def test_challenge16(self):
key = encryption_key()
init_vector = iv()
ciphertext = encrypt_kvps('admin=true', key, init_vector)
self.assertFalse(
is_admin(ciphertext, key, init_vector),
'Should have escaped the ='
)
# 0 1 2 3 4
# comment1=cooking%20MCs;userdata=true;comment2=%20like%20a%20pound%20of%20bacon
# comment1=cooking%20MCs;us;admin=true;comment2=%20like%20a%20pound%20of%20bacon
ciphertext = encrypt_kvps('true', key, init_vector)
# Block 0 ciphertext is XOR'ed against the decryptor output of block 1
# to produce the plaintext output. Twiddling a bit in the block 0
# ciphertext will invert the corresponding bit in the block 1 plaintext
combined = zip(
ciphertext[:16],
'%20MCs;userdata=',
'%20MCs;us;admin='
)
twiddled = ''.join(
twiddle_bits(iv_byte, actual_char, target_char)
for iv_byte, actual_char, target_char in combined
)
ciphertext = twiddled + ciphertext[16:]
self.assertTrue(is_admin(ciphertext, key, init_vector))
def encrypt_random_choice(key):
selection = secret_messages[random.randint(0, len(secret_messages) - 1)]
init_vector = iv()
return (
cbc_encrypt(selection, key, init_vector),
init_vector,
)
def multimode_oracle(plaintext, mode):
tampered = random_padding() + plaintext + random_padding()
key = encryption_key()
if mode == 'ecb':
return ecb_encrypt(tampered, key)
elif mode == 'cbc':
return cbc_encrypt(tampered, key, iv())
raise ValueError('Unknown mode')
def echo(self, message):
s = pow(self.A, self.b, self.p)
key = Sha1Hash().update(to_bytes_le(s)).digest()[:16]
ciphertext = message[:-16]
iv = message[-16:]
plaintext = crypto.cbc_decrypt(ciphertext, key, iv)
self.messages.append(plaintext)
iv = crypto.iv()
return crypto.cbc_encrypt(plaintext, key, iv) + iv
def send(self, receiver, message):
self.messages.append(message)
B = receiver.handshake(self.p, self.g, self.A)
s = pow(B, self.a, self.p)
key = Sha1Hash().update(to_bytes_le(s)).digest()[:16]
iv = crypto.iv()
ciphertext = crypto.cbc_encrypt(message, key, iv) + iv
returned_message = receiver.echo(ciphertext)
returned_ciphertext = returned_message[:-16]
returned_iv = returned_message[-16:]
decrypted = crypto.cbc_decrypt(
returned_ciphertext,
key,
returned_iv
)
return decrypted