Path: utzoo!mnetor!uunet!husc6!think!ames!amdahl!oliveb!sun!moria!dunc
From: dunc%moria@Sun.COM (duncs home)
Newsgroups: sci.crypt
Subject: Re: how do you tell encrytped data from random data?
Message-ID: <39394@sun.uucp>
Date: 20 Jan 88 15:54:15 GMT
References: <660@bucket.UUCP>
Sender: news@sun.uucp
Reply-To: dunc@sun.UUCP (duncs home)
Organization: Sun Microsystems, Mountain View
Lines: 18
Summary: Flat distribution does not imply random data.

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!)