blowfish

blowfish

This module implements the Blowfish cipher using only Python (3.4+).

Blowfish is a block cipher that can be used for symmetric-key encryption. It has a 8-byte block size and supports a variable-length key, from 4 to 56 bytes. It's fast, free and has been analyzed considerably. It was designed by Bruce Schneier and more details about it can be found at https://www.schneier.com/blowfish.html.

.. contents:: :local: :backlinks: top

Dependencies

• Python 3.4+

Features

• Fast (well, as fast you can possibly go using only Python 3.4+)
• Efficient; generators/iterators are used liberally to reduce memory usage
• Electronic Codebook (ECB) mode
• Electronic Codebook with Ciphertext Stealing (ECB-CTS) mode
• Cipher-Block Chaining (CBC) mode
• Cipher-Block Chaining with Ciphertext Stealing (CBC-CTS) mode
• Propagating Cipher-Block Chaining (PCBC) mode
• Cipher Feedback (CFB) mode
• Output Feedback (OFB) mode
• Counter (CTR) mode

Installation

If you just need a Blowfish cipher in your Python project, feel free to manually copy blowfish.py to your package directory (license permitting).

distutils ######### To install the module to your Python distribution, use the included distutils script::

$ python setup.py install

pip

Stable versions can be installed from pypi_ using pip::

$ pip install blowfish

pip can also install the latest development version directly from git::

$ pip install 'git+https://github.com/jashandeep-sohi/python-blowfish.git'

.. _pypi: https://pypi.python.org/pypi/blowfish

Development

Want to add a mode of operation? Speed up encryption?

Make your changes to a clone of the repository at https://github.com/jashandeep-sohi/python-blowfish and send me a pull request.

Tests

Tests are written using the Python unittest framework. All tests are in the test.py file and can be run using::

$ python -m unittest test.py

Bugs

Are you having problems? Please let me know at https://github.com/jashandeep-sohi/python-blowfish/issues

Usage

.. warning::

Cryptography is complex, so please don't use this module in anything
*critical* without understanding what you are doing and checking the source
code to make sure it is doing what you want it to.

.. note::

This is just a quick overview on how to use the module. For detailed
documentation please see the `docstrings` in the module.

First create a Cipher object with a key.

.. code:: python3

import blowfish

cipher = blowfish.Cipher(b"Key must be between 4 and 56 bytes long.")

By default this initializes a Blowfish cipher that will interpret bytes using the big-endian byte order. Should the need arrise to use the little-endian byte order, provide "little" as the second argument.

.. code:: python3

cipher_little = blowfish.Cipher(b"my key", byte_order = "little")

Block

To encrypt or decrypt a block of data (8 bytes), use the encrypt_block or decrypt_block methods of the Cipher object.

.. code:: python3

from os import urandom

block = urandom(8)

ciphertext = cipher.encrypt_block(block)
plaintext = cipher.decrypt_block(ciphertext)

assert block == plaintext

As these methods can only operate on 8 bytes of data, they're of little practical use. Instead, use one of the implemented modes of operation.

Electronic Codebook Mode (ECB) ############################## To encrypt or decrypt data in ECB mode, use encrypt_ecb or decrypt_ecb methods of the Cipher object. ECB mode can only operate on data that is a multiple of the block-size in length.

.. code:: python3

data = urandom(10 * 8) # data to encrypt

data_encrypted = b"".join(cipher.encrypt_ecb(data))
data_decrypted = b"".join(cipher.decrypt_ecb(data_encrypted)

assert data == data_decrypted

Electronic Codebook Mode with Cipher Text Stealing (ECB-CTS) ############################################################ To encrypt or decrypt data in ECB-CTS mode, use encrypt_ecb_cts or decrypt_ebc_cts methods of the Cipher object. ECB-CTS mode can operate on data of any length greater than 8 bytes.

