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Date: Tue, 31 Mar 2015 12:26:36 +0300
From: Solar Designer <solar@...nwall.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Another PHC candidates "mechanical" tests (ROUND2)

On Tue, Mar 31, 2015 at 10:28:26AM +0200, Jean-Philippe Aumasson wrote:
> I second Hongjun's argument about strong crypto: as-fast-as-possible
> primitives need as few operations as possible, thus short chains of
> operations need be crypto strong. OTOH, password hashing doesn't need
> ultra high speed, thus can afford a high number of operations to
> ensure that, even with suboptimally-secure building blocks, many
> iterations will guarantee collision/preimage/pseudorandomness strength
> etc.

I mostly agree with you and with Hongjun on the above.  Yes, we "can
afford" "to ensure" so that this "will guarantee" (and this is in fact
trivial compared to the challenges and tradeoffs faced with
"as-fast-as-possible primitives"), but on its own "a high number of
operations" does not necessarily "guarantee" those security properties.
I'm sure this is obvious to you, too.

> MD5 with 256 instead of 64 rounds would probably be secure, for example :-)

Perhaps, although since MD5's rounds are not all the same "MD5 with 256
rounds" is non-specific.  But I get what you mean, and I agree.

For an example of the opposite, and in PHC context, I wasn't necessarily
being overly paranoid when I tested revisions of what eventually became
pwxform not only at low, but also at ridiculously high round counts like
1000000.  In earlier revisions (not what I ended up submitting), at
round counts like this biases would accumulate arguably too much, so
yescrypt's V array as a whole would be significantly non-random.  This
would not affect security of yescrypt as a whole, at least due to the
strong crypto on the outer layer, but I preferred to avoid it anyway.
Without strong crypto on the outer layer, and without the Salsa20/8
invocations at the end of each block, and if someone actually used
pwxform round counts like this (that's three things that are currently
false for yescrypt as submitted), this could have been important.  In
yescrypt as submitted, V passes my randomness tests even at 1 or 1000000
rounds of pwxform (although this still depends on some initial whitening
of the inputs, which yescrypt obviously has).

Of course, having biases accumulate is less likely with larger internal
state (one pwxform lane has just 64 bits of state passing from round to
round).  In fact, larger internal state is what we have in pwxform too,
when the rounds count is small (more state gets mixed in at each
sub-block).  And of course, POMELO uses much larger internal state
between rounds.  I am merely referring to this example to show that, in
general, having too many rounds can actually expose problems, and this
isn't too far away from PHC (has been considered and has affected the
way yescrypt's pwxform is currently defined).

Alexander

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