This package implements an algorithm for breaking the PkZip cipher that was
devised by Eli Biham and Paul Kocher.

This program applies a known plaintext attack to an encrypted file.
A known-plaintext-attack recovers a password using the encrypted file and
(part of) the unencrypted file.

Please note that cryptographers use the word 'plaintext' for any kind of
unencrypted data - not necessarily readable ASCII text.

Before you ask why somebody may want to know the password when he already knows
the plaintext think of the following situations:

 - Usually there's a large number of files in a ZIP-archive. Usually all these
   files are encrypted using the same password. So if you know one of the files,
   you can recover the password and decrypt the other files.
 - You need to know only a part of the plaintext (at least 13 bytes). Many files
   have commonly known headers, like DOS .EXE-files. Knowing a reasonably long
   header you can recover the password and decrypt the entire file.

WWW: http://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html