implementing DES-X (mode CBC) using PyCrypto

desx.py

'''
This code implements DES-X mode CBC efficiently using PyCrypto
'''
from Crypto.Cipher import DES

def des_x(plaintext, key, iv='\0'*8):
    'key must be 24 bytes, plaintext must be multiple of 8 bytes'
    pre_whiten  = XOR.new(key[ 8:16]).encrypt #XOR -- encrypt/decrypt are the same
    post_whiten = XOR.new(key[16:24]).encrypt
    encrypt = DES.new(key[:8], mode=DES.MODE_ECB).encrypt
    out = []
    prev_xor = XOR.new(iv).encrypt
    for i in xrange(len(key)/8):
        cur_plain = plaintext[i*8:i*8+8]
        cur_cipher = post_whiten(encrypt(pre_whiten(prev_xor(cur_plain))))
        out.append(cur_cipher)
        prev_xor = XOR.new(cur_cipher).encrypt
    return b"".join(out)

def un_des_x(ciphertext, key, iv='\0'*8):
    'key must be 24 bytes, plaintext must be multiple of 8 bytes'
    pre_whiten  = XOR.new(key[ 8:16]).decrypt
    post_whiten = XOR.new(key[16:24]).decrypt
    decrypt = DES.new(key[:8], mode=DES.MODE_ECB).decrypt
    out = []
    prev_xor = XOR.new(iv).decrypt
    for i in xrange(len(ciphertext)/8):
        cur_cipher = ciphertext[i*8:i*8+8]
        cur_plain = prev_xor(pre_whiten(decrypt(post_whiten(cur_cipher))))
        out.append(cur_plain)
        prev_xor = XOR.new(cur_cipher).decrypt
    return b"".join(out)