49 lines
1.9 KiB
Python
49 lines
1.9 KiB
Python
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class AESModeOfOperationCBC(AESBlockModeOfOperation):
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'''AES Cipher-Block Chaining Mode of Operation.
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o The Initialization Vector (IV)
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o Block-cipher, so data must be padded to 16 byte boundaries
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o An incorrect initialization vector will only cause the first
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block to be corrupt; all other blocks will be intact
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o A corrupt bit in the cipher text will cause a block to be
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corrupted, and the next block to be inverted, but all other
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blocks will be intact.
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Security Notes:
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o This method (and CTR) ARE recommended.
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Also see:
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o https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher-block_chaining_.28CBC.29
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o See NIST SP800-38A (http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf); section 6.2'''
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name = "Cipher-Block Chaining (CBC)"
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def __init__(self, key, iv = None):
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if iv is None:
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self._last_cipherblock = [ 0 ] * 16
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elif len(iv) != 16:
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raise ValueError('initialization vector must be 16 bytes')
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else:
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self._last_cipherblock = _string_to_bytes(iv)
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AESBlockModeOfOperation.__init__(self, key)
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def encrypt(self, plaintext):
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if len(plaintext) != 16:
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raise ValueError('plaintext block must be 16 bytes')
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plaintext = _string_to_bytes(plaintext)
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precipherblock = [ (p ^ l) for (p, l) in zip(plaintext, self._last_cipherblock) ]
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self._last_cipherblock = self._aes.encrypt(precipherblock)
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return _bytes_to_string(self._last_cipherblock)
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def decrypt(self, ciphertext):
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if len(ciphertext) != 16:
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raise ValueError('ciphertext block must be 16 bytes')
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cipherblock = _string_to_bytes(ciphertext)
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plaintext = [ (p ^ l) for (p, l) in zip(self._aes.decrypt(cipherblock), self._last_cipherblock) ]
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self._last_cipherblock = cipherblock
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return _bytes_to_string(plaintext)
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