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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.
# ===================================================================
from Crypto.Util.python_compat import *
from Crypto.Util import _counter
import struct
# Factory function
def new(nbits, prefix="", suffix="", initial_value=1, overflow=0, little_endian=False, allow_wraparound=False, disable_shortcut=False):
# TODO: Document this
# Sanity-check the message size
(nbytes, remainder) = divmod(nbits, 8)
if remainder != 0:
# In the future, we might support arbitrary bit lengths, but for now we don't.
raise ValueError("nbits must be a multiple of 8; got %d" % (nbits,))
if nbytes < 1:
raise ValueError("nbits too small")
elif nbytes > 0xffff:
raise ValueError("nbits too large")
initval = _encode(initial_value, nbytes, little_endian)
if little_endian:
return _counter._newLE(str(prefix), str(suffix), initval, allow_wraparound=allow_wraparound, disable_shortcut=disable_shortcut)
else:
return _counter._newBE(str(prefix), str(suffix), initval, allow_wraparound=allow_wraparound, disable_shortcut=disable_shortcut)
def _encode(n, nbytes, little_endian=False):
retval = []
n = long(n)
for i in range(nbytes):
if little_endian:
retval.append(chr(n & 0xff))
else:
retval.insert(0, chr(n & 0xff))
n >>= 8
return "".join(retval)
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