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# -*- coding: ascii -*-
#
# FortunaGenerator.py : Fortuna's internal PRNG
#
# 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.
# ===================================================================
__revision__ = "$Id$"
from Crypto.Util.python_compat import *
import struct
from Crypto.Util.number import ceil_shift, exact_log2, exact_div
from Crypto.Util import Counter
from Crypto.Cipher import AES
import SHAd256
class AESGenerator(object):
"""The Fortuna "generator"
This is used internally by the Fortuna PRNG to generate arbitrary amounts
of pseudorandom data from a smaller amount of seed data.
The output is generated by running AES-256 in counter mode and re-keying
after every mebibyte (2**16 blocks) of output.
"""
block_size = AES.block_size # output block size in octets (128 bits)
key_size = 32 # key size in octets (256 bits)
# Because of the birthday paradox, we expect to find approximately one
# collision for every 2**64 blocks of output from a real random source.
# However, this code generates pseudorandom data by running AES in
# counter mode, so there will be no collisions until the counter
# (theoretically) wraps around at 2**128 blocks. Thus, in order to prevent
# Fortuna's pseudorandom output from deviating perceptibly from a true
# random source, Ferguson and Schneier specify a limit of 2**16 blocks
# without rekeying.
max_blocks_per_request = 2**16 # Allow no more than this number of blocks per _pseudo_random_data request
_four_kiblocks_of_zeros = "\0" * block_size * 4096
def __init__(self):
self.counter = Counter.new(nbits=self.block_size*8, initial_value=0, little_endian=True)
self.key = None
# Set some helper constants
self.block_size_shift = exact_log2(self.block_size)
assert (1 << self.block_size_shift) == self.block_size
self.blocks_per_key = exact_div(self.key_size, self.block_size)
assert self.key_size == self.blocks_per_key * self.block_size
self.max_bytes_per_request = self.max_blocks_per_request * self.block_size
def reseed(self, seed):
if self.key is None:
self.key = "\0" * self.key_size
self._set_key(SHAd256.new(self.key + seed).digest())
self.counter() # increment counter
assert len(self.key) == self.key_size
def pseudo_random_data(self, bytes):
assert bytes >= 0
num_full_blocks = bytes >> 20
remainder = bytes & ((1<<20)-1)
retval = []
for i in xrange(num_full_blocks):
retval.append(self._pseudo_random_data(1<<20))
retval.append(self._pseudo_random_data(remainder))
return "".join(retval)
def _set_key(self, key):
self.key = key
self._cipher = AES.new(key, AES.MODE_CTR, counter=self.counter)
def _pseudo_random_data(self, bytes):
if not (0 <= bytes <= self.max_bytes_per_request):
raise AssertionError("You cannot ask for more than 1 MiB of data per request")
num_blocks = ceil_shift(bytes, self.block_size_shift) # num_blocks = ceil(bytes / self.block_size)
# Compute the output
retval = self._generate_blocks(num_blocks)[:bytes]
# Switch to a new key to avoid later compromises of this output (i.e.
# state compromise extension attacks)
self._set_key(self._generate_blocks(self.blocks_per_key))
assert len(retval) == bytes
assert len(self.key) == self.key_size
return retval
def _generate_blocks(self, num_blocks):
if self.key is None:
raise AssertionError("generator must be seeded before use")
assert 0 <= num_blocks <= self.max_blocks_per_request
retval = []
for i in xrange(num_blocks >> 12): # xrange(num_blocks / 4096)
retval.append(self._cipher.encrypt(self._four_kiblocks_of_zeros))
remaining_bytes = (num_blocks & 4095) << self.block_size_shift # (num_blocks % 4095) * self.block_size
retval.append(self._cipher.encrypt(self._four_kiblocks_of_zeros[:remaining_bytes]))
return "".join(retval)
# vim:set ts=4 sw=4 sts=4 expandtab:
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