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This is a simple python implementation of the XTEA block encryption algorithm (http://www.cix.co.uk/~klockstone/xtea.pdf). For details please see docstrings in the code.

Python, 128 lines
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""" 
XTEA Block Encryption Algorithm

Author: Paul Chakravarti (paul_dot_chakravarti_at_gmail_dot_com)
License: Public Domain

This module provides a Python implementation of the XTEA block encryption
algorithm (http://www.cix.co.uk/~klockstone/xtea.pdf). 

The module implements the basic XTEA block encryption algortithm
(`xtea_encrypt`/`xtea_decrypt`) and also provides a higher level `crypt`
function which symmetrically encrypts/decrypts a variable length string using
XTEA in OFB mode as a key generator. The `crypt` function does not use
`xtea_decrypt` which is provided for completeness only (but can be used
to support other stream modes - eg CBC/CFB).

This module is intended to provide a simple 'privacy-grade' Python encryption
algorithm with no external dependencies. The implementation is relatively slow
and is best suited to small volumes of data. Note that the XTEA algorithm has
not been subjected to extensive analysis (though is believed to be relatively
secure - see http://en.wikipedia.org/wiki/XTEA). For applications requiring
'real' security please use a known and well tested algorithm/implementation.

The security of the algorithm is entirely based on quality (entropy) and
secrecy of the key. You should generate the key from a known random source and
exchange using a trusted mechanism. In addition, you should always use a random
IV to seed the key generator (the IV is not sensitive and does not need to be
exchanged securely)

    >>> import os
    >>> iv = 'ABCDEFGH'
    >>> z = crypt('0123456789012345','Hello There',iv)
    >>> z.encode('hex')
    'fe196d0a40d6c222b9eff3'
    >>> crypt('0123456789012345',z,iv)
    'Hello There'

""" 

import struct

def crypt(key,data,iv='\00\00\00\00\00\00\00\00',n=32):
    """
        Encrypt/decrypt variable length string using XTEA cypher as
        key generator (OFB mode)
        * key = 128 bit (16 char) 
        * iv = 64 bit (8 char)
        * data = string (any length)

        >>> import os
        >>> key = os.urandom(16)
        >>> iv = os.urandom(8)
        >>> data = os.urandom(10000)
        >>> z = crypt(key,data,iv)
        >>> crypt(key,z,iv) == data
        True

    """
    def keygen(key,iv,n):
        while True:
            iv = xtea_encrypt(key,iv,n)
            for k in iv:
                yield ord(k)
    xor = [ chr(x^y) for (x,y) in zip(map(ord,data),keygen(key,iv,n)) ]
    return "".join(xor)

def xtea_encrypt(key,block,n=32,endian="!"):
    """
        Encrypt 64 bit data block using XTEA block cypher
        * key = 128 bit (16 char) 
        * block = 64 bit (8 char)
        * n = rounds (default 32)
        * endian = byte order (see 'struct' doc - default big/network) 

        >>> z = xtea_encrypt('0123456789012345','ABCDEFGH')
        >>> z.encode('hex')
        'b67c01662ff6964a'

        Only need to change byte order if sending/receiving from 
        alternative endian implementation 

        >>> z = xtea_encrypt('0123456789012345','ABCDEFGH',endian="<")
        >>> z.encode('hex')
        'ea0c3d7c1c22557f'

    """
    v0,v1 = struct.unpack(endian+"2L",block)
    k = struct.unpack(endian+"4L",key)
    sum,delta,mask = 0L,0x9e3779b9L,0xffffffffL
    for round in range(n):
        v0 = (v0 + (((v1<<4 ^ v1>>5) + v1) ^ (sum + k[sum & 3]))) & mask
        sum = (sum + delta) & mask
        v1 = (v1 + (((v0<<4 ^ v0>>5) + v0) ^ (sum + k[sum>>11 & 3]))) & mask
    return struct.pack(endian+"2L",v0,v1)

def xtea_decrypt(key,block,n=32,endian="!"):
    """
        Decrypt 64 bit data block using XTEA block cypher
        * key = 128 bit (16 char) 
        * block = 64 bit (8 char)
        * n = rounds (default 32)
        * endian = byte order (see 'struct' doc - default big/network) 

        >>> z = 'b67c01662ff6964a'.decode('hex')
        >>> xtea_decrypt('0123456789012345',z)
        'ABCDEFGH'

        Only need to change byte order if sending/receiving from 
        alternative endian implementation 

        >>> z = 'ea0c3d7c1c22557f'.decode('hex')
        >>> xtea_decrypt('0123456789012345',z,endian="<")
        'ABCDEFGH'

    """
    v0,v1 = struct.unpack(endian+"2L",block)
    k = struct.unpack(endian+"4L",key)
    delta,mask = 0x9e3779b9L,0xffffffffL
    sum = (delta * n) & mask
    for round in range(n):
        v1 = (v1 - (((v0<<4 ^ v0>>5) + v0) ^ (sum + k[sum>>11 & 3]))) & mask
        sum = (sum - delta) & mask
        v0 = (v0 - (((v1<<4 ^ v1>>5) + v1) ^ (sum + k[sum & 3]))) & mask
    return struct.pack(endian+"2L",v0,v1)

if __name__ == "__main__":
    import doctest
    doctest.testmod()

I originally implementaed this module due to the lack of a simple encryption module in the Python library which could be used for 'light-duty' encyption of small volumes of data. Please ensure that you understand the limitations described in the module. For 'serious' requirements you should use a known algorithm/library - eg. PyCrypto (http://www.amk.ca/python/code/crypto).

9 Jan 2007 - Added support for endian parameter in xtea_encrypt/xtea_decrypt (patch from Christian Heimes - thanks) - this allows compatibility with other XTEA implementations which may use a different byte order (ie. not big/network endian)

1 Mar 2007 - Christian Heimes has released PEP 272 compatible XTEA implementation with optional pyrex and c-based optimisation - this is available at http://pymoul.svn.sourceforge.net/viewvc/pymoul/crypttea/trunk/

5 comments

emily maison 15 years, 6 months ago  # | flag

error. when i started this code, i got:

File "xtea.py", line 33, in __main__ Failed example: z.encode('hex') Expected: 'fe196d0a40d6c222b9eff3' Got: 'dbb956b5e4c7be33f6cb5d'


1 items had failures: 1 of 5 in __main__ Test Failed 1 failures.

Exit code: 0

Paul Chakravarti (author) 15 years, 6 months ago  # | flag

Doctests Fixed. Sorry for the problem - I made some changes to the doctest to the master version and tried to edit the version here to match however got this wrong. This should now work correctly.

Thanks for pointing this out.

Paul

Josiah Carlson 15 years, 4 months ago  # | flag

With the extended Euclid GCD algorithm and large (likely) prime generation recipes here in the cookbook, it would be fairly trivial to write an RSA cryptosystem implementation in Python, which is usable for more than just obfuscation.

Collin Stocks 13 years, 2 months ago  # | flag

Thanks, Josiah, for the inspiration.

My ("Vanilla") RSA implementation: http://code.activestate.com/recipes/572196/

ztwaker 8 years, 6 months ago  # | flag
Created by Paul Chakravarti on Thu, 25 May 2006 (PSF)
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