PHDays Quals 2014: ecc (not really ECC..)

import hashlib
import sys

def xor(a, b):
    return "".join([chr(ord(a[i]) ^ ord(b[i % len(b)])) for i in xrange(len(a))])

def derive(i):
    return hashlib.sha256(str(i)).digest()

def decrypt(msg, key):
    iv = msg[:8]
    msg = msg[8:]
    stream = hashlib.sha256(iv + key).digest()
    stream = hashlib.sha256(stream + iv + key).digest()
    cipher = xor(msg, stream)
    return cipher

sa = set()
sb = set()
ea = "e86e73ce95d5be01428522838f464257af5f3f97577226b8ce".decode("hex")
eb = "d4c17fa1087b1bedca95579b783c43a8b5a18156036f36bbe8".decode("hex")

for i in xrange(100000) :
    key = derive(i)
    sa.add(decrypt(ea, key))
    sb.add(decrypt(eb, key))
    if sa & sb :
        print (sa & sb).pop()
        sys.exit(0)