phc-discussions - Re: [PHC] Lyra, Password Key Derivation Based On The Sponge Construction

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Date: Wed, 08 Jan 2014 23:08:04 +0000
From: "Poul-Henning Kamp" <phk@....freebsd.dk>
To: discussions@...sword-hashing.net, Bill Cox <waywardgeek@...il.com>
Subject: Re: [PHC] Lyra, Password Key Derivation Based On The Sponge Construction

In message <CAOLP8p5wwnaOpPGW0rA+Q9nz-jYtKhEL0aujMALuRuG=8zQtRg@...l.gmail.com>
, Bill Cox writes:

>Thanks for this very interesting link.  Lyra first fills a matrix with hash
>data which is derived from the password, and then randomly picks a "row"
>and for each location it updates the hash state from the location's value,
>and then XORs into the location the next output of the hashing engine.

Two things worry me about the general approach Lyra takes.

My first thought was that this sounds vulnerable to the same issue
RC4 suffers from:   It takes more entropy to "randomize" it properly
than is typically available for the purpose.

Lack of entropy is a major issue in any password context, and therefore
I think it is wise to pay attention to the:

	bits of entropy
	---------------
	bits of state

ratio not getting too low.

The second thought is that a large memory footprint, desirable for
all the reasons the Lyra presentation mentions, vastly increases
the ways and means to discover what is going on through covert
channels.

So as a general principle, I'm personally not going to be very
impressed by claims on the general form:

	"The ${datestructure} can be sized however large you like"

even if it comes with a mathematically proof of ${something}, unless
it also comes with plan for how it gets initialized with only limited
entropy being available, and analysis of to what extent the access
patterns may reveal its state.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@...eBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.