Once upon a time

One-Time-Passwords always fascinated me. Long long time ago in a land far far away I suddenly had this idea. The idea was simple and in today’s terms pretty common, perhaps trivial. One-Time-Password without the need for an extra token. After the user keys in their username and password, they get sent a random password via SMS. Ten years ago there wasn’t anything that did that. I created a basic RADIUS implementation with support for different SMS gateways, all in Java. Sadly however, with no funding, no clue how to turn it into a business, and just finishing my computer science degree, it had to be abandoned for an easier day job.

I was recently pulled into looking at two-factor-authentication (2FA) solutions. I used SecurID at a previous job, and know of several solutions in this area. I was quite pleased to discover many soft-token solutions working on mobile phones (iphone, blackberry, HTC, Nokia) and USB-based ideas like Yubikey. I was even more pleased to discover open source initiatives in this area, and OATH HOTP in particular.

HMAC-based One-Time-Password (HOTP) is reasonably simple to implement. It’s open, free and secure. Perhaps not militrary-grade security, but pretty good security nevertheless. I also must have found half a dozen free HOTP apps for my iphone (I’m using oathtoken), and it seems like there are apps for almost any phone. When coming to implement it on the server though, I felt a little disappointed. There are a couple of pam modules for linux (HOTP toolkit and Barada), but neither seem very stable or widely used.

What about python implementations then?? I was genuinely surprised there were virtually none. I found one, but it seemed rather specific to the Hebrew University and slightly complex. Or maybe I was too lazy to work out what it actually did…

So I then decided to write my own. It’s a very basic implementation, and by no means ‘production ready’, but it seems to work. I tested it against the test vectors on the rfc, and with my oathtoken iphone app, and it seems to match. I even implemented TOTP, a time-based variant instead of counter based – and that worked too!

#!/usr/bin/env python
"""
OATH HOTP + TOTP Implementation in python.

Based on http://tools.ietf.org/html/rfc4226

Parameter and function names kept inline with the RFC
(e.g. HOTP, Truncate, K, C etc)
"""

import hmac
import hashlib
import array
import time
import unittest

def HOTP(K, C, digits=6):
    """
    HOTP accepts key K and counter C
    optional digits parameter can control the response length

    returns the OATH integer code with {digits} length
    """
    C_bytes = _long_to_byte_array(C)
    hmac_sha1 = hmac.new(key=K, msg=C_bytes,
                         digestmod=hashlib.sha1).hexdigest()
    return Truncate(hmac_sha1)[-digits:]

def TOTP(K, digits=6, window=30):
    """
    TOTP is a time-based variant of HOTP.
    It accepts only key K, since the counter is derived from the current time
    optional digits parameter can control the response length
    optional window parameter controls the time window in seconds

    returns the OATH integer code with {digits} length
    """
    C = long(time.time() / window)
    return HOTP(K, C, digits=digits)

def Truncate(hmac_sha1):
    """
    Truncate represents the function that converts an HMAC-SHA-1
    value into an HOTP value as defined in Section 5.3.

http://tools.ietf.org/html/rfc4226#section-5.3

    """
    offset = int(hmac_sha1[-1], 16)
    binary = int(hmac_sha1[(offset * 2):((offset * 2) + 8)], 16) & 0x7fffffff
    return str(binary)

def _long_to_byte_array(long_num):
    """
    helper function to convert a long number into a byte array
    """
    byte_array = array.array('B')
    for i in reversed(range(0, 8)):
        byte_array.insert(0, long_num & 0xff)
        long_num >>= 8
    return byte_array

class HotpTest(unittest.TestCase):
    """
    a very simple test case for HOTP.
    Based on test vectors from http://www.ietf.org/rfc/rfc4226.txt
    """
    def setUp(self):
        self.key_string = '12345678901234567890'

    def test_hotp_vectors(self):
        hotp_result_vector = [755224, 287082, 359152,
                              969429, 338314, 254676,
                              287922, 162583, 399871,
                              520489, 060613]
        for i in range(0, 10):
            self.assertEquals(HOTP(self.key_string, i),
                              str(hotp_result_vector[i]))

    def test_totp(self):
        """
        a simple test for TOTP.
        since TOTP depends on the time window, we cannot predict the value.
        However, if we execute it several times, we should expect the
        same response most of the time.
        We only expect the value to change
        once or not at all within a reasonable time window.
        """
        value = TOTP(self.key_string, digits=8, window=20)
        value_changes = 0  # counting the number of changes to TOTP value
        for i in range(0, 100000):
            new_totp = TOTP(self.key_string, digits=8, window=20)
            if new_totp != value:
                value_changes += 1
                value = new_totp
        self.assertTrue(value_changes <= 1)

if __name__ == '__main__':
    unittest.main()