phc-discussions - Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel Attacks

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Date: Mon, 4 May 2015 14:03:31 +0200 (CEST)
From: Stefan.Lucks@...-weimar.de
To: "discussions@...sword-hashing.net" <discussions@...sword-hashing.net>
Subject: Re: [PHC] Maximising Pseudo-Entropy versus resistance to Side-Channel
 Attacks

On Thu, 30 Apr 2015, Bill Cox wrote:

> On Thu, Apr 30, 2015 at 5:43 AM, <Stefan.Lucks@...-weimar.de> wrote:

>       As an example, let the password hashing function H1 be ten times faster than H2, i.e., if Mallory's costs for H1 are be ten times
>       higher than his costs for H2 (for the same budget on the defender's side). Ten times faster sounds like a big deal, doesn't it?
> 
> 
> Mallory's cost will be 100X higher in this case.  Memory*time defense goes as the square of the runtime.  So, that's 6.64 additional bits of
> strength, not 3.32.

No! Say H2 runs in 1 second allocating 10 MB, while H2, allocating the 
same 10 MB is ten times faster 0.1 second. If the defender is willing 
to wait 1 second, for either H1 or H2, H1 could then allocate 100 MB.

cost for H1: 100 sMB (seconds * Megabyte)

cost for H2: 10 sMb (sconds * Megabyte)

So the pseudo-entropy for H1 is 3.32 bit larger than the pseudo-entropy 
for H2, not 6.64.

Stefan

------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
uni-weimar.de/de/medien/professuren/mediensicherheit/people/stefan-lucks
--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--