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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.
# ===================================================================
"""Fast counter functions for CTR cipher modes.
CTR is a chaining mode for symmetric block encryption or decryption.
Messages are divideded into blocks, and the cipher operation takes
place on each block using the secret key and a unique *counter block*.
The most straightforward way to fulfil the uniqueness property is
to start with an initial, random *counter block* value, and increment it as
the next block is processed.
The block ciphers from `Crypto.Cipher` (when configured in *MODE_CTR* mode)
invoke a callable object (the *counter* parameter) to get the next *counter block*.
Unfortunately, the Python calling protocol leads to major performance degradations.
The counter functions instantiated by this module will be invoked directly
by the ciphers in `Crypto.Cipher`. The fact that the Python layer is bypassed
lead to more efficient (and faster) execution of CTR cipher modes.
An example of usage is the following:
>>> from Crypto.Cipher import AES
>>> from Crypto.Util import Counter
>>>
>>> pt = b'\x00'*1000000
>>> ctr = Counter.new(128)
>>> cipher = AES.new(b'\x00'*16, AES.MODE_CTR, counter=ctr)
>>> ct = cipher.encrypt(pt)
:undocumented: __package__
"""
import sys
if sys.version_info[0] == 2 and sys.version_info[1] == 1:
from Crypto.Util.py21compat import *
from Crypto.Util.py3compat import *
from Crypto.Util import _counter
import struct
# Factory function
def new(nbits, prefix=b(""), suffix=b(""), initial_value=1, overflow=0, little_endian=False, allow_wraparound=False, disable_shortcut=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 one at each call.
:Parameters:
nbits : integer
Length of the desired counter, 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.
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
If True, the function will raise an *OverflowError* exception as soon
as the counter wraps around. If False (default), the counter will
simply restart from zero.
disable_shortcut : boolean
If True, do not make ciphers from `Crypto.Cipher` bypass the Python
layer when invoking the counter block function.
If False (default), bypass the Python layer.
:Returns:
The counter block function.
"""
# 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(bstr(prefix), bstr(suffix), initval, allow_wraparound=allow_wraparound, disable_shortcut=disable_shortcut)
else:
return _counter._newBE(bstr(prefix), bstr(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(bchr(n & 0xff))
else:
retval.insert(0, bchr(n & 0xff))
n >>= 8
return b("").join(retval)
# vim:set ts=4 sw=4 sts=4 expandtab:
|
