Path: utzoo!mnetor!uunet!husc6!bloom-beacon!oberon!cit-vax!elroy!devvax!jackm
From: jackm@devvax.JPL.NASA.GOV (Jack Morrison)
Newsgroups: sci.crypt
Subject: Re: how do you tell encrytped data from random data?
Message-ID: <1098@devvax.JPL.NASA.GOV>
Date: 22 Jan 88 16:16:46 GMT
References: <660@bucket.UUCP> <39394@sun.uucp>
Reply-To: jackm@devvax.JPL.NASA.GOV (Jack Morrison)
Organization: Jet Propulsion Laboratory, Pasadena, CA.
Lines: 44
Summary: Flat != Random

In article <39394@sun.uucp> dunc@sun.UUCP (duncs home) writes:
>In article <660@bucket.UUCP> leonard@bucket.UUCP (Leonard Erickson) writes:
>>                               ...
>>I know that transposition and *simple* substitution can be detected by
>>letter frequency analysis. But is a "flat" distibution evidence of random
>>data?
>
>No.  Consider this simple substitution cipher:
>
>'a' = 'abcdefghijklmnopqrstuvwxyz'
>'b' = 'bcdefghijklmnopqrstuvwxyza'
>'c' = 'cdefghijklmnopqrstuvwxyzab'
>             ...
>'z' = 'zabcdefghijklmnopqrstuvwxy'
>
>The frequency distribution of the letters will be absolutely flat, yet it
>does contain a message.  This shows that a flat distribution does not mean
>absence of a message (and also that flat distribution, by itself, does not
>guarantee a good cipher!)


Cute, but keep in mind that a "distribution" can be more than just a histogram
of letter frequencies; the example cipher would have a very lopsided
distibution in other measurements, for example digram frequencies.
(I realize the proposal was not meant as a good cipher, and I'm just 
adding to the discussion; this is NOT a flame, I just don't see where to
stick the :-) !)

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Jack C. Morrison	Jet Propulsion Laboratory
(818)354-1463		jackm@jpl-devvax.jpl.nasa.gov
"The paycheck is part government property, but the opinions are all mine."