lib/Crypto/Util/Counter.py

# -*- 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
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# ===================================================================
"""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)

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