bkcrack is a command-line tool which implements a known plaintext attack
against legacy encrypted ZIP archives. The vulnerability was shown by Eli Biham
and Paul C. Kocher in the research paper "A known plaintext attack on the PKZIP
stream cipher." The main features are:

* Recover internal state from ciphertext and plaintext.
* Remove or change a ZIP archive's password using the internal state.
* Recover the original password from the internal state.