exercise_13

#!/usr/bin/env python3

# Copyright 2014 Carl Winbäck. See the COPYING file at the top-level directory
# of this distribution.

from Crypto.Cipher import AES
from os import urandom
from random import SystemRandom
import re
from libpals.util import pkcs7pad, pkcs7pad_remove

# Some assumptions:
#
# A) The attacker is aware of the ”profile format”. (E.g.
# ”email=foo@bar.com&uid=10&role=user”)
#
# B) The attacker-provided string doesn’t have to be a valid e-mail address.
#
# C) The attacker can insert padding. (I.e. non-printable ASCII characters.)

SECRET_KEY = urandom(16)


def parse(input_string):
    list_of_pairs = input_string.split(sep="&")
    result = {}
    for kv_string in list_of_pairs:
        kv_pair = kv_string.split(sep="=")
        key = kv_pair[0]
        value = kv_pair[1]
        result[key] = value
    return result


def generate_profile(email_address):
    sanitized_address = re.sub('[&=]', '', email_address)
    uid = SystemRandom().randint(1, 99)
    result = """email={}&uid={:0>2d}&role=user""".format(sanitized_address,
                                                         uid)
    return result


def encrypt(input_string):
    cipher = AES.new(SECRET_KEY, AES.MODE_ECB)
    input_bytes = input_string.encode('ascii')
    plaintext = pkcs7pad(input_bytes, 16)
    ciphertext = cipher.encrypt(plaintext)
    return ciphertext


def decrypt(ciphertext):
    cipher = AES.new(SECRET_KEY, AES.MODE_ECB)
    plaintext_bytes = pkcs7pad_remove(cipher.decrypt(ciphertext))
    plaintext_string = plaintext_bytes.decode('ascii')
    return plaintext_string


attacker_input = ('XXXXXXXXXXadmin\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b\x0b'
                  'XXXXXXX@example.com')

original_ciphertext = encrypt(generate_profile(attacker_input))

# By rearranging the ciphertext, Mallory can influence the resulting plaintext.
evil_ciphertext = original_ciphertext[0:64] + original_ciphertext[16:32]

# Creating a profile from the modified ciphertext
profile = parse(decrypt(evil_ciphertext))

output_message = """SET 02 CHALLENGE 13: ECB cut-and-paste

Attacker input: {}

Resulting profile: {}"""

print(output_message.format(repr(attacker_input),
                            profile))