Newsgroups: comp.compression Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!batcomputer!cornell!wayner From: wayner@CS.Cornell.EDU (Peter Wayner) Subject: Re: Compression as encryption? Message-ID: <1991Jun18.002240.27628@cs.cornell.edu> Sender: news@cs.cornell.edu (USENET news user) Nntp-Posting-Host: kama.cs.cornell.edu Organization: Cornell Univ. CS Dept, Ithaca NY 14853 References: Date: Tue, 18 Jun 1991 00:22:40 GMT Lines: 42 dfs@doe.carleton.ca (David F. Skoll) writes: >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? No. Just read the cryptography literature before doing this if you are curious. People have been worrying about this problem for a long time. There is plenty of good analysis of the problem. One good book was written by Dorthy Denning but the others are quite good as well. I wrote a paper on one twist, in fact. It describes a way of making Huffman compression somewhat more difficult to decrypt. I'll send you a copy if you want, or you can find it in Cryptologia, April 1988. >-- >David F. Skoll -- Peter Wayner Department of Computer Science Cornell Univ. Ithaca, NY 14850 EMail:wayner@cs.cornell.edu Office: 607-255-9202 or 255-1008 Home: 116 Oak Ave, Ithaca, NY 14850 Phone: 607-277-6678