Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!hao!gatech!bloom-beacon!mit-eddie!bbn!rochester!PT.CS.CMU.EDU!NL.CS.CMU.EDU!mlm
From: mlm@NL.CS.CMU.EDU (Michael Mauldin)
Newsgroups: sci.crypt
Subject: Re: One time pads?
Message-ID: <976@PT.CS.CMU.EDU>
Date: 26 Feb 88 23:02:54 GMT
References: <4209@june.cs.washington.edu> <1988Feb15.151522.5094@utzoo.uucp> <1857@phoenix.Princeton.EDU>
Sender: netnews@PT.CS.CMU.EDU
Organization: Carnegie-Mellon University, CS/RI
Lines: 21
Summary: using real text as a key

In article <1857@phoenix.Princeton.EDU>, amlovell@phoenix.Princeton.EDU (Anthony M Lovell) writes:
> What if the n+5th message is the key (again adulterated in some
> form) for the nth message?  Any scheme like this will be impregnable
> until guessed, and its patterns are certainly unlike those typically
> searched for.

When using English text as the "key" in a stream cipher, the patterns
in English show through into the ciphertext.  For example, when the
word 'the' is encrypted against the word 'the', you get repeated
occurrences of a three letter word.  Denning82 (Cryptography and Data
Security) discuesses ways to crack these kinds of systems.

"Impregnable until guessed" reminds me of a quote from my old DiffyQ
text in college:

	"Bacteria are virtually indestructable unless killed"

Aren't we all.

Michael L. Mauldin (Fuzzy)		Department of Computer Science
ARPA: Michael.Mauldin@NL.CS.CMU.EDU	Carnegie-Mellon University
Phone: (412) 268-3065			Pittsburgh, PA  15213-3890