2-factor authentication code generator in Python 3 Google has an Authenticator app that, given a secret and the time, shows a 6-digit code. The code cycles every 30 seconds. I wrote an implementation in Python 3 (can be run in Python 2 with some changes) that optionally generates a qrcode for a new user and/or just shows the current code displayed in the Authenticator app.

import hmac, base64, struct, hashlib, time, pyqrcode, random, sys, os

def get_hotp_token(secret, intervals_no):
    key = base64.b32decode(secret)
    msg = struct.pack("&gt;Q", intervals_no)
    h = hmac.new(key, msg, hashlib.sha1).digest()
    o = h[19] &amp; 15
    h = (struct.unpack("&gt;I", h[o:o+4])[0] &amp; 0x7fffffff) % 1000000
    return h

def get_totp_token(secret):
    return get_hotp_token(secret, intervals_no=int(time.time())//30)

# This works whenever I run it, but I'm not 100% sure it conforms to RFC standards
# Also, I'm not sure if the secret is every allowed to not be 16 bytes
def generate_secret(length=16):
    random_secret = ""
    characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567"
    for _ in range(length):
        random_secret += random.choice(characters)
    return random_secret

# Pads secret with zeroes and separates with a space
def format_secret(secret, to_length=6):
    padded = '0'*(6-len(str(secret)))+str(secret)
    return padded[:3] + ' ' + padded[3:]

# Sets up a user given a label, username, and generates/takes a secret
# Is there a better way to do the "y/n: " stuff?
def setup_user():
    label = input("Label: ").replace(' ', '%20')
    user = input("User: ").replace(' ', '%20')
    secret = input("Auto-generate secret? y/n: ")

    if secret.lower() == "y":
        secret = generate_secret()
    else: 
        secret = input("Secret: ")

    return label, user, secret

secret = "DID NOT SET SECRET"

# Again, is there a better way to do the "y/n: "?
if input("Setup user? y/n: ").lower() == "y":
    label, user, secret = setup_user()

    if input("Show qr-code? y/n: ").lower().strip() == "y":
        # I'm unsure if this URL breaks when certain characters are in the label or username
        qr_url = "otpauth://totp/%s:%s?secret=%s&amp;issuer=%s" % (label, user, secret, label)
        url = pyqrcode.create(qr_url)
        os.system('cls' if os.name == 'nt' else 'clear')
        # Is it possible to make the qrcode smaller when printed in Terminal/Console?
        print(url.terminal(quiet_zone=1))
else:
    secret = input("Secret: ")
    os.system('cls' if os.name == 'nt' else 'clear')

print("User secret: ", secret)

sys.stdout.write("\r%s" % format_secret(get_totp_token(secret)))
sys.stdout.flush()
# This next line is my code to time-sync the timer. I'm not sure if it actually works though. 
# I think it might have to do with using modular arithmetic on a float
time.sleep(1 - time.time() % 1)

while True: 
    # These next couple of lines format a progress bar for how many seconds are left until a new code.
    seconds = int(30 - time.time() % 30)
    lines = "-" * int(seconds)# / 3)
    blocks = "█" * (30 - len(lines))
    progress = "|" + blocks + lines + "|"

    sys.stdout.write("\r%s" % format_secret(get_totp_token(secret)) + "\t" + progress)
    sys.stdout.flush()

    time.sleep(1)


This uses the pyqrcode library, which does not ship standard with Python.

I noticed that sometimes, the code does not change exactly when it does in the Authenticator app. This may be a problem with my time-syncing code.

What are your thoughts on quality of code, usability, pythonic-ness, and correctness?
Code quality


Each import should have its own, seperate line:

import hmac
import base64
import struct
import hashlib
import time
import pyqrcode
import random
import sys
import os

Top-level functions should be seperated using two blank lines:

def foo():
    foobar()


def bar():
    foobar()

A lot of your comments should actually be docstrings.


Security


Don't use the random module for security purposes. Skimming through the documentation, red banners warn you not to use it for security purposes and instead to use the secrets module. Note that secrets is only part of the standard library in Python 3.6 and upwards. If you need to run this on other versions of Python, which is quite likely, make use of random.SystemRandom (or os.urandom() directly).
Use getpass.getpass() instead of plain input() for gathering secrets like passwords or private codes. [getpass documentation]


General recommendations


Since pyqrcode isn't part of the standard library, you may want to wrap import pyqrcode in a try:/except: statement:

try:
    import pyqrcode
except ModuleNotFoundError:
    print("Couldn't import pyqrcode.")
    sys.exit(1)

If you plan to release this as a module, it's a very good idea to add documentation (docstrings, as well as a README for quick reference), so others understand how the code works and how they should use it.