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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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"""
Ciphertext Block Chaining (CBC) mode.
"""
__all__ = ['CbcMode']
from Crypto.Util.py3compat import _copy_bytes
from Crypto.Util._raw_api import (load_pycryptodome_raw_lib, VoidPointer,
create_string_buffer, get_raw_buffer,
SmartPointer, c_size_t, c_uint8_ptr,
is_writeable_buffer)
from Crypto.Random import get_random_bytes
raw_cbc_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_cbc", """
int CBC_start_operation(void *cipher,
const uint8_t iv[],
size_t iv_len,
void **pResult);
int CBC_encrypt(void *cbcState,
const uint8_t *in,
uint8_t *out,
size_t data_len);
int CBC_decrypt(void *cbcState,
const uint8_t *in,
uint8_t *out,
size_t data_len);
int CBC_stop_operation(void *state);
"""
)
class CbcMode(object):
"""*Cipher-Block Chaining (CBC)*.
Each of the ciphertext blocks depends on the current
and all previous plaintext blocks.
An Initialization Vector (*IV*) is required.
See `NIST SP800-38A`_ , Section 6.2 .
.. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
:undocumented: __init__
"""
def __init__(self, block_cipher, iv):
"""Create a new block cipher, configured in CBC mode.
:Parameters:
block_cipher : C pointer
A smart pointer to the low-level block cipher instance.
iv : bytes/bytearray/memoryview
The initialization vector to use for encryption or decryption.
It is as long as the cipher block.
**The IV must be unpredictable**. Ideally it is picked randomly.
Reusing the *IV* for encryptions performed with the same key
compromises confidentiality.
"""
self._state = VoidPointer()
result = raw_cbc_lib.CBC_start_operation(block_cipher.get(),
c_uint8_ptr(iv),
c_size_t(len(iv)),
self._state.address_of())
if result:
raise ValueError("Error %d while instantiating the CBC mode"
% result)
# Ensure that object disposal of this Python object will (eventually)
# free the memory allocated by the raw library for the cipher mode
self._state = SmartPointer(self._state.get(),
raw_cbc_lib.CBC_stop_operation)
# Memory allocated for the underlying block cipher is now owed
# by the cipher mode
block_cipher.release()
self.block_size = len(iv)
"""The block size of the underlying cipher, in bytes."""
self.iv = _copy_bytes(None, None, iv)
"""The Initialization Vector originally used to create the object.
The value does not change."""
self.IV = self.iv
"""Alias for `iv`"""
self._next = [ self.encrypt, self.decrypt ]
def encrypt(self, plaintext, output=None):
"""Encrypt data with the key and the parameters set at initialization.
A cipher object is stateful: once you have encrypted a message
you cannot encrypt (or decrypt) another message using the same
object.
The data to encrypt can be broken up in two or
more pieces and `encrypt` can be called multiple times.
That is, the statement:
>>> c.encrypt(a) + c.encrypt(b)
is equivalent to:
>>> c.encrypt(a+b)
That also means that you cannot reuse an object for encrypting
or decrypting other data with the same key.
This function does not add any padding to the plaintext.
:Parameters:
plaintext : bytes/bytearray/memoryview
The piece of data to encrypt.
Its lenght must be multiple of the cipher block size.
:Keywords:
output : bytearray/memoryview
The location where the ciphertext must be written to.
If ``None``, the ciphertext is returned.
:Return:
If ``output`` is ``None``, the ciphertext is returned as ``bytes``.
Otherwise, ``None``.
"""
if self.encrypt not in self._next:
raise TypeError("encrypt() cannot be called after decrypt()")
self._next = [ self.encrypt ]
if output is None:
ciphertext = create_string_buffer(len(plaintext))
else:
ciphertext = output
if not is_writeable_buffer(output):
raise TypeError("output must be a bytearray or a writeable memoryview")
if len(plaintext) != len(output):
raise ValueError("output must have the same length as the input"
" (%d bytes)" % len(plaintext))
result = raw_cbc_lib.CBC_encrypt(self._state.get(),
c_uint8_ptr(plaintext),
c_uint8_ptr(ciphertext),
c_size_t(len(plaintext)))
if result:
if result == 3:
raise ValueError("Data must be padded to %d byte boundary in CBC mode" % self.block_size)
raise ValueError("Error %d while encrypting in CBC mode" % result)
if output is None:
return get_raw_buffer(ciphertext)
else:
return None
def decrypt(self, ciphertext, output=None):
"""Decrypt data with the key and the parameters set at initialization.
A cipher object is stateful: once you have decrypted a message
you cannot decrypt (or encrypt) another message with the same
object.
The data to decrypt can be broken up in two or
more pieces and `decrypt` can be called multiple times.
That is, the statement:
>>> c.decrypt(a) + c.decrypt(b)
is equivalent to:
>>> c.decrypt(a+b)
This function does not remove any padding from the plaintext.
:Parameters:
ciphertext : bytes/bytearray/memoryview
The piece of data to decrypt.
Its length must be multiple of the cipher block size.
:Keywords:
output : bytearray/memoryview
The location where the plaintext must be written to.
If ``None``, the plaintext is returned.
:Return:
If ``output`` is ``None``, the plaintext is returned as ``bytes``.
Otherwise, ``None``.
"""
if self.decrypt not in self._next:
raise TypeError("decrypt() cannot be called after encrypt()")
self._next = [ self.decrypt ]
if output is None:
plaintext = create_string_buffer(len(ciphertext))
else:
plaintext = output
if not is_writeable_buffer(output):
raise TypeError("output must be a bytearray or a writeable memoryview")
if len(ciphertext) != len(output):
raise ValueError("output must have the same length as the input"
" (%d bytes)" % len(plaintext))
result = raw_cbc_lib.CBC_decrypt(self._state.get(),
c_uint8_ptr(ciphertext),
c_uint8_ptr(plaintext),
c_size_t(len(ciphertext)))
if result:
if result == 3:
raise ValueError("Data must be padded to %d byte boundary in CBC mode" % self.block_size)
raise ValueError("Error %d while decrypting in CBC mode" % result)
if output is None:
return get_raw_buffer(plaintext)
else:
return None
def _create_cbc_cipher(factory, **kwargs):
"""Instantiate a cipher object that performs CBC encryption/decryption.
:Parameters:
factory : module
The underlying block cipher, a module from ``Crypto.Cipher``.
:Keywords:
iv : bytes/bytearray/memoryview
The IV to use for CBC.
IV : bytes/bytearray/memoryview
Alias for ``iv``.
Any other keyword will be passed to the underlying block cipher.
See the relevant documentation for details (at least ``key`` will need
to be present).
"""
cipher_state = factory._create_base_cipher(kwargs)
iv = kwargs.pop("IV", None)
IV = kwargs.pop("iv", None)
if (None, None) == (iv, IV):
iv = get_random_bytes(factory.block_size)
if iv is not None:
if IV is not None:
raise TypeError("You must either use 'iv' or 'IV', not both")
else:
iv = IV
if len(iv) != factory.block_size:
raise ValueError("Incorrect IV length (it must be %d bytes long)" %
factory.block_size)
if kwargs:
raise TypeError("Unknown parameters for CBC: %s" % str(kwargs))
return CbcMode(cipher_state, iv)
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