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From: truett@cup.portal.com
Newsgroups: sci.crypt
Subject: Re: how do you tell encrytped data from
Message-ID: <2415@cup.portal.com>
Date: 12 Jan 88 10:45:02 GMT
References: <660@bucket.UUCP> <3600002@osiris.cso.uiuc.edu>
Organization: The Portal System (TM)
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XPortal-User-Id: 1.1001.2190

Randomness or flatness of the probability distribution of the result is neither
necessary nor sufficient for a good encryptor.

First, it is not necessary.  Consider the one time pad.  In a binary stream, a
one time pad will convert "random" streams to "nonrandom" ones just as often as
it converts "nonrandom" (presumably plaintext) streams to apparently "random"
ones.  The only way to camouflage this effect is to increase the size of the
apparent alphabet or state space of the message channel (somebody correct me if
I'm wrong here.).

It isn't sufficient either.  Consider the linear congruential pseudorandum
number generator:  y = (ax + b) mod (n).  For suitable a,b,n, you can use the
successive characters of the message as values of x and the resulting stream of
y values will be quite random.  Another way is to use a data compression scheme
like the ARC system used on many electronic bulletin board systems.  The
reduction of redundancy in a data stream makes it look more like noise than the
original.

Actually, good random number generators make quite good encryptors.  So do
ECC generators.  Remember that Shannon demonstrated that the closer a noisy
channel got to zero error, the more the data stream has to look like noise.
Ergo:  apply a really good ECC to the data stream (thus adding redundant data,
see my comment above) and "BINGO" you have a good encryptor.

Again, the answer is NO, you can't tell noise and encrypted data apart without
a known encryption system.  In the latter case, you tell by decrypting.

Truett Lee Smith, Sunnyvale, CA
UUCP:  truett@cup.portal.com