moth/puzzles/crypto/cbc.py

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2009-10-15 12:34:58 -06:00
def cipherBlockChainingE(key, IV, C, text):
"""Cypher block chaining encryption. Works in blocks the size of IV.
@param key: the key for the Cipher.
@param IV: initialization vector (bytes object).
@param C: the cypher function C(text, key).
@param text: A bytes object of the text. The length of the text
must be a multiple of the length of the IV.
"""
mod = len(text) % len(IV)
assert mod == 0, 'The text length needs to be a multiple of the key '\
'length. %d of %d' % (mod, len(IV))
feedback = IV
block = len(IV)
out = bytearray()
while text:
p, text = text[:block], text[block:]
c = bytearray(block)
for i in range(block):
c[i] = p[i] ^ feedback[i]
c2 = C(c, key)
out.extend(c2)
feedback = c2
return bytes(out)
def cipherBlockChainingD(key, IV, C, text):
"""Cipher block chaining decryption. Arguments are the same as for the
encrypting function."""
mod = len(text) % len(IV)
assert mod == 0, 'The text length needs to be a multiple of the IV '\
'length. %d of %d' % (mod, len(IV))
feedback = IV
block = len(IV)
out = bytearray()
while text:
c, text = text[:block], text[block:]
p = C(c, key)
p = bytearray(p)
for i in range(block):
p[i] = p[i] ^ feedback[i]
out.extend(p)
feedback = c
return bytes(out)