 8).  In practice,
the machine used initially and the machine expected to be used
subsequently will often be the same.

For password hashing use, we should use 1 lane currently (and thus only
1 thread per instance).  The parallelism will come from concurrent
requests.  As the number of CPU cores grows, or with implementations on
current many-core devices, we might have to deviate from that in order
to keep the latency low.  But until/unless we have to, I think we should
just use 1 and thereby avoid the drawbacks of multi-threaded hash
computation.  I mean primarily greater sensitivity of password hashing
throughput to other server load (such sensitivity results from needing
to synchronize the threads, which becomes inefficient under load).

I recognize that use of multiple threads for computation of one hash
increases the attacker's memory requirements per instance (even though
it also decreases the running time by the same factor, to maintain the
server's throughput, so this isn't an area-time product increase).  This
might mean the attacker will have to use more distant, more expensive,
or/and slower memory (and memory bus).  However, I think this effect is
more profound for KDF use, whereas for password hashing use the improved
locality of reference for single-lane hashes benefits CPU defenders too
(as the per instance sizes are on par with those of CPU caches).
For small lane (and thread) counts (higher than 1), I think the cost to
defender from thread synchronization on servers under load and from
higher cache miss rate is likely greater than cost to attacker from
needing to use larger memories per instance.

> It is important to require both numbers to be inputs if they differ. If we
> fixed the number of lanes to some high default value, such as 8, then any
> application that can not support this number of threads will immediately
> give the thread-richer cracker an advantage.

Right.

Moreover, I think hard-coding a number of lanes higher than 1 would
hamper password hashing use on current systems with not-too-many cores
(even if there are enough cores to support that number in hardware), as
per my explanation above.

> We welcome the comments as well as discussion of other threading issues.
> For example, is there any need for external thread handlers?

What do you mean by that?

As to "other threading issues", why is the number of lanes currently
limited to 255?  This is already barely sufficient for Knights Corner
(which runs up to 244 hardware threads per chip), and will be
insufficient for Knights Landing (expected to support 288 hardware
threads).  (And yes, it's those many-core devices where use of multiple
threads might be worthwhile even with password hashing use.  With proper
implementation, there isn't expected to be any unrelated load on those.)

Alexander

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