Implementing TOTP and Dynamic Truncation

totp.py

import base64
import time
import math
import hmac
import hashlib

def print_bytes(bytes):
    print(",".join([str(byt) for byt in bytes]))

def base32_decode(encoded_str):
    return base64.b32decode(s=encoded_str)

def counter_array():
    timestamp = time.time()
    counter = math.floor(timestamp/30)
    counter_bytes = counter.to_bytes(8, byteorder="big")
    return counter_bytes

def hmac_sha1_bytes(secretkey_bytes, counter_bytes):
    hmac_obj = hmac.new(secretkey_bytes, counter_bytes, digestmod=hashlib.sha1)
    digest_bytes = hmac_obj.digest()
    return digest_bytes

def generate_code(secretkey_bytes, counter_bytes):
    digest_bytes = hmac_sha1_bytes(secretkey_bytes, counter_bytes)
    last_byte = digest_bytes[-1]

    four_digit_mask = 0b1111
    last_four_digits = last_byte & four_digit_mask
    offset = last_four_digits
    random_four_bytes = digest_bytes[offset:offset+4]  # from the offset take 4 bytes
    number = int.from_bytes(random_four_bytes, byteorder="big")
    mask = 0x7FFFFFFF
    positive_number = number & mask
    six_digit_codes = positive_number % 1000000
    return str(six_digit_codes).zfill(6)

def main():
    secret_key = "JBSWY3DPEHPK3PXP"
    secretkey_byte_array = base32_decode(secret_key)
    counter_bytes = counter_array()
    code = generate_code(secretkey_byte_array, counter_bytes)
    print(code)

if __name__ == "__main__":
    main()