def decode_repeating_byte_xor(ciphertext: bytes):
c = ciphertext
edit_distances = defaultdict(list)
for keysize in range(2, 41):
k, e = keysize, []
e.append(normalised_hamming_distance(c[:k], c[k:k * 2]))
e.append(normalised_hamming_distance(c[k * 2:k * 3], c[k * 3:k * 4]))
e.append(normalised_hamming_distance(c[k * 3:k * 4], c[k * 4:k * 5]))
e.append(normalised_hamming_distance(c[k * 4:k * 5], c[k * 5:k * 6]))
edit_distances[sum(e) / 4].append(keysize)
likely_keysizes = sorted(edit_distances.items())[:5]
def reducing_func(x: List[int], y: Tuple[Any, List[int]]) -> List[Any]:
return x + y[1]
possible_keysizes = reduce(reducing_func, likely_keysizes, [])
keys = [] # type: List[str]
for keysize in possible_keysizes:
blocks = split_into_groups(ciphertext, keysize)
if len(blocks[-1]) != keysize:
del blocks[-1]
transposed_blocks = zip(*blocks)
key = []
for block in transposed_blocks:
char, _ = decode_1_byte_xor(block)
key.append(char)
keys.append(''.join(key))
keys = [x for x in keys if x]
plaintexts = {} # Dict[str, str]
for key in keys:
plaintexts[
repeating_key_xor(
ciphertext, bytes(key, 'ascii')).decode('ascii')] = key
plaintext = find_english_text(plaintexts.keys())
return plaintexts[plaintext], plaintext
def aes_cbc_encode(plaintext: bytes, password: bytes, iv: bytes) -> bytes:
blocks = split_into_groups(plaintext, 16)
res = []
prev_block = iv
for block in blocks:
prev_block = aes_ecb_encode(xor(prev_block, block), password)
res.append(prev_block)
return b''.join(res)
def detect_aes_ecb_encrypted_texts(ciphertexts: List[bytes]):
max_repeats = defaultdict(list)
for text in ciphertexts:
counts = Counter(split_into_groups(text, 16))
most_no_of_repeats = counts.most_common(1)[0][1]
max_repeats[most_no_of_repeats].append(text)
most_repeats = max(max_repeats.keys())
return most_repeats, max_repeats[most_repeats]
def aes_cbc_decode(ciphertext: bytes, password: bytes, iv: bytes) -> bytes:
blocks = split_into_groups(ciphertext, 16)
res = []
# Sort out the first block on its own, as it requires iv
res.append(xor(aes_ecb_decode(blocks[0], password), iv))
# Sort out all other blocks
for i, block in enumerate(blocks[1:], 1):
res.append(xor(aes_ecb_decode(block, password), blocks[i - 1]))
return b''.join(res)
