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✏️ Editing: _mode_cbc.py

# ===================================================================
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# Copyright (c) 2014, Legrandin <helderijs@gmail.com>
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# 1. Redistributions of source code must retain the above copyright
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#    the documentation and/or other materials provided with the
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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 = ["encrypt", "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 "encrypt" not in self._next:
            raise TypeError("encrypt() cannot be called after decrypt()")
        self._next = ["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 "decrypt" not in self._next:
            raise TypeError("decrypt() cannot be called after encrypt()")
        self._next = ["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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