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Date: Sat, 5 Apr 2014 20:19:56 -0400
From: Bill Cox <waywardgeek@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Mechanical tests (was: POMELO fails the dieharder tests)

On Sat, Apr 5, 2014 at 7:10 PM, Solar Designer <solar@...nwall.com> wrote:
> On Sat, Apr 05, 2014 at 12:58:48PM -0400, Bill Cox wrote:
>> One thing we can prove is that a PHS that attempts to be a secure
>> memory-hard algorithm is "strongly secure" in the sense that we've
>> hashed all the inputs to meet that definition, and have preserved
>> entropy all the way to the output.  Both yescript and TwoCats pass
>> this test, as well as several others, I think.  All you have to show
>> is your code works something like:
>>
>> PRK = H(all inputs, including lengths)
>> X = whateverHashAlgorithm(PRK)
>> output H(PRK || X)
>>
>> I would call anything like this a "strongly secure PHS".  I'm not
>> really thinking about the non-memory-hard PHS's which seem to take
>> radically different approaches.  Some of the memory-hard PHS's look
>> more like:
>>
>> PRK = H(all inputs, including lengths)
>> X = whateverHashAlgorithm(PRK)
>> output H(X)
>>
>> For example, I think Scrypt, Catena, and Gambit look more like this.
>
> scrypt includes the password in the final H(), but it doesn't include
> the salt in there.  So if anything goes wrong in scrypt's
> whateverHashAlgorithm(), it might lose salt entropy, but not password
> entropy (except to the extent H() loses entropy).
>
> yescrypt includes the entire PRK in the final H(), but the initial
> calculation of PRK includes {password, salt} yet doesn't include {N, r,
> p, t, flags}.  I think this is a good choice, but if this community
> feels the parameters also need to be included in the entropy bypass to
> final H hash (why?), I am willing to add that in the tweaks period.
>
>> I'd have to double check to be sure.  These schemes require more proof
>> that entropy is not lost, but since they use familiar cryptographic
>> primitives for all hashing, they seem pretty safe to me.  The ones
>> that don't use any cryptographic primitives at all are the scary ones
>> to me.  Using the dieharder tests, we're already showing that some of
>> those are not very well put together, at least in their submitted
>> form.
>
> Thank you for these tests you're running.
>
> Once you're done with the basic tests, I think we should also attack the
> "strongly secure" hashes in a weakened form: removing PRK from the final
> H().  My intent is that yescrypt should pass randomness tests in that
> weakened form as well (if it doesn't, I'll tweak it until it does, but I
> expect that it does already).  This is similar e.g. to attacking
> reduced-round versions of ciphers and fast hashes, or attacking their
> individual components (a closer analogy perhaps).
>
> Taking this a step further, we can test X instead of H(PRK || X).
>
> If a "strongly secure" hash weakened as above fails randomness tests,
> this likely indicates that its whateverHashAlgorithm()'s cost may be
> fully or partially avoided by the attacker in a subset of cases.  Then
> we could analyze whether this has practical impact (which will depend on
> the size of this subset and how quickly an attacker is able to trigger
> the bypass and with what overhead on inputs that don't trigger the
> bypass), or/and count it as a reason against that scheme (until it's
> tweaked to avoid this problem).
>
> Alexander

Ok, this is all really unfair.  Of course we should attack X exactly
as you say.  I think I should skip attacking H(X) (without PRK).  An
author with a weak X has work to do.  Now I'm not going to be able to
sleep for another week!  Aren't we supposed to shooting the s*, and
talk about integrating FPGA programmability into multi-CPU thingies at
this point :-)

Bill

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