square_and_multiply.py

#!/usr/bin/env python
# coding=utf-8
# -*- Mode: python; c-basic-offset: 4 -*-

def convert_into_binary(c):
    # We return the binary of c as a STRING
    # Each character of the string can be converted it into integer using python's int() function.
    return bin(c)[2:]

def square_and_multiply(x,c,n):
    '''
    Calculates the z = x^c mod n
    :param x: Integer
    :param c: Integer
    :param n: Integer
    :return: Integer
    '''
    c=convert_into_binary(c)
    z=1

    for bit in c:
        z=z*z%n
        if bit is '1': #Remember: bit is a character
            z=(z*x) % n

    return z

print square_and_multiply(11,23,187)