from Crypto.Cipher import AES

from lib.Message import Message

from lib.utils import get_passwd

class NonCompilantAsciiTextException(Exception):

def __init__(self, text):

self.text = text

def get_cypher():

return AES.new(get_passwd(), AES.MODE_CBC, IV=get_passwd())

def check_if_high_ascii_values(text):

try:

text.encode("utf8")

except UnicodeDecodeError:

raise NonCompilantAsciiTextException(text)

def main():

plaintext = 'a' * AES.block_size + 'b' * AES.block_size + 'c' * AES.block_size

ciphertext = get_cypher().encrypt(plaintext)

modified_ciphertext = \

ciphertext[:AES.block_size] + \

'\x00' * AES.block_size + \

ciphertext[:AES.block_size]

try:

check_if_high_ascii_values(

get_cypher().decrypt(modified_ciphertext)

)

except NonCompilantAsciiTextException as e:

c1 = e.text[:AES.block_size]

c2 = e.text[AES.block_size*2:AES.block_size*3]

password = Message(c1).xor(c2).to_str()

assert password == get_passwd()

print 'password is', repr(password)

if __name__ == '__main__':

import argparse

parser = argparse.ArgumentParser(

description='Recover the key from CBC with IV=Key - Challenge 27 (Set 4) of Matasano Crypto Challenge.')

args = parser.parse_args()

main()