80 lines
3.1 KiB
Python
80 lines
3.1 KiB
Python
|
# -*- 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.
|
||
|
|
An ``OverflowError`` exception is always raised when the counter wraps
|
||
|
|
around to zero.
|
||
|
|
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
|
||
|
|
}
|