mirror of
https://github.com/Legrandin/pycryptodome.git
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268 lines
10 KiB
Python
268 lines
10 KiB
Python
# -*- coding: utf-8 -*-
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#
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# Cipher/mode_ctr.py : CTR 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 (CTR) mode.
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"""
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__all__ = ['CtrMode']
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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.Util.py3compat import *
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raw_ctr_lib = load_pycryptodome_raw_lib("Crypto.Cipher._raw_ctr", """
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int CTR_start_operation(void *cipher,
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uint8_t initialCounterBlock[],
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size_t initialCounterBlock_len,
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size_t prefix_len,
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unsigned counter_len,
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unsigned littleEndian,
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void **pResult);
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int CTR_encrypt(void *ctrState,
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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 CTR_decrypt(void *ctrState,
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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 CTR_stop_operation(void *ctrState);"""
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)
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class CtrMode(object):
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"""*CounTeR (CTR)* mode.
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This mode is very similar to ECB, in that
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encryption of one block is done independently of all other blocks.
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Unlike ECB, the block *position* contributes to the encryption
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and no information leaks about symbol frequency.
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Each message block is associated to a *counter* which
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must be unique across all messages that get encrypted
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with the same key (not just within the same message).
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The counter is as big as the block size.
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Counters can be generated in several ways. The most
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straightword one is to choose an *initial counter block*
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(which can be made public, similarly to the *IV* for the
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other modes) and increment its lowest **m** bits by one
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(modulo *2^m*) for each block. In most cases, **m** is
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chosen to be half the block size.
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See `NIST SP800-38A`_, Section 6.5 (for the mode) and
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Appendix B (for how to manage the *initial counter block*).
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.. _`NIST SP800-38A` : http://csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf
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"""
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def __init__(self, block_cipher, initial_counter_block,
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prefix_len, counter_len, little_endian):
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"""Create a new block cipher, configured in CTR 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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initial_counter_block : byte string
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The initial plaintext to use to generate the key stream.
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It is as large as the cipher block, and it embeds
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the initial value of the counter.
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This value must not be reused.
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It shall contain a nonce or a random component.
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Reusing the *initial counter block* for encryptions
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performed with the same key compromises confidentiality.
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prefix_len : integer
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The amount of bytes at the beginning of the counter block
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that never change.
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counter_len : integer
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The length in bytes of the counter embedded in the counter
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block.
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little_endian : boolean
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True if the counter in the counter block is an integer encoded
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in little endian mode. If False, it is big endian.
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"""
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expect_byte_string(initial_counter_block)
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self._state = VoidPointer()
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result = raw_ctr_lib.CTR_start_operation(block_cipher.get(),
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initial_counter_block,
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c_size_t(len(initial_counter_block)),
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c_size_t(prefix_len),
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counter_len,
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little_endian,
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self._state.address_of())
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if result:
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raise ValueError("Error %X while instatiating the CTR mode"
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% 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_ctr_lib.CTR_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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#: The block size of the underlying cipher, in bytes.
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self.block_size = len(initial_counter_block)
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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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expect_byte_string(plaintext)
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ciphertext = create_string_buffer(len(plaintext))
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result = raw_ctr_lib.CTR_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 %X while encrypting in CTR 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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expect_byte_string(ciphertext)
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plaintext = create_string_buffer(len(ciphertext))
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result = raw_ctr_lib.CTR_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 %X while decrypting in CTR mode" % result)
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return get_raw_buffer(plaintext)
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def _create_ctr_cipher(factory, **kwargs):
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"""Instantiate a cipher object that performs CTR 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 CBC.
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IV : byte string
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Alias for ``iv``.
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counter : object
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Instance of ``Crypto.Util.Counter``.
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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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try:
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counter = kwargs.pop("counter")
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except KeyError:
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# Required by unit test
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raise TypeError("Missing 'counter' parameter for CTR mode")
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# 'counter' used to be a callable object, but now it is
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# just a dictionary for backward compatibility.
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counter_len = counter.pop("counter_len")
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prefix = counter.pop("prefix")
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suffix = counter.pop("suffix")
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initial_value = counter.pop("initial_value")
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little_endian = counter.pop("little_endian")
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# Compute initial counter block
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words = []
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while initial_value > 0:
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words.append(bchr(initial_value & 255))
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initial_value >>= 8
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words += [bchr(0)] * max(0, counter_len - len(words))
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if not little_endian:
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words.reverse()
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initial_counter_block = prefix + b("").join(words) + suffix
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if kwargs:
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raise ValueError("Unknown parameters for CTR mode: %s"
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% str(kwargs))
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return CtrMode(cipher_state, initial_counter_block,
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len(prefix), counter_len, little_endian)
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