103 lines
4 KiB
Python
103 lines
4 KiB
Python
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# -*- coding: ascii -*-
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#
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# Util/Counter.py : Fast counter for use with CTR-mode ciphers
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#
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# Written in 2008 by Dwayne C. Litzenberger <dlitz@dlitz.net>
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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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"""Fast counter functions for CTR cipher modes.
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CTR is a chaining mode for symmetric block encryption or decryption.
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Messages are divideded into blocks, and the cipher operation takes
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place on each block using the secret key and a unique *counter block*.
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The most straightforward way to fulfil the uniqueness property is
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to start with an initial, random *counter block* value, and increment it as
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the next block is processed.
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The block ciphers from `Crypto.Cipher` (when configured in *MODE_CTR* mode)
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invoke a callable object (the *counter* parameter) to get the next *counter block*.
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Unfortunately, the Python calling protocol leads to major performance degradations.
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The counter functions instantiated by this module will be invoked directly
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by the ciphers in `Crypto.Cipher`. The fact that the Python layer is bypassed
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lead to more efficient (and faster) execution of CTR cipher modes.
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An example of usage is the following:
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>>> from Crypto.Cipher import AES
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>>> from Crypto.Util import Counter
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>>> from Crypto import Random
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>>>
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>>> nonce = Random.get_random_bytes(8)
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>>> ctr = Counter.new(64, nonce)
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>>> key = b'AES-128 symm key'
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>>> plaintext = b'X'*1000000
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>>> cipher = AES.new(key, AES.MODE_CTR, counter=ctr)
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>>> ciphertext = cipher.encrypt(plaintext)
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"""
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from Crypto.Util.py3compat import *
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def new(nbits, prefix=b(""), suffix=b(""), initial_value=1, little_endian=False, allow_wraparound=False):
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"""Create a stateful counter block function suitable for CTR encryption modes.
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Each call to the function returns the next counter block.
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Each counter block is made up by three parts::
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prefix || counter value || postfix
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The counter value is incremented by 1 at each call.
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:Parameters:
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nbits : integer
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Length of the desired counter value, in bits. It must be a multiple of 8.
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prefix : byte string
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The constant prefix of the counter block. By default, no prefix is
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used.
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suffix : byte string
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The constant postfix of the counter block. By default, no suffix is
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used.
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initial_value : integer
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The initial value of the counter. Default value is 1.
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little_endian : boolean
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If *True*, the counter number will be encoded in little endian format.
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If *False* (default), in big endian format.
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allow_wraparound : boolean
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This parameter is ignored.
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:Returns:
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An object that can be passed with the 'counter' parameter to a CTR mode
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cipher.
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It must hold that ``len(prefix) + nbits//8 + len(suffix)`` matches the
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block size of the underlying block cipher.
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"""
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if (nbits % 8) != 0:
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raise ValueError("'nbits' must be a multiple of 8")
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# Ignore wraparound
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return {"counter_len": nbits // 8,
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"prefix": prefix,
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"suffix": suffix,
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"initial_value": initial_value,
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"little_endian": little_endian
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}
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