# -*- coding: ascii -*-
#
# FortunaAccumulator.py : Fortuna's internal accumulator
#
# 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__ = "$Id$"
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 binascii import b2a_hex
import time
import warnings
from Crypto.pct_warnings import ClockRewindWarning
from . import SHAd256
from . import FortunaGenerator
class FortunaPool(object):
"""Fortuna pool type
This object acts like a hash object, with the following differences:
- It keeps a count (the .length attribute) of the number of bytes that
have been added to the pool
- It supports a .reset() method for in-place reinitialization
- The method to add bytes to the pool is .append(), not .update().
"""
digest_size = SHAd256.digest_size
def __init__(self):
self.reset()
def append(self, data):
self._h.update(data)
self.length += len(data)
def digest(self):
return self._h.digest()
def hexdigest(self):
if sys.version_info[0] == 2:
return b2a_hex(self.digest())
else:
return b2a_hex(self.digest()).decode()
def reset(self):
self._h = SHAd256.new()
self.length = 0
def which_pools(r):
"""Return a list of pools indexes (in range(32)) that are to be included during reseed number r.
According to _Practical Cryptography_, chapter 10.5.2 "Pools":
"Pool P_i is included if 2**i is a divisor of r. Thus P_0 is used
every reseed, P_1 every other reseed, P_2 every fourth reseed, etc."
"""
# This is a separate function so that it can be unit-tested.
assert r >= 1
retval = []
mask = 0
for i in range(32):
# "Pool P_i is included if 2**i is a divisor of [reseed_count]"
if (r & mask) == 0:
retval.append(i)
else:
break # optimization. once this fails, it always fails
mask = (mask << 1) | 1
return retval
class FortunaAccumulator(object):
# An estimate of how many bytes we must append to pool 0 before it will
# contain 128 bits of entropy (with respect to an attack). We reseed the
# generator only after pool 0 contains `min_pool_size` bytes. Note that
# unlike with some other PRNGs, Fortuna's security does not rely on the
# accuracy of this estimate---we can accord to be optimistic here.
min_pool_size = 64 # size in bytes
# If an attacker can predict some (but not all) of our entropy sources, the
# `min_pool_size` check may not be sufficient to prevent a successful state
# compromise extension attack. To resist this attack, Fortuna spreads the
# input across 32 pools, which are then consumed (to reseed the output
# generator) with exponentially decreasing frequency.
#
# In order to prevent an attacker from gaining knowledge of all 32 pools
# before we have a chance to fill them with enough information that the
# attacker cannot predict, we impose a rate limit of 10 reseeds/second (one
# per 100 ms). This ensures that a hypothetical 33rd pool would only be
# needed after a minimum of 13 years of sustained attack.
reseed_interval = 0.100 # time in seconds
def __init__(self):
self.reseed_count = 0
self.generator = FortunaGenerator.AESGenerator()
self.last_reseed = None
# Initialize 32 FortunaPool instances.
# NB: This is _not_ equivalent to [FortunaPool()]*32, which would give
# us 32 references to the _same_ FortunaPool instance (and cause the
# assertion below to fail).
self.pools = [FortunaPool() for i in range(32)] # 32 pools
assert(self.pools[0] is not self.pools[1])
def _forget_last_reseed(self):
# This is not part of the standard Fortuna definition, and using this
# function frequently can weaken Fortuna's ability to resist a state
# compromise extension attack, but we need this in order to properly
# implement Crypto.Random.atfork(). Otherwise, forked child processes
# might continue to use their parent's PRNG state for up to 100ms in
# some cases. (e.g. CVE-2013-1445)
self.last_reseed = None
def random_data(self, bytes):
current_time = time.time()
if (self.last_reseed is not None and self.last_reseed > current_time): # Avoid float comparison to None to make Py3k happy
warnings.warn("Clock rewind detected. Resetting last_reseed.", ClockRewindWarning)
self.last_reseed = None
if (self.pools[0].length >= self.min_pool_size and
(self.last_reseed is None or
current_time > self.last_reseed + self.reseed_interval)):
self._reseed(current_time)
# The following should fail if we haven't seeded the pool yet.
return self.generator.pseudo_random_data(bytes)
def _reseed(self, current_time=None):
if current_time is None:
current_time = time.time()
seed = []
self.reseed_count += 1
self.last_reseed = current_time
for i in which_pools(self.reseed_count):
seed.append(self.pools[i].digest())
self.pools[i].reset()
seed = b("").join(seed)
self.generator.reseed(seed)
def add_random_event(self, source_number, pool_number, data):
assert 1 <= len(data) <= 32
assert 0 <= source_number <= 255
assert 0 <= pool_number <= 31
self.pools[pool_number].append(bchr(source_number))
self.pools[pool_number].append(bchr(len(data)))
self.pools[pool_number].append(data)
# vim:set ts=4 sw=4 sts=4 expandtab: