Newsgroups: comp.compression
Path: utzoo!utgpu!cunews!dfs
From: dfs@doe.carleton.ca (David F. Skoll)
Subject: Compression as encryption?
Message-ID: <dfs.677194041@crusher>
Sender: news@cunews.carleton.ca
Organization: Carleton University
Date: Mon, 17 Jun 1991 21:27:21 GMT

It occurred to me that the standard Unix 'compress' utility would make
a good encryption tool, used as follows:

- Compress your text, using "compress"

- Strip out any headers or "magic numbers" from the .Z file

- Exclusive-or each character of the file with successive characters
from a password, preferably 8 to 10 characters long.  (In fact, if you
use two passwords whose lengths are relatively prime and are "well
chosen", you can achieve a password whose effective length is the
product of the lengths of the two original passwords, by cycling
through successive characters of both passwords and exclusive-oring
the original data with both characters.)

Now, the "exclusive-or" technique is very poor if you just apply it to
straight text, because it can be broken by guessing the length of the
password key, lining the codetext up into columns and using letter
frequencies as a guide to decryption.  However, the compress utility
tends to even out the frequencies of the characters, thus making this
approach very difficult.

Is there a catch?

--
David F. Skoll