.. code:: python3

data = urandom(10 * 8 + 5) # data to encrypt

data_encrypted = b"".join(cipher.encrypt_ecb_cts(data))
data_decrypted = b"".join(cipher.decrypt_ecb_cts(data_encrypted))

assert data == data_decrypted

Cipher-Block Chaining Mode (CBC) ################################ To encrypt or decrypt data in CBC mode, use encrypt_cbc or decrypt_cbc methods of the Cipher object. CBC mode can only operate on data that is a multiple of the block-size in length.

.. code:: python3

data = urandom(10 * 8) # data to encrypt
iv = urandom(8) # initialization vector

data_encrypted = b"".join(cipher.encrypt_cbc(data, iv))
data_decrypted = b"".join(cipher.decrypt_cbc(data_encrypted, iv))

assert data == data_decrypted

Cipher-Block Chaining with Ciphertext Stealing (CBC-CTS) ######################################################## To encrypt or decrypt data in CBC-CTS mode, use encrypt_cbc_cts or decrypt_cbc_cts methods of the Cipher object. CBC-CTS mode can operate on data of any length greater than 8 bytes.

.. code:: python3

data = urandom(10 * 8 + 6) # data to encrypt
iv = urandom(8) # initialization vector

data_encrypted = b"".join(cipher.encrypt_cbc_cts(data, iv))
data_decrypted = b"".join(cipher.decrypt_cbc_cts(data_encrypted, iv))

assert data == data_decrypted

Propagating Cipher-Block Chaining Mode (PCBC) ############################################# To encrypt or decrypt data in PCBC mode, use encrypt_pcbc or decrypt_pcbc methods of the Cipher object. PCBC mode can only operate on data that is a multiple of the block-size in length.

.. code:: python3

data = urandom(10 * 8) # data to encrypt
iv = urandom(8) # initialization vector

data_encrypted = b"".join(cipher.encrypt_pcbc(data, iv))
data_decrypted = b"".join(cipher.decrypt_pcbc(data_encrypted, iv))

assert data == data_decrypted

Cipher Feedback Mode (CFB) ########################## To encrypt or decrypt data in CFB mode, use encrypt_cfb or decrypt_cfb methods of the Cipher object. CFB mode can operate on data of any length.

.. code:: python3

data = urandom(10 * 8 + 7) # data to encrypt
iv = urandom(8) # initialization vector

data_encrypted = b"".join(cipher.encrypt_cfb(data, iv))
data_decrypted = b"".join(cipher.decrypt_cfb(data_encrypted, iv))

assert data == data_decrypted

Output Feedback Mode (OFB) ########################## To encrypt or decrypt data in OFB mode, use encrypt_ofb or decrypt_ofb methods of the Cipher object. OFB mode can operate on data of any length.

.. code:: python3

data = urandom(10 * 8 + 1) # data to encrypt
iv = urandom(8) # initialization vector

data_encrypted = b"".join(cipher.encrypt_ofb(data, iv))
data_decrypted = b"".join(cipher.decrypt_ofb(data_encrypted, iv))

assert data == data_decrypted

Counter Mode (CTR) ################## To encrypt or decrypt data in CTR mode, use encrypt_ctr or decrypt_ctr methods of the Cipher object. CTR mode can operate on data of any length. Although you can use any counter you want, a simple increment by one counter is secure and the most popular. So for convenience sake a simple increment by one counter is implemented by the blowfish.ctr_counter function. However, you should implement your own for optimization purposes.

.. code:: python3

from operator import xor

data = urandom(10 * 8 + 2) # data to encrypt

# increment by one counters
nonce = int.from_bytes(urandom(8), "big")
enc_counter = blowfish.ctr_counter(nonce, f = xor)
dec_counter = blowfish.ctr_counter(nonce, f = xor)

data_encrypted = b"".join(cipher.encrypt_ctr(data, enc_counter))
data_decrypted = b"".join(cipher.decrypt_ctr(data_encrypted, dec_counter))

assert data == data_decrypted

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