Token Spike

friendly_token.py

"""                                                                                                                                                                                                                                                                                
Spike for the token creation and                                                                                                                                                                                                                                                   
associated probability calculation.                                                                                                                                                                                                                                                
"""

import math
import random
import string


def _dec_alphabet():
    ret_val = list(string.ascii_uppercase)
    for _char in ['I','O','Q','S', 'Z']:
        ret_val.remove(_char)
    return ''.join(ret_val)

def _dec_digits():
    ret_val = list(string.digits)
    for _digit in ['0','1','2','5']:
        ret_val.remove(_digit)
    return ''.join(ret_val)


def _chars():
    return _dec_alphabet() + _dec_digits()


def dec_token(no_chars):
    chars = _chars()
    print("There are {0} characters in the hash".format(len(chars),))
    token = ''.join([random.SystemRandom().choice(chars) for i in range(no_chars)])
    return token


def bp(n, d):
    # The Birthday Problem                                                                                                                                                                                                                                                         
    v = 1.0
    for i in range(n):
        v = v * (1 - float(i)/d)
    return 1 - v


def single_collision_probability(hash_length):
    no_of_chars = len(_chars())
    combinations = math.pow(no_of_chars, hash_length)
    return combinations


def main():
    import sys

    if len(sys.argv) <2:
        print
        print("Usage:")
        print("$ python friendly_token.py <no-of-borrowers> <hash-length>")
        sys.exit(1)

    no_of_borrowers = int(sys.argv[1])
    hash_length = int(sys.argv[2])

    print("Calculating for {0} borrowers with a hash lenth of {1}".format(no_of_borrowers, hash_length))

    scp = single_collision_probability(hash_length)
    prob = bp(no_of_borrowers, scp) * 100

    print("Probability of a Single Collision: " + str(1./scp))
    print("There is a {0:.4f}% chance of a collision with {1} borrowers with a hash length of {2}".format(prob, no_of_borrowers, hash_length))
    print("Example token: '{0}'".format(dec_token(hash_length)))


if __name__ == "__main__":
    main()