252 lines
8.8 KiB
Python
252 lines
8.8 KiB
Python
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# -*- coding: utf-8 -*-
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#
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# Cipher/mode_cfb.py : CFB mode
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#
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# ===================================================================
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# The contents of this file are dedicated to the public domain. To
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# the extent that dedication to the public domain is not available,
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# everyone is granted a worldwide, perpetual, royalty-free,
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# non-exclusive license to exercise all rights associated with the
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# contents of this file for any purpose whatsoever.
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# No rights are reserved.
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#
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
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# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
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# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
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# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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# SOFTWARE.
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# ===================================================================
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"""
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Counter Feedback (CFB) mode.
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"""
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__all__ = ['CfbMode']
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from Crypto.Util._raw_api import (load_pycryptodome_raw_lib, VoidPointer,
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create_string_buffer, get_raw_buffer,
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SmartPointer, c_size_t, expect_byte_string)
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from Crypto.Random import get_random_bytes
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raw_cfb_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_cfb","""
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int CFB_start_operation(void *cipher,
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const uint8_t iv[],
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size_t iv_len,
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size_t segment_len, /* In bytes */
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void **pResult);
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int CFB_encrypt(void *cfbState,
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const uint8_t *in,
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uint8_t *out,
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size_t data_len);
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int CFB_decrypt(void *cfbState,
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const uint8_t *in,
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uint8_t *out,
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size_t data_len);
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int CFB_stop_operation(void *state);"""
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)
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class CfbMode(object):
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"""*Cipher FeedBack (CFB)*.
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This mode is similar to CFB, but it transforms
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the underlying block cipher into a stream cipher.
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Plaintext and ciphertext are processed in *segments*
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of **s** bits. The mode is therefore sometimes
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labelled **s**-bit CFB.
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An Initialization Vector (*IV*) is required.
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See `NIST SP800-38A`_ , Section 6.3.
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.. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
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:undocumented: __init__
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"""
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def __init__(self, block_cipher, iv, segment_size):
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"""Create a new block cipher, configured in CFB mode.
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:Parameters:
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block_cipher : C pointer
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A smart pointer to the low-level block cipher instance.
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iv : byte string
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The initialization vector to use for encryption or decryption.
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It is as long as the cipher block.
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**The IV must be unpredictable**. Ideally it is picked randomly.
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Reusing the *IV* for encryptions performed with the same key
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compromises confidentiality.
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segment_size : integer
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The number of bytes the plaintext and ciphertext are segmented in.
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"""
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expect_byte_string(iv)
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self._state = VoidPointer()
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result = raw_cfb_lib.CFB_start_operation(block_cipher.get(),
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iv,
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c_size_t(len(iv)),
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c_size_t(segment_size),
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self._state.address_of())
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if result:
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raise ValueError("Error %d while instatiating the CFB mode" % result)
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# Ensure that object disposal of this Python object will (eventually)
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# free the memory allocated by the raw library for the cipher mode
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self._state = SmartPointer(self._state.get(),
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raw_cfb_lib.CFB_stop_operation)
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# Memory allocated for the underlying block cipher is now owed
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# by the cipher mode
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block_cipher.release()
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self.block_size = len(iv)
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"""The block size of the underlying cipher, in bytes."""
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self.iv = iv
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"""The Initialization Vector originally used to create the object.
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The value does not change."""
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self.IV = iv
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"""Alias for `iv`"""
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self._next = [ self.encrypt, self.decrypt ]
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def encrypt(self, plaintext):
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"""Encrypt data with the key and the parameters set at initialization.
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A cipher object is stateful: once you have encrypted a message
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you cannot encrypt (or decrypt) another message using the same
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object.
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The data to encrypt can be broken up in two or
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more pieces and `encrypt` can be called multiple times.
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That is, the statement:
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>>> c.encrypt(a) + c.encrypt(b)
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is equivalent to:
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>>> c.encrypt(a+b)
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This function does not add any padding to the plaintext.
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:Parameters:
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plaintext : byte string
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The piece of data to encrypt.
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It can be of any length.
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:Return:
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the encrypted data, as a byte string.
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It is as long as *plaintext*.
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"""
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if self.encrypt not in self._next:
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raise TypeError("encrypt() cannot be called after decrypt()")
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self._next = [ self.encrypt ]
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expect_byte_string(plaintext)
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ciphertext = create_string_buffer(len(plaintext))
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result = raw_cfb_lib.CFB_encrypt(self._state.get(),
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plaintext,
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ciphertext,
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c_size_t(len(plaintext)))
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if result:
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raise ValueError("Error %d while encrypting in CFB mode" % result)
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return get_raw_buffer(ciphertext)
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def decrypt(self, ciphertext):
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"""Decrypt data with the key and the parameters set at initialization.
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A cipher object is stateful: once you have decrypted a message
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you cannot decrypt (or encrypt) another message with the same
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object.
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The data to decrypt can be broken up in two or
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more pieces and `decrypt` can be called multiple times.
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That is, the statement:
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>>> c.decrypt(a) + c.decrypt(b)
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is equivalent to:
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>>> c.decrypt(a+b)
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This function does not remove any padding from the plaintext.
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:Parameters:
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ciphertext : byte string
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The piece of data to decrypt.
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It can be of any length.
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:Return: the decrypted data (byte string).
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"""
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if self.decrypt not in self._next:
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raise TypeError("decrypt() cannot be called after encrypt()")
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self._next = [ self.decrypt ]
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expect_byte_string(ciphertext)
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plaintext = create_string_buffer(len(ciphertext))
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result = raw_cfb_lib.CFB_decrypt(self._state.get(),
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ciphertext,
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plaintext,
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c_size_t(len(ciphertext)))
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if result:
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raise ValueError("Error %d while decrypting in CFB mode" % result)
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return get_raw_buffer(plaintext)
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def _create_cfb_cipher(factory, **kwargs):
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"""Instantiate a cipher object that performs CFB encryption/decryption.
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:Parameters:
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factory : module
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The underlying block cipher, a module from ``Crypto.Cipher``.
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:Keywords:
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iv : byte string
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The IV to use for CFB.
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IV : byte string
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Alias for ``iv``.
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segment_size : integer
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The number of bit the plaintext and ciphertext are segmented in.
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If not present, the default is 8.
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Any other keyword will be passed to the underlying block cipher.
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See the relevant documentation for details (at least ``key`` will need
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to be present).
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"""
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cipher_state = factory._create_base_cipher(kwargs)
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iv = kwargs.pop("IV", None)
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IV = kwargs.pop("iv", None)
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if (None, None) == (iv, IV):
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iv = get_random_bytes(factory.block_size)
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if iv is not None:
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if IV is not None:
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raise TypeError("You must either use 'iv' or 'IV', not both")
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else:
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iv = IV
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segment_size_bytes, rem = divmod(kwargs.pop("segment_size", 8), 8)
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if segment_size_bytes == 0 or rem != 0:
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raise ValueError("'segment_size' must be positive and multiple of 8 bits")
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if kwargs:
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raise TypeError("Unknown parameters for CFB: %s" % str(kwargs))
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return CfbMode(cipher_state, iv, segment_size_bytes)
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