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From: outer@utcsrgv.UUCP (Richard Outerbridge)
Newsgroups: net.crypt
Subject: Re: why 16 rounds in DES
Message-ID: <4005@utcsrgv.UUCP>
Date: Mon, 23-Apr-84 14:25:10 EST
Article-I.D.: utcsrgv.4005
Posted: Mon Apr 23 14:25:10 1984
Date-Received: Mon, 23-Apr-84 14:30:03 EST
Organization: CSRG, University of Toronto
Lines: 10

That raises another question: why use 32 "S"-boxes?  You have to use enough
to avoid invertible one-to-one mappings, and the DES fudges eight groups
of four bits in parallel.  From Konheim, exercise 6.1, "any least three"
(at least three?  any three?) of these eight must be many-(ie two)-to-one
mappings.  That's a minimum of sixteen.  Are the others only along to make
life interesting?  

Richard Outerbridge	<outer@utcsrgv>
-- 
Richard Outerbridge	<outer@utcsrgv>	416 978 2742