phc-discussions - Re: [PHC] A (naively?) simple PHC submission using hash chains

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Message-ID: <CAK9dnSzWr59uS0aUWfpGKu=TXjAut21v-czihBjUhP8OGyAwdg@mail.gmail.com>
Date: Wed, 7 Aug 2013 11:57:37 +0200
From: CodesInChaos <codesinchaos@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] A (naively?) simple PHC submission using hash chains

The complete algorithm with a sequential implementation runs in pmem time
using pmem memory even with efficient prepending.
=> cost is pmem^2 using (memory+cores)*time as cost metric.

One can argue that (memory+cores)*time is only a good metric for an
attacker, not for the defender because the defender is often time-bound,
not memory bound.
In that case using pmem as cost for the defender might  be a better metric,
as long as they don't run out of RAM.
But even when you compare with equal defender time, this algorithm isn't as
good as scrypt, since scrypt costs an attacker pmem^2,
but this algo costs the attacker pmem^{3/2}, or even less.

 (I'm using ptime=1 here, since ptime isn't relevant for this attack)

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