pyDES

Author:	Todd Whiteman
Issue Date:	28th April, 2010
Version:	2.0.1
Compatibility:	Requires Python 2.2 or higher, an older Python 1.5.2 compatible module can be found in the CVS source.

Download (Windows):	pyDes-2.0.1.zip
Download (Unix):	pyDes-2.0.1.tar.gz

License:	Free - Public Domain
Source code:	SourceForge: pyDes
Last updated:	28th April, 2010

About pyDES

This is a pure python implementation of the DES encryption algorithm. It is in pure python to avoid portability issues, since most DES implementations are programmed in C (for performance reasons).

Triple DES class is also implemented, utilising the DES base. Triple DES is either DES-EDE3 with a 24 byte key, or DES-EDE2 with a 16 byte key. See the "About triple DES" section below more info on this algorithm.

The code below is not written for speed or performance, so not for those needing a fast des implementation, but rather a handy portable solution ideal for small usage. It takes my AMD2000+ machine 1 second per 2.5 kilobyte to encrypt or decrypt using the DES method. That is extremely SLOW!!

About triple DES

Triple DES is just running the DES algorithm 3 times over the data with the specified key. The supplied key is split up into 3 parts, each part being 8 bytes long (the mandatory key size for DES).

The triple DES algorithm uses the DES-EDE3 method when a 24 byte key is supplied. This means there are three DES operations in the sequence encrypt-decrypt-encrypt with the three different keys. The first key will be bytes 1 to 8, the second key bytes 9 to 16 and the third key bytes 17 to 24.

If a 16 byte key is supplied instead, the triple DES method used will be DES-EDE2. This means there are three DES operations in the sequence encrypt-decrypt-encrypt, but the first and third operations use the same key. The first/third key will be bytes 1 to 8 and the second key bytes 9 to 16.

Credits

Thanks go to:

David Broadwell for his ideas, comments and suggestions.
Mario Wolff for finding errors in triple des CBC.
Santiago Palladino for providing the PKCS5 padding technique.
Shaya for correcting the PAD_PKCS5 triple des CBC errors.
Yoav Aner for spotting a triple des CBC IV error.

Installation

Extract the files from the pyDes archive.
Run the following command: python setup.py install
To test, run: python test_pydes.py

Note: On Unix, you'd run this command from a shell prompt; on Windows, you have to open a command prompt window (``DOS box'') and do it there;

pyDes Usage

Class initialization
--------------------
pyDes.des(key, [mode], [IV], [pad], [padmode])
pyDes.triple_des(key, [mode], [IV], [pad], [padmode])

key     -> Bytes containing the encryption key. 8 bytes for DES, 16 or 24 bytes
	   for Triple DES
mode    -> Optional argument for encryption type, can be either
	   pyDes.ECB (Electronic Code Book) or pyDes.CBC (Cypher Block Chaining)
IV      -> Optional Initial Value bytes, must be supplied if using CBC mode.
	   Length must be 8 bytes.
pad     -> Optional argument, set the pad character (PAD_NORMAL) to use during
	   all encrypt/decrpt operations done with this instance.
padmode -> Optional argument, set the padding mode (PAD_NORMAL or PAD_PKCS5)
	   to use during all encrypt/decrpt operations done with this instance.

I recommend to use PAD_PKCS5 padding, as then you never need to worry about any
padding issues, as the padding can be removed unambiguously upon decrypting
data that was encrypted using PAD_PKCS5 padmode.

Common methods
--------------
encrypt(data, [pad], [padmode])
decrypt(data, [pad], [padmode])

data    -> Bytes to be encrypted/decrypted
pad     -> Optional argument. Only when using padmode of PAD_NORMAL. For
	   encryption, adds this characters to the end of the data block when
	   data is not a multiple of 8 bytes. For decryption, will remove the
	   trailing characters that match this pad character from the last 8
	   bytes of the unencrypted data block.
padmode -> Optional argument, set the padding mode, must be one of PAD_NORMAL
	   or PAD_PKCS5). Defaults to PAD_NORMAL.
	  

Example
-------
from pyDes import *

# For Python3, you'll need to use bytes, i.e.:
#   data = b"Please encrypt my data"
#   k = des(b"DESCRYPT", CBC, b"\0\0\0\0\0\0\0\0", pad=None, padmode=PAD_PKCS5)

data = "Please encrypt my data"
k = des("DESCRYPT", CBC, "\0\0\0\0\0\0\0\0", pad=None, padmode=PAD_PKCS5)
d = k.encrypt(data)
print "Encrypted: %r" % d
print "Decrypted: %r" % k.decrypt(d)
assert k.decrypt(d, padmode=PAD_PKCS5) == data



See the module source (pyDes.py) for more examples of use.
You can slo run the pyDes.py file without and arguments to see a simple test.

Note: This code was not written for high-end systems needing a fast
      implementation, but rather a handy portable solution with small usage.