Path: illuminati.io.com!uunet!MathWorks.Com!europa.eng.gtefsd.com!news.umbc.edu! olson From: olson@umbc.edu (Bryan G. Olson; CMSC (G)) Newsgroups: sci.crypt Subject: Re: Toward Axiomatic Fenced DES (long!) Date: 2 Jun 1994 18:53:05 GMT Organization: University of Maryland, Baltimore County Lines: 16 Message-ID: <2sl9qh$nbj@news.umbc.edu> References: <199405261829.NAA24225@indial1.io.com> <2sjjjg$he0@news.umbc.edu> NNTP-Posting-Host: umbc7.umbc.edu X-Newsreader: TIN [version 1.2 PL2] Bryan G. Olson; CMSC (G) (olson@umbc.edu) wrote: : THEOREM: Given n-bit strings a and b with a <> b, if P(x) is chosen : (independently of (x,y) ) from a uniform distribution of all : permutations of the set of n-bit strings, then the expected Hamming : weight of P(a) xor P(b) is: : 2**(n-1) / (2**n -1) : (The Hamming weight of the xor is the number of different bits.) Oops. That's the probability a given bit has changed. The expected number of changed bits is n times this. : --Bryan --Bryan