def test_challenge26(self):
key = crypto.encryption_key()
nonce = '\x00' * 8
ciphertext = encrypt_kvps('admin=true', key, nonce)
self.assertFalse(is_admin(ciphertext, key, nonce),
'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, nonce)
self.assertFalse(is_admin(ciphertext, key, nonce))
# ciphertext ^ orig plaintext = keystream
# keystream ^ (desired plaintext) = new ciphertext to splice in
start = 25
orig_plaintext = 'erdata'
keystream = bitops.xor(ciphertext[start:start + len(orig_plaintext)],
orig_plaintext)
tampered_chunk = bitops.xor(keystream, ';admin')
tampered_ciphertext = (ciphertext[:start] + tampered_chunk +
ciphertext[start + len(tampered_chunk):])
self.assertTrue(is_admin(tampered_ciphertext, key, nonce))
def hmac(key, message):
padding_length = sha1.Sha1Hash.block_size - len(key)
if padding_length > 0:
key += '\x00' * padding_length
else:
key = sha1.Sha1Hash().update(key).digest()
full_message = (
xor(key, '\x5c' * len(key)) +
sha1.Sha1Hash().update(xor(key, '\x36' * len(key))).digest() + message)
return sha1.Sha1Hash().update(full_message).digest()
def set_plaintext_char(self, index, pos, plaintext_val):
if plaintext_val is None:
self.keystream[pos] = None
else:
self.keystream[pos] = xor(
self.ciphertexts[index][pos],
plaintext_val
)
def test_challenge2(self):
hex_input1 = '1c0111001f010100061a024b53535009181c'
hex_input2 = '686974207468652062756c6c277320657965'
xor_result = xor(hex_to_bytes(hex_input1), hex_to_bytes(hex_input2))
self.assertEquals(
'746865206b696420646f6e277420706c6179',
bytes_to_hex(xor_result)
)
def test_challenge20(self):
plaintexts = [
base64_to_bytes(line)
for line in utils.readlines('20.txt')
]
key = encryption_key()
nonce = '\0' * 8
ciphertexts = [
ctr_encrypt(m, key, nonce)
for m in plaintexts
]
# Because of the fixed-nonce, the encrypted keystream bytes are
# repeated for every plaintext message.
#
# ciphertext[i] ^ keystream[i] = plaintext[i]
#
# We can create a transposed ciphertext message by concatenating
# ciphertext[i] from every encrypted message and then xor'ing that
# against a guessed keystream byte. Then we can test whether the
# resulting plaintext looks like english based on character
# distributions. If so, then we've figured out the keystream byte.
keystream = ''
for index in itertools.count():
transposed = ''.join(m[index:index+1] for m in ciphertexts)
if not transposed:
break
allowed_chars = None
if index == 0:
allowed_chars = string.ascii_uppercase + '"\''
score, _, key = crack.find_best_single_byte_key(
transposed,
allowed_chars=allowed_chars
)
# print 'Best score for index {}: {}'.format(index, score)
keystream += key[0]
recovered_plaintexts = [
bitops.xor(m, keystream) for m in ciphertexts
]
# for m in recovered_plaintexts:
# print m
self.assertIn(
'\'Cause my girl is definitely mad / \'Cause it took us too long to do this album',
recovered_plaintexts
)
def cbc_decrypt(ciphertext, key, iv):
backend = default_backend()
cipher = Cipher(algorithms.AES(key), modes.ECB(), backend=backend)
decryptor = cipher.decryptor()
plaintext = ''
block_size = len(iv)
for start in range(0, len(ciphertext), block_size):
end = start + block_size
ciphertext_block = ciphertext[start:end]
plaintext += xor(iv, decryptor.update(ciphertext_block))
iv = ciphertext_block
return strip_pkcs_7(plaintext)
def find_best_single_byte_key(rawbytes, freq_vector=None, allowed_chars=None):
best_key = None
best_translation = None
best_score = -1.0
for key in single_byte_keys(len(rawbytes)):
translation = xor(rawbytes, key)
score = english_score(translation,
freq_vector=freq_vector,
allowed_chars=allowed_chars)
if score > best_score:
best_score = score
best_key = key
best_translation = translation
return (best_score, best_translation, best_key)
def cbc_encrypt(plaintext, key, iv):
backend = default_backend()
cipher = Cipher(algorithms.AES(key), modes.ECB(), backend=backend)
encryptor = cipher.encryptor()
padded = pkcs_7(plaintext, len(iv))
ciphertext = ''
for start in range(0, len(padded), len(iv)):
end = start + len(iv)
scrambled = xor(iv, padded[start:end])
ciphertext_block = encryptor.update(scrambled)
ciphertext += ciphertext_block
iv = ciphertext_block
return ciphertext
def test_challenge25(self):
ciphertext = convert.base64_to_bytes(utils.read('25.txt'))
plaintext = crypto.ecb_decrypt(ciphertext, 'YELLOW SUBMARINE')
key = crypto.encryption_key()
nonce = '\x00' * 8
ciphertext = crypto.ctr_encrypt(plaintext, key, nonce)
# The edit function lets us recover the original keystream because
# in CTR mode: plaintext ^ keystream = ciphertext and then through
# the magic of xor:
#
# plaintext ^ ciphertext = keystream
#
# Through the edit function, we know the plaintext and ciphertext
# for every index in the byte stream.
keystream = ''
for offset in range(len(ciphertext)):
new_ciphertext = edit(ciphertext, key, nonce, offset, 'A')
keystream += bitops.xor(new_ciphertext[offset], 'A')
recovered_plaintext = bitops.xor(ciphertext, keystream)
self.assertEquals(plaintext, recovered_plaintext)
def test_challenge27(self):
key = crypto.encryption_key()
ciphertext = encrypt_kvps_cbc('hello', key)
tampered = (ciphertext[:16] + ('\x00' * 16) + ciphertext[:16] +
ciphertext[16:])
try:
is_admin_cbc(tampered, key)
except ValueError as e:
expected_prefix = 'Invalid message '
plaintext = e.message[len(expected_prefix):]
# C1 = 0
# P2 = D(E(P0 ^ IV)) ^ C1
# P2 = P0 ^ IV ^ 0
# P2 = P0 ^ IV
# The first block in the recovered plaintext is P0 and iv = key so:
# P0 ^ P2 = IV = KEY
#
# The 3rd plaintext block is
# D(E(P0 ^ IV)) ^ 0
recovered_key = bitops.xor(plaintext[:16], plaintext[32:48])
self.assertEqual(key, recovered_key)
def test_xor_trim_to_shortest(self):
self.assertEqual('\x00', bitops.xor('\x01', '\x01\x02'))
def _decrypt_char(self, index, ciphertext_val):
if self.keystream[index] is None:
return None
else:
return xor(self.keystream[index], ciphertext_val)
def twister_convert(text, key):
r = myrandom.MT19937(key & 0xffff)
keystream = (chr(r.next() & 0xff) for _ in itertools.count())
return ''.join(xor(a, b) for a, b in zip(text, keystream))
def ctr_convert(text, key, nonce):
blocks = (text[s:(s + 16)] for s in range(0, len(text), 16))
return ''.join(
xor(a, b) for a, b in zip(blocks, _ctr_keystream(key, nonce)))
