InfoSec Assignment 7 - LFSR.py

#!/usr/bin/env python

import sys

def main():
    if len(sys.argv) > 1:
        registers = sys.argv[1]
    else:
        registers = '1001'

    # Ensure only 1's and 0's are present
    for reg in registers:
        if reg not in '01':
            print('Must only use 1s and 0s', file=sys.stderr)
            sys.exit(1)

    # Convert registers to a list of ints
    registers = [int(x) for x in registers]

    output = ""
    for i in range(0, 15):
        # Display the registers before any manipulation
        print("[registers]", *registers, end=" ")
        # XOR the last 2 registers
        r1 = registers[-2]
        r2 = registers[-1]
        xor = r1 ^ r2
        # Add the last register to the end of the output
        output += str(registers.pop())
        # Display infoz
        print("[xor]", "{} ^ {} = {}".format(r1, r2, xor), end=" ")
        print("[output]", output)
        # Insert the XOR result to the beginning of the registers
        registers.insert(0, xor)

if __name__ == '__main__':
    main()