dec.py

import hashlib, string, sys

def h(x):
    x = hashlib.sha256(x).digest()
    x = xor(x[:16], x[16:])
    return x

def xor(a, b):
    l = min(len(a), len(b))
    return ''.join([chr(ord(x) ^ ord(y)) for x, y in zip(a[:l], b[:l])])


def crypt(msg, k): 
    out = ''
    for i in xrange(0, len(msg), 16):
        out += xor(msg[i:i+16], k)
        k = h(k + str(len(msg)))
 
    return out

def verify(x):
    return all([ord(c) < 127 for c in x])


#plaintext = 'From: Baron van '
plaintext = 'From: Vlugge Jap'
print len(plaintext)
#get first 16 bytes from msg002.enc
file_contents = open('msg002.enc','r').read().decode('base64') 
ciphertext = file_contents[0:16]
key = xor(plaintext,ciphertext)

out = crypt(file_contents,key)
if verify(out):
	print out
else:
    print 'Bad decrypt!'