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Date: Wed, 25 Mar 2015 09:48:06 +0100
From: Milan Broz <>
Subject: Re: [PHC] Another PHC candidates "mechanical" tests (ROUND2)

On 03/25/2015 05:49 AM, Solar Designer wrote:
> On Tue, Mar 24, 2015 at 11:50:44PM +0100, Milan Broz wrote:
>> The updated test report (draft) is here
> Thank you!
> I think you should include more info on your two test platforms.  Most
> notably, the specific CPU types and clock rate.

OK, I am trying to avoid these test to be presented as a performance tests
because with reference implementations it would be unfair.

Anyway, there is already lscpu for both platforms in git
(This is quite a big machine, actually running Fedora for some Red Hat
distro development I have access to)
> The charts showing real memory usage and real time vs. the cost
> parameter values might be useful, but it would also be useful to see a
> cross-chart of real memory usage vs. real time (with the lowest time cost
> setting supported by each scheme) and with growing memory cost setting
> (this is how you'd control the real time).

Isn't it what Figure 1 already shows? Or you mean some combination?
(tcost is minimal for each candidate there, memory cost is increasing and chart
shows real memory use + real run time).


>> - I added some test case for my intended use (KDF for LUKS) for few functions.
> Lyra2 is surprisingly fast.  That's nice.  Your table does not give
> exact numbers for candidates meeting your requirements, though - maybe
> you can add that (for their lowest supported t_cost)?

For the last KDF test the exact input parameters are in Table 5, times
are in log directory. For example with the minimal t_cost:

> As to yescrypt taking 1.3s for 1 GB, I'd say that consuming 1 GB isn't a
> goal per se.  The goal is attack resistance.  Quite possibly yescrypt's
> 6 sequential pwxform rounds (each with two S-box lookups and one
> multiply) per sub-block provide more attack resistance in the time
> factor of the area-time product than Lyra2's higher memory filling speed
> does in the area factor (if you measured these two at the same running
> time, so with different memory usage).  Besides, yescrypt tries not to
> bump into the memory bandwidth prematurely, thereby letting you use
> multiple threads to a greater advantage.  If you do not intend to use
> multiple threads and want to maximize the memory filling speed by your
> one thread (even if at expense of possibly reducing the attacker's
> area-time product, via the potentially much lower time factor), you may
> reduce yescrypt's pwxform round count from 6 to 1 as Bill suggested.  In
> yescrypt-simd.c, simply delete 5 of the 6 repetitions of PWXFORM_ROUND
> in the PWXFORM cpp macro.  I got to make this runtime tunable (easy),
> even if just to show that yescrypt is (more) competitive. ;-)

TBH I think yescrypt is very competitive here :)

I did not want to modify default internal configurations (and I think it
should not be even needed. Give users some knobs and they'll find insecure
combination and will use it as the default... well, joking :-)

Why I do not use multiple threads is based on disk-encryption my use case:

In general, FDE-enabled disk can be moved between several machines with
completely different performance and hardware configurations.

On slow machine unlocking should take more time and it would be
probably acceptable (in some reasonable limits).

But if it cannot be unlocked at all (cannot run parallel threads) it is
unacceptable for the user (usability fail). Security/usability tradeoff...

The second reason is that it can could run in a very limited environment
(a bootloader for full system encryption) where threads are not easily manageable.

But it is just my opinion.


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