File size: 1,782 Bytes
55e2289 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 | from typing import Callable
BLOCK_SIZE = 16 # 128 bit for Twofish
def xor_bytes(a: bytes, b: bytes) -> bytes:
return bytes(x ^ y for x, y in zip(a, b))
def left_shift_one(bitstring: bytes) -> bytes:
out = bytearray(len(bitstring))
carry = 0
for i in reversed(range(len(bitstring))):
new = (bitstring[i] << 1) & 0xFF
out[i] = new | carry
carry = (bitstring[i] & 0x80) >> 7
return bytes(out)
def generate_subkeys(encrypt_block: Callable[[bytes], bytes]):
const_rb = 0x87
zero = bytes(BLOCK_SIZE)
L = encrypt_block(zero)
K1 = left_shift_one(L)
if L[0] & 0x80:
K1 = xor_bytes(K1, b'\x00' * 15 + bytes([const_rb]))
K2 = left_shift_one(K1)
if K1[0] & 0x80:
K2 = xor_bytes(K2, b'\x00' * 15 + bytes([const_rb]))
return K1, K2
def pad(block: bytes) -> bytes:
padded = block + b'\x80'
return padded + b'\x00' * (BLOCK_SIZE - len(padded))
class CMAC:
def __init__(self, encrypt_block: Callable[[bytes], bytes]):
self.encrypt_block = encrypt_block
self.K1, self.K2 = generate_subkeys(encrypt_block)
def digest(self, data: bytes) -> bytes:
if len(data) == 0:
last = xor_bytes(pad(b''), self.K2)
blocks = []
else:
blocks = [data[i:i+BLOCK_SIZE] for i in range(0, len(data), BLOCK_SIZE)]
if len(blocks[-1]) == BLOCK_SIZE:
last = xor_bytes(blocks[-1], self.K1)
blocks = blocks[:-1]
else:
last = xor_bytes(pad(blocks[-1]), self.K2)
blocks = blocks[:-1]
X = bytes(BLOCK_SIZE)
for block in blocks:
X = self.encrypt_block(xor_bytes(X, block))
return self.encrypt_block(xor_bytes(X, last))
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