TOTP implementation in python

totp.py

import base64
import hashlib
import hmac
import math
import time


def dynamic_truncation(raw_key: hmac.HMAC, length: int) -> str:
    hex_digest: str = raw_key.hexdigest()
    int_repr: int = int(hex_digest, base=16)
    bitstring: str = bin(int_repr)
    last_four_bits: str = bitstring[-4:]
    offset: int = int(last_four_bits, base=2)
    chosen_32_bits: str = bitstring[offset * 8: offset * 8 + 32]
    full_totp: str = str(int(chosen_32_bits, base=2))

    return full_totp[-length:]


def generate_totp(shared_key: str, length: int = 6) -> str:
    now_in_seconds: int = math.floor(time.time())
    step_in_seconds = 30
    t: int = math.floor(now_in_seconds / step_in_seconds)
    hmac_hash = hmac.new(
        base64.b32decode(shared_key, True),
        t.to_bytes(length=8, byteorder="big"),
        hashlib.sha1,
    )

    return dynamic_truncation(hmac_hash, length)


def validate_totp(totp: str, shared_key: str) -> bool:
    return totp == generate_totp(shared_key)


if __name__ == "__main__":
    secret = "VNIGBG35N7XUHCZZ"

    print("Generating One-Time Password...")
    totp = generate_totp(secret)
    print(f"Done. It is: {totp}")

    print("Validating One-Time Password...")
    if validate_totp(totp, secret):
        print("It is valid!")
    else:
        print("It is invalid.")