def cbc_decrypt(key, data, pad=True, sbox=DEFAULT_SBOX, mesh=False):
""" CBC decryption mode of operation
:param bytes key: encryption key
:param bytes data: ciphertext
:type bool pad: perform ISO/IEC 7816-4 unpadding after decryption
:param sbox: S-box parameters to use
:type sbox: str, SBOXES'es key
:param bool mesh: enable key meshing
:returns: plaintext
:rtype: bytes
"""
validate_key(key)
validate_sbox(sbox)
if not data or len(data) % BLOCKSIZE != 0:
raise ValueError("Data is not blocksize aligned")
if len(data) < 2 * BLOCKSIZE:
raise ValueError("There is no either data, or IV in ciphertext")
iv = data[:BLOCKSIZE]
plaintext = []
for i in xrange(BLOCKSIZE, len(data), BLOCKSIZE):
if (
mesh and
(i - BLOCKSIZE) >= MESH_MAX_DATA and
(i - BLOCKSIZE) % MESH_MAX_DATA == 0
):
key, _ = meshing(key, iv, sbox=sbox)
plaintext.append(strxor(
ns2block(decrypt(sbox, key, block2ns(data[i:i + BLOCKSIZE]))),
data[i - BLOCKSIZE:i],
))
if pad:
plaintext[-1] = unpad2(plaintext[-1], BLOCKSIZE)
return b"".join(plaintext)
def test_symmetric(self):
for _ in range(100):
for blocksize in (8, 16):
data = urandom(randint(0, blocksize * 3))
self.assertSequenceEqual(
unpad2(pad2(data, blocksize), blocksize),
data,
)
def __init__(self, blockcipher, pad=None, unpad=None):
self.enc = blockcipher.encrypt
self.dec = blockcipher.decrypt
self.blen = blockcipher.blen
if pad == None:
self.padding = lambda x: gost3413.pad2(x, self.blen)
else:
self.padding = pad
if unpad == None:
self.unpadding = lambda x: gost3413.unpad2(x, self.blen)
else:
self.unpadding = unpad