gistfile1.pyw

#!/usr/bin/env python
# -*- coding: utf-8 -*-

"""
Implementacion para entender cómo es el algoritmo de los Multiplicative 
Scramblers. 

Referencias:

- http://en.wikipedia.org/wiki/Scrambler#Multiplicative_.28self-synchronizing.29_scramblers

- http://books.google.com.ar/books?id=-ZAccwyQeXMC&pg=PA385&lpg=PA385&dq=%22multiplicative+scrambler%22+example+simple&source=bl&ots=OWO4F6c6E1&sig=8BSR2FGegYThmekEUpoerdjuie4&hl=es&ei=lTIlTNbXBMKB8gb59YnKDw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCEQ6AEwAg#v=onepage&q&f=false

Gracias Miguel por el link del libro.
"""

class MultScrambler(object):
    def __init__(self, terms):
        self.terms = terms
        self.size = max(self.terms)
        self.shiftreg = [0] * self.size

    def process(self, scramble, bit):
        if bit not in [0, 1]:
            raise Exception('bit no es un bit')

        tmp = 0
        for t in self.terms:
            tmp = tmp ^ self.shiftreg[t-1]

        tmp = tmp ^ bit

        self.shiftreg.pop(-1)

        if scramble:
            self.shiftreg.insert(0, tmp)
        else:
            self.shiftreg.insert(0, bit)

        return tmp

    def scramble(self, bit):
        return self.process(True, bit)

    def descramble(self, bit):
        return self.process(False, bit)

if __name__ == '__main__':
    ms = MultScrambler([3, 4])
    bits = [1,0,1,1,0,0,0,1,0,1,1,0]
    result = []

    print "Original: \t", bits

    for b in bits:
        result.append(ms.scramble(b))

    print "Scrambled: \t", result

    # Para poder obtener la secuencia original, el estado inicial del registro
    # de desplazamiento tiene que ser el mismo.

    ms = MultScrambler([3, 4])
    bits = result
    result = []

    for b in bits:
        result.append(ms.descramble(b))

    print "Descrambled: \t", result