Crack legacy zip encryption with Biham and Kocher's known plaintext attack.

You can get the latest official release on GitHub.

Precompiled packages for Ubuntu, MacOS and Windows are available for download. Extract the downloaded archive wherever you like.

Alternatively, you can compile the project with CMake.

First, download the source files or clone the git repository. Then, running the following commands in the source tree will create an installation in the install folder.

cmake -S . -B build -DCMAKE_INSTALL_PREFIX=install
cmake --build build --config Release
cmake --build build --config Release --target install

An unofficial package bkcrack-git is available in AUR.

Install it with any AUR helpers you like.

The attack requires at least 12 bytes of known plaintext. At least 8 of them must be contiguous. The larger the contiguous known plaintext, the faster the attack.

Having a zip archive encrypted.zip with the entry cipher being the ciphertext and plain.zip with the entry plain as the known plaintext, bkcrack can be run like this:

bkcrack -C encrypted.zip -c cipher -P plain.zip -p plain

Having a file cipherfile with the ciphertext (starting with the 12 bytes corresponding to the encryption header) and plainfile with the known plaintext, bkcrack can be run like this:

bkcrack -c cipherfile -p plainfile

If the plaintext corresponds to a part other than the beginning of the ciphertext, you can specify an offset. It can be negative if the plaintext includes a part of the encryption header.

bkcrack -c cipherfile -p plainfile -o offset

If you know little contiguous plaintext (between 8 and 11 bytes), but know some bytes at some other known offsets, you can provide this information to reach the requirement of a total of 12 known bytes. To do so, use the -x flag followed by an offset and bytes in hexadecimal.

bkcrack -c cipherfile -p plainfile -x 25 4b4f -x 30 21

If bkcrack was built with parallel mode enabled, the number of threads used can be set through the environment variable OMP_NUM_THREADS.

If the attack is successful, the deciphered data associated to the ciphertext used for the attack can be saved:

bkcrack -c cipherfile -p plainfile -d decipheredfile

If the keys are known from a previous attack, it is possible to use bkcrack to decipher data:

bkcrack -c cipherfile -k 12345678 23456789 34567890 -d decipheredfile

The deciphered data might be compressed depending on whether compression was used or not when the zip file was created. If deflate compression was used, a Python 3 script provided in the tools folder may be used to decompress data.

python3 tools/inflate.py < decipheredfile > decompressedfile

It is also possible to generate a new encrypted archive with the password of your choice:

bkcrack -C encrypted.zip -k 12345678 23456789 34567890 -U unlocked.zip password

The archive generated this way can be extracted using any zip file utility with the new password. It assumes that every entry was originally encrypted with the same password.

Given the internal keys, bkcrack can try to find the original password up to a given length:

bkcrack -k 1ded830c 24454157 7213b8c5 -r 10 ?p

A tutorial is provided in the example folder.

For more information, have a look at the documentation and read the source.

Do not hesitate to suggest improvements or submit pull requests on GitHub.

This project is provided under the terms of the zlib/png license.