Edit File: FortunaAccumulator.py
# -*- 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: