Pkcrack

Utility for breaking pkzip encryption

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.