phc-discussions - Re: [PHC] Dumb fast file digest idea...

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Date: Thu, 24 Apr 2014 10:30:51 +0200
From: Dmitry Khovratovich <khovratovich@...il.com>
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Dumb fast file digest idea...

Bill,

this is not the way the things are done in symmetric crypto. As you see,
one can invent hundreds of hash functions per day, many of them being very
fast and seemingly secure. However, the cryptanalysis resources are
limited, so the entire community can properly study only a handful of
functions per year. Hence it's designer's job to select one (rarely two) of
many possible alternatives, study it himself, publish the analysis and
motivation (10+ pages usually), implement it and only then invite the
others to look at the design.

In your particular example, before anyone starts analysing it, you should
have answered the following questions publicly and convincingly:
1) why two states (not one, not three?)?
2) why and which rand_const (why one constant? why here constant? any
constant?)
3) why 127-i? (not 2i, not 3i, not pi*i)
4) why inverse_round?
5) why low and high (why not xor the two, why not only low, why not only
high)?

Similar questions apply to most of the PHC submissions, in fact.

Best regards,
Dmitry

On Wed, Apr 23, 2014 at 11:48 PM, Bill Cox <waywardgeek@...il.com> wrote:

> On Wed, Apr 23, 2014 at 2:33 PM, Bill Cox <waywardgeek@...il.com> wrote:
>
>> I think there are various possibilities for hashing functions like the
>> one above.  For example, how about using a large block hash function
>> more like:
>>
>> hashBlock(uint512 block[128], uint1024 state) {
>>     state1 = state
>>     state2 = state ^ RAND_CONST
>>     for(i = 0; i < 128; i++) {
>>         state1 = Blake2b_ROUND(state1, block[i])
>>         state2 = Blake2b_Inverse_ROUND(state2, block[127-i])
>>     }
>>     // Full Blake2b hash to combine state1 and state2
>>     return Blake2b(Blake2b(state1, low(state2)), high(state2))
>> }
>>
>
> I coded this for fun, based on Lyra2's modified version of Blake2b.  I'm
> not confident it's secure, but it does run fast.  It hashes files almost
> 3.5 GB/s, or just over 1 byte per clock.  Here's some speed comparisons:
>
> wghash (as I'm calling this hack): 3.48 GB/s
> md5sum: 0.72 GB/s
> sha256sum: 0.29 GB/s
>
> It's almost 5X faster than md5sum, and 12X faster than sha256sum.  It
> would be very cool of something like this works out.
>
> Bill
>

-- 
Best regards,
Dmitry Khovratovich

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