Shrinking Generator - An Implementation of A Shrinking Generator in Python

An Implementation of A Shrinking Generator in Python

This example uses two Galois LFRSs to produce the output pseudorandom bitstream. The python code can be used to encrypt and decrypt a file or any bytestream.

#!/usr/bin/python import sys # ---------------------------------------------------------------------------- # Crypto4o functions start here # ---------------------------------------------------------------------------- class glfsr: def __init__(self, polynom, initial_value): self.polynom = polynom | 1 self.data = initial_value tmp = polynom self.mask = 1 while tmp != 0: if tmp & self.mask != 0: tmp = tmp ^ self.mask; if tmp == 0: break self.mask = self.mask << 1 def next_state(self): self.data = self.data << 1 retval = 0 if self.data & self.mask != 0: retval = 1 self.data = self.data ^ self.polynom return retval class sprng: def __init__(self, polynom_d, init_value_d, polynom_c, init_value_c): self.glfsr_d = glfsr(polynom_d, init_value_d) self.glfsr_c = glfsr(polynom_c, init_value_c) def next_byte(self): byte = 0 bitpos = 7 while 1 == 1: bit_d = self.glfsr_d.next_state bit_c = self.glfsr_c.next_state if bit_c != 0: bit_r = bit_d byte = byte | (bit_r << bitpos) bitpos = bitpos - 1 if bitpos < 0: break return byte # ---------------------------------------------------------------------------- # Crypto4o functions end here # ---------------------------------------------------------------------------- def main: prng = sprng(int(sys.argv, 16), int(sys.argv, 16), int(sys.argv, 16), int(sys.argv, 16)) print "GLFSR D0: using polynom 0x%X, initial value: 0x%X." % (int(sys.argv, 16), int(sys.argv, 16)) print "GLFSR C0: using polynom 0x%X, initial value: 0x%X." % (int(sys.argv, 16), int(sys.argv, 16)) f = open(sys.argv, "rb") g = open(sys.argv, "wb") while 1 == 1: input_ch = f.read(1) if input_ch == "": break random_ch = prng.next_byte & 0xff g.write(chr(ord(input_ch) ^ random_ch)) f.close g.close main

The C code is also available, see External links.