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Date: Sat, 17 Oct 2015 20:09:53 +0000
From: Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Argon2 PHC release...

One of my concerns with ds is, as you point out, the immaturity of its
design and of its code. There's already a lot to do to make the code
reliable.

Let's not exclude an S-box-based version later, but I'd like to deliver a
first usable release by end of this month.

BTW the code in https://github.com/P-H-C/phc-winner-argon2 is now less
C++-looking, with C-style naming convention, and we're getting rid of
C++isms and misused keywords (working with Samuel on that). We've fixed a
couple of of bugs too.

On Sat, Oct 17, 2015 at 10:04 PM Solar Designer <solar@...nwall.com> wrote:

> On Sat, Oct 17, 2015 at 08:55:27AM -0700, Bill Cox wrote:
> > I prefer Argon2id over Argon2d.  Is there any reason to use 2d instead of
> > 2id?
>
> I think we should include 2i and 2ds.  I wouldn't mind 2i and 2ids, if
> there's demand and that flavor is introduced.  2d and 2id are probably
> not worth it: they don't take full advantage of data dependent access,
> yet also don't provide the peace of mind that 2i does.
>
> Also, I just started looking at 2ds, and I think it might need a little
> bit more work:
>
> Do I understand it right that right now the S-boxes are shared for all
> lanes (when there's more than one)?  That's not great if so.  They
> should be per lane (they are in yescrypt).  OTOH, it isn't a fatal
> drawback either, because for password hashing use on current CPUs we'll
> typically have lanes=threads=1.  (We may need more on many-core devices
> like Xeon Phi, and on future CPUs, though.)
>
> The source code comment in argon2.h says that 2ds is "20% slower" than
> 2d.  I think it's a property of the current implementations, which keep
> the MAXFORM loop in a separate "for" loop, and most importantly within
> an "if".  I think this "if" doesn't let a compiler mix that loop's body
> with the following code even when the loop is unrolled due to compiler
> optimization flags.  Thus, we get the sum of two latencies (for MAXFORM
> and for Argon2's block compression function) rather than max of the two.
> (I am simplifying, as scalar and SIMD aren't entirely independent: they
> compete for instruction decode, etc.  So slight slowdown is expected,
> but 20% is worse than expected.)  To optimize this, we may try declaring
> an inline FillBlock_body() function, and calling it twice from a wrapper
> FillBlock() function, passing a Boolean constant for the "if".  The
> intent would be to tell the compiler to specialize the code for the two
> cases.  We can do this post-PHC, unless this 20% thing determines our
> (non-)inclusion of 2ds in a PHC-official implementation.
>
> Right now, 2ds doesn't appear to be included in the would-be PHC release
> that JP et al. started work on (BTW, thanks for the effort!)  I think
> that ideally we'd include it instead of 2d, and make its S-boxes per-lane.
>
> Alexander
>

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