auth.py

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

import hmac, base64, struct, hashlib, time, sys


def get_hotp_token(secret, intervals_no):
    """This is where the magic happens."""
    key = base64.b32decode(
        normalize(secret), True
    )  # True is to fold lower into uppercase
    msg = struct.pack(">Q", intervals_no)
    h = bytearray(hmac.new(key, msg, hashlib.sha1).digest())
    o = h[19] & 15
    h = str((struct.unpack(">I", h[o : o + 4])[0] & 0x7FFFFFFF) % 1000000)
    return prefix0(h)


def get_totp_token(secret):
    """The TOTP token is just a HOTP token seeded with every 30 seconds."""
    return get_hotp_token(secret, intervals_no=int(time.time()) // 30)


def normalize(key):
    """Normalizes secret by removing spaces and padding with = to a multiple of 8"""
    k2 = key.strip().replace(" ", "")
    # k2 = k2.upper()	# skipped b/c b32decode has a foldcase argument
    if len(k2) % 8 != 0:
        k2 += "=" * (8 - len(k2) % 8)
    return k2


def prefix0(h):
    """Prefixes code with leading zeros if missing."""
    if len(h) < 6:
        h = "0" * (6 - len(h)) + h
    return h


if __name__ == "__main__":
    values = sys.argv
    if len(values) > 1:
        print(get_totp_token(values[1]))