phc-discussions - Re: [PHC] my pre (if) submit proposal

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Date: Mon, 20 Jan 2014 09:13:44 +0100
From: Krisztián Pintér <pinterkr@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] my pre (if) submit proposal

Steve Thomas (at Monday, January 20, 2014, 12:37:58 AM):

>> https://docs.google.com/document/d/18R-qEAmL9WWh5zhGeBlvI7C6ikBAz6TF7MEtfPJK7m0

> This is broken:  
>     * Mem is only read once. So once read you can discard it.     

>     * In the paper you state f=-1 is a perfect choice. Except from i=0 to t/2-1 you
>     are just reading zeros. So you don't even need the second half of Mem.

this is not broken and also not true. the number of zero-reads is m/2,
independent of t. the number of unnecessary writes is also m/2. it is
explained in the document. it is not a break, because it does not
affect the minimum memory needed, and omitting reads and writes does
not matter when the algorithm is cpu-bound.

i considered some tuning to address this, but discarded for its
awkwardness and negative impact on simplicity.

>     * With (C * (t/2) ** (1/2)) ram it will take 2 times longer. C is the size of the
>     context of your sponge function. In general this is (C * size ** (1/N)) ram and
>     N times more computations. Max N is ln(size). I keep hearing about "parallel
>     cores" I believe it's the same thing or at least similar.      

it is not a break, and i'm not sure if true.

not a break, because sequential memory hardness is not claimed.

i don't understand your algorithm. by context, i assume you mean
state. if you want to calculate a missing state, you need to evaluate
a whole bunch of other states since it relies on r other states, and
those also rely on other states, and so on.

i don't have the math, but i believe with the maximum m, the cost is
indeed n^1.5, but if you go higher with the t/m ratio, it should rise,
hopefully toward n^2 as limit.

it is questionable whether sequential memory hard functions exist
without indexing by secret.