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myrand.py
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myrand.py
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#!/usr/bin/env python
# myrand.py - Implement MT19937
#
# Copyright (C) 2015 Andrew J. Zimolzak <andyzimolzak@gmail.com>,
# and licensed under GNU GPL version 3. Full notice is found in
# the file 'LICENSE' in the same directory as this file.
import cryptopals
import time
w = 32 # bit word
n = 624 # degree of recurrence
m = 397 # "middle word"
r = 31 # bits in lower bit mask / sep point of 1 wd
a = 0x9908B0DF # "coefficients of the rational normal form twist matrix"
u = 11 # mt tem bit shift
d = 0xffffffff # mt tem bitmask
s = 7 # gfsr temper bit shift
b = 0x9D2C5680 # gfsr temper bitmask
t = 15 # gfsr temper bit shift
c = 0xEFC60000 # gfsr temper bitmask
l = 18 # mt tem bit shift
f = 1812433253
def low_word(x):
return int((2**w - 1) & x)
lower_mask = (1 << r) - 1 # usually 0x7fffffff
upper_mask = low_word(lower_mask ^ 0xffffffff) # usually 0x80000000
class BadSeed(Exception):
def __init__(self, value):
self.value = value
def __str__(self):
return "Cannot seed RNG with this value: " + repr(self.value)
class MTRNG():
def __init__(self, seed):
if type(seed) == type(1):
self.mt = [0] * n
self.index = n
self.mt[0] = seed
for i in range(1, n):
self.mt[i] = low_word(f * (self.mt[i-1]
^ (self.mt[i-1] >> (w-2))) + i)
elif type(seed) == type(list()):
assert len(seed) == n
self.mt = seed
self.index = n
else:
raise BadSeed(seed)
def extract_number(self):
if self.index >= n:
self.twist()
y = self.mt[self.index] # retrieve from MT[]
# begin tempering
y = y ^ ((y >> u) & d) # note d = 0xffffffff
y = y ^ ((y << s) & b)
y = y ^ ((y << t) & c)
y = y ^ (y >> l) # lowercase L, not numeral 1
self.index += 1
return low_word(y)
def twist(self):
for i in range(n):
x = (self.mt[i] & upper_mask) + (self.mt[(i + 1) % n] & lower_mask)
xa = x >> 1
if (x % 2) != 0:
xa = xa ^ a
self.mt[i] = self.mt[(i+m) % n] ^ xa # store new into MT[]
self.index = 0
class NoTimeSeed(Exception):
def __init__(self, value):
self.value = value
def __str__(self):
return "Number may be from RNG not seeded with time" + repr(self.value)
def find_time_seed(target_num, now):
for s in range(now - 60 * 35, now + 60 * 2):
m = MTRNG(s)
if m.extract_number() == target_num:
return s
raise NoTimeSeed(target_num)
#### tests, if any ####
cryptopals.warn("No errors:", __file__)