Single-file script for generating TOTP with https://github.com/pyauth/pyotp

Example usage

> python -mtotp DGLTPWEUERUUDCEC SWPKQCKEWRXPCRXE
628502
674329

totp.py

import base64
import calendar
import datetime
import hashlib
import hmac
import sys
import time
from typing import Any

"""
Copyright (C) 2011-2021 Mark Percival <m@mdp.im>,
Nathan Reynolds <email@nreynolds.co.uk>, Andrey Kislyuk <kislyuk@gmail.com>,
and PyOTP contributors

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
"""


def int_to_bytestring(i: int, padding: int = 8) -> bytes:
    """
    Turns an integer to the OATH specified
    bytestring, which is fed to the HMAC
    along with the secret
    """
    result = bytearray()
    while i != 0:
        result.append(i & 0xFF)
        i >>= 8
    # It's necessary to convert the final result from bytearray to bytes
    # because the hmac functions in python 2.6 and 3.3 don't work with
    # bytearray
    return bytes(bytearray(reversed(result)).rjust(padding, b"\0"))


def timecode(for_time: datetime.datetime, interval: int = 30) -> int:
    """
    Accepts either a timezone naive (`for_time.tzinfo is None`) or
    a timezone aware datetime as argument and returns the
    corresponding counter value (timecode).
    """
    if for_time.tzinfo:
        return int(calendar.timegm(for_time.utctimetuple()) / interval)
    else:
        return int(time.mktime(for_time.timetuple()) / interval)


def byte_secret(secret) -> bytes:
    missing_padding = len(secret) % 8
    if missing_padding != 0:
        secret += "=" * (8 - missing_padding)
    return base64.b32decode(secret, casefold=True)


def generate_otp(
    secret: bytes, input: int, digest: Any = hashlib.sha1, digits: int = 6
) -> str:
    """
    :param input: the HMAC counter value to use as the OTP input.
        Usually either the counter, or the computed integer based on the Unix timestamp
    """
    if input < 0:
        raise ValueError("input must be positive integer")

    hasher = hmac.new(secret, int_to_bytestring(input), digest)
    hmac_hash = bytearray(hasher.digest())

    offset = hmac_hash[-1] & 0xF

    code = (
        (hmac_hash[offset] & 0x7F) << 24
        | (hmac_hash[offset + 1] & 0xFF) << 16
        | (hmac_hash[offset + 2] & 0xFF) << 8
        | (hmac_hash[offset + 3] & 0xFF)
    )

    str_code = str(code % 10**digits)

    while len(str_code) < digits:
        str_code = "0" + str_code

    return str_code


def easy_totp(secret):
    return generate_otp(byte_secret(secret), timecode(datetime.datetime.now()))


if __name__ == "__main__":
    for secret in sys.argv[1:]:
        print(easy_totp(secret))