Totp.py

#!/usr/bin/python3.5
# -*- coding: utf-8 -*-

"""
Copyright (c) 2015 James Booth

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program.  If not, see <http://www.gnu.org/licenses/>.

Python V3.5 Google two-factor authentication Class.
"""

from base64 import b32decode
import time
import struct
from hashlib import sha1
import hmac

class Totp(object):    
    class TotpException(Exception):
        def __init__(self, msg):
            self.msg = msg
    
        def __str__(self):
            return repr(self.msg)

   
    def __init__(self, interval=30, pin_length=6):
        self.__interval = interval
        self.__pin_length = pin_length


    def generate_pin(self, base32secret):
        if not base32secret:
            raise TOTP.TOTPException('No base32secret supplied')
        try:
            secret_bytes = b32decode(base32secret.upper())
            time_seconds = int(int(time.time())/ self.__interval)
            time_bytes = struct.pack('>Q', time_seconds)
            hmacsha1 = hmac.new(secret_bytes, time_bytes, sha1)
            hash_bytes = bytearray.fromhex(str(hmacsha1.hexdigest()))
            offset = hash_bytes[-1] & 0xF
            data = hash_bytes[offset:offset + 4][::-1]
            integer = int.from_bytes(data, byteorder='little', signed=True) & 0x7FFFFFFF
            return str(integer)[-6:]
        except:
            raise Totp.TotpException('Invalid base32secret supplied')


# Test Code
totp = Totp()
pin = totp.generate_pin('fzwa7rpj2v4i7xocxnrdys4t4swzkn5g')
print (pin)