71 lines
2.7 KiB
Python
71 lines
2.7 KiB
Python
# -*- 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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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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+------+--------------+-------+
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|prefix| counter value|postfix|
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+------+--------------+-------+
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The counter value is incremented by 1 at each call.
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Args:
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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 :data:`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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