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Counter.py
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# -*- coding: ascii -*- # # Util/Counter.py : Fast counter for use with CTR-mode ciphers # # Written in 2008 by Dwayne C. Litzenberger <dlitz@dlitz.net> # # =================================================================== # The contents of this file are dedicated to the public domain. To # the extent that dedication to the public domain is not available, # everyone is granted a worldwide, perpetual, royalty-free, # non-exclusive license to exercise all rights associated with the # contents of this file for any purpose whatsoever. # No rights are reserved. # # THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, # EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF # MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND # NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS # BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN # ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE # SOFTWARE. # =================================================================== def new(nbits, prefix=b"", suffix=b"", initial_value=1, little_endian=False, allow_wraparound=False): """Create a stateful counter block function suitable for CTR encryption modes. Each call to the function returns the next counter block. Each counter block is made up by three parts: +------+--------------+-------+ |prefix| counter value|postfix| +------+--------------+-------+ The counter value is incremented by 1 at each call. Args: nbits (integer): Length of the desired counter value, in bits. It must be a multiple of 8. prefix (byte string): The constant prefix of the counter block. By default, no prefix is used. suffix (byte string): The constant postfix of the counter block. By default, no suffix is used. initial_value (integer): The initial value of the counter. Default value is 1. Its length in bits must not exceed the argument ``nbits``. little_endian (boolean): If ``True``, the counter number will be encoded in little endian format. If ``False`` (default), in big endian format. allow_wraparound (boolean): This parameter is ignored. Returns: An object that can be passed with the :data:`counter` parameter to a CTR mode cipher. It must hold that *len(prefix) + nbits//8 + len(suffix)* matches the block size of the underlying block cipher. """ if (nbits % 8) != 0: raise ValueError("'nbits' must be a multiple of 8") iv_bl = initial_value.bit_length() if iv_bl > nbits: raise ValueError("Initial value takes %d bits but it is longer than " "the counter (%d bits)" % (iv_bl, nbits)) # Ignore wraparound return {"counter_len": nbits // 8, "prefix": prefix, "suffix": suffix, "initial_value": initial_value, "little_endian": little_endian }