2fa-buddy.py

#!/usr/bin/env python3

import base64
import getpass
import hmac
import struct
import time


def make_hotp(secret_bytes, counter, length):
    counter_bytes = struct.pack('>Q', counter)

    hotp_bytes = hmac.new(
        secret_bytes, msg=counter_bytes, digestmod='sha1').digest()
    offset = hotp_bytes[-1] & 0xF
    hotp = (struct.unpack(
        '>I', hotp_bytes[offset:offset + 4]
    )[0] & 0x7FFFFFFF) % (10 ** length)

    return str(hotp).zfill(length)


def make_totp(secret_bytes, interval, length):
    counter, rem = divmod(int(time.time()), interval)
    ttl = interval - rem

    return make_hotp(secret_bytes, counter, length), ttl


def main():
    secret = getpass.getpass('TOTP secret seed: ')
    interval = input('Interval (seconds, default 30): ') or 30
    length = input('Output length (default 6): ') or 6

    print()
    print('Generating codes below. Press Ctrl+C to stop.')

    secret_bytes = base64.b32decode(secret, casefold=True)
    endl = (' ' * len(str(interval))) + '\r'

    while True:
        try:
            totp, ttl = make_totp(secret_bytes, int(interval), int(length))
            print('  {0} ({1})'.format(totp, ttl), end=endl)
            time.sleep(1)
        except KeyboardInterrupt:
            print()
            break


if __name__ == '__main__':
    main()