Using Google Authenticator For Your Website

Google has started offering two-factor authentication for Google logins, using Google Authenticator. They have applications available for iPhone, Android, and Blackberry that give time-based passwords based on the proposed TOTP (Time-based One Time Password) draft standard.

The Google code provides a command line program that can generate secret keys as well as a PAM module, but it turns out to be very little code to authenticate a TOTP, thereby providing two-factor authentication to your website very easily.

To give the user the key, you’ll need to generate a cryptographically-secure 10 byte random key, presented to the user as a base32 16-character string. They can either enter this string directly, or you can use Google charts to provide a barcode that they can scan into the Google Authenticator application:

def get_barcode_image(username, domain, secretkey):
    url = "https://www.google.com/chart"
    url += "?chs=200x200&chld=M|0&cht=qr&chl=otpauth://totp/"
    url += username + "@" + domain + "%3Fsecret%3D" + secretkey
    return url

For an example of what a code looks like, click here, or, look below:

After the user has a secret key from you and has entered it into Google Authenticator either by typing it in directly or scanning in the barcode, you have to be able to verify the key during login (for example). The code to authenticate is only a few lines in Python:

import time
import struct
import hmac
import hashlib
import base64
 
def authenticate(secretkey, code_attempt):
    tm = int(time.time() / 30)
 
    secretkey = base64.b32decode(secretkey)
 
    # try 30 seconds behind and ahead as well
    for ix in [-1, 0, 1]:
        # convert timestamp to raw bytes
        b = struct.pack(">q", tm + ix)
 
        # generate HMAC-SHA1 from timestamp based on secret key
        hm = hmac.HMAC(secretkey, b, hashlib.sha1).digest()
 
        # extract 4 bytes from digest based on LSB
        offset = ord(hm[-1]) & 0x0F
        truncatedHash = hm[offset:offset+4]
 
        # get the code from it
        code = struct.unpack(">L", truncatedHash)[0]
        code &= 0x7FFFFFFF;
        code %= 1000000;
 
        if ("%06d" % code) == str(code_attempt):
            return True
 
    return False

25 thoughts on “Using Google Authenticator For Your Website

  1. Lior Gradstein

    Really nice implementation! What about the 10 bytes generator? Can I just use os.urandom(10) (in fact: base64.b32encode(os.urandom(10)) ), or is there something more cryptographically secure? (/dev/random is really too slow).

    I think using a Radius server like FreeRadius and use its plugin infrastructure to authenticate using your code would be really easy, and really practical as radius plugins are very common (pam, apache module, cisco embedded, etc.)

    I’ll try to implement it next week.

    Thanks,
    Lior

    Reply
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  3. Ben Poliakoff

    Great post! We’ll be integrating some of this to support second factor auth within our web SSO.

    Minor nit: the “if code == code_attempt” bit didn’t work for me properly until I forced both variables to be integers (“if int(code) == int(code_attempt)”).

    Thanks again!

    Reply
  4. James

    @Todd Yep. The example link is missing a semicolon before the chld parameter. Add that and it’s fixed.

    Reply
    1. tim Post author

      @Todd, @James: Can you believe it was a bug in the pretty-printer? Rearranged the code for now so that it doesn’t hit the bug. Thanks.

  5. Erinn Looney-Triggs

    There appears to be a bug in your implementation. The problem arises when the code is less than X digits. In the reference implementation in the RFE the number is padded with preceding zeros up to a specified code length. To do this I believe you will need to convert the int to a str, and then you can use zfill like this:

    code = str(code).zfill(code_length)

    Then you will need to make sure that the comparison operator is working on like types, so this:

    if code == code_attempt:

    Should probably be changed to this:

    if code == str(code_attempt):

    Anyway this problem only occurs when less than code_length worth of digits is returned, it looks like in Google’s case the code_length = 6.

    Reply
  6. Bastian Hoyer

    For extra security you should make sure that every token is only accepted once. If you won’t an attacker might get a 1.30 Minute time frame to login with the same code again.

    Reply
  7. Pingback: .log : ?? ????? ????????

  8. tim Post author

    @Todd: Argh. Thanks, fixed. Also added a copy of the image itself to the article, let’s see Google break that, hah!

    Reply
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  10. noah

    umm I’m using python through terminal and for the creating of the barcode when I click enter this is what it looks like (Hid login)
    python
    Python 2.7.3 (v2.7.3:70274d53c1dd, Apr 9 2012, 20:52:43)
    [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
    Type “help”, “copyright”, “credits” or “license” for more information.
    >>> def get_barcode_image(Noah, jeff, n223f546g79245ft257j74h39u):
    … url = “https://chart.googleapis.com/chart?”
    … url += “chs=200×200&chld=M|0&cht=qr&chl=otpauth:/totp/”
    … url += username + “@” + domain + “%3Fsecret%3D” + secretkey
    … return url

    Reply
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  12. Aaron

    I’m using Python 3.2.

    I generated a test secret using:
    base64.b32encode(os.urandom(10))

    When I run your code, I get the following error:
    offset = ord(hm[-1]) & 0x0F
    TypeError: ord() expected string of length 1, but int found

    Reply
  13. tim Post author

    @Aaron: Yeah, it looks like Python 3.2 changed some type signatures to take bytes instead of strings. Deleting the “ord” should make it work for you — I’ll take a look tonight and find a more elegant fix that is compatible with both Python 2.x and 3.x.

    Reply
  14. wirefreak

    Any news on porting this on Python 3? My server only runs 3.3.1 and I get an error on struct.pack:
    b = struct.pack(“>q”, tm + ix)
    ^ (the cursor is under the last parenthesis)
    TabError: inconsistent use of tabs and spaces in indentation

    What can I do?

    Reply
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