This package implements an algorithm that was developed by Eli Biham and Paul Kocher and that is described in this paper (Postscript, 80k).
The attack is a known plaintext attack, which means you have to know part of the encrypted data in order to break the cipher.
You can take a look at the README-file or the changelog of the package, or just download the sources (Gzip'd tar-file, 170k).