From: "Brendan McKenna" <brendan@moe.oc3s-emh1.army.mil>
Date: Thu, 12 Jan 95 05:22:32 PST
To: cypherpunks@toad.com
Subject: Cryptanalysis
Message-ID: <9501121322.AA22913@toad.com>
MIME-Version: 1.0
Content-Type: text/plain


Hi,

	I light of recent threads about recognising whether or not a given 
message/file is encrypted, and using CBW and things along those lines, is there
any way to determine how something was encrypted?  For example, I know that
a statistical analysis of the cyphertext will uncover simple substitution
cyphers fairly quickly.  Does the same sort of analysis apply to determining
whether something was encrypted using IDEA or DES or RSA?  I realize that they
attempt to maximize the entropy of the cyphertext -- perhaps there is some
characteristic amount or range of amounts of entropy associated with these
cyphers?  Not every package is as nice as PGP in labeling everything it
encrypts with headers...   Any pointers would be greatly appreciated....


						Brendan

PS.  What I'd like to be able to do is take a given chunk of cyphertext and
     analyze it and say: "There is an x% probability that this was encrypted
     using method y...."  Hopefully I'd have a reasonable chance of recognizing
     how it was encrypted, and not all of the percentages would be so low as
     to make the exercise meaningless.