Tag Archives: SSL

Using google authenticator with OpenBSD SSH logins

For an introduction on two factor authentication please see: http://en.wikipedia.org/wiki/Two-step_verification


NOTE: Make sure you leave a terminal with root access if you are using a remote system until you have tested that you can indeed authenticate to it!

Steps tested on OpenBSD 5.4, used tools on EL6 client to generate QR code. I’m mainly documenting this here so I can remember how to do this again.

1. Install: login_oauth-.tgz eg.

# pkg_add -v http://mirror.planetunix.net/pub/OpenBSD/5.4/packages/i386/login_oath-0.8p1.tgz

Make sure to look over the readme: /usr/local/share/doc/pkg-readmes/login_oath-version

2. Add a new login class by editing /etc/login.conf and add change user(s) to use it.

# The TOTPPW login class requires both TOTP and passwd; the user must
# supply the password in the format OTP/password, e.g. 816721/axlotl.

totppw:\
        :auth=-totp-and-pwd:\
        :tc=default:

Change user(s) login class eg.

# usermod -L totppw username

3. Generate a random key in the users home directory

$ openssl rand -hex 20 > ~/.totp-key

Note: Make sure your home directory and this file are not world readable! Otherwise you will be prevented from logging in.

4. Convert hex string key to base32

Documents show perl, I have supplied some code in python.

import binascii
import base64
import sys

if __name__ == '__main__':
    if len(sys.argv) != 2:
        print 'Syntax: %s ' % ( sys.argv[0])
        sys.exit(1)

    print base64.b32encode(binascii.unhexlify(sys.argv[1]))

5. Generate QR code so you don’t have to type in a big random sequence on your smart phone. Free web based ones exist, but only use if you trust.

$ qrencode -o ~/.totp-key.png 
"otpauth://totp/?secret=BASE 32 SECRET&issuer=Your name, etc."

More info: http://code.google.com/p/google-authenticator/wiki/KeyUriFormat

6. Using google authenticator create a new software key using your created QR code. If you don’t want to create a QR code, then create a new entry, set to time based and enter the 64 characters. I’m guessing it may take a few tries to get it correct.

7. Test that google authenticator and oathtool –totp `cat ~/.totp-key` match. If they don’t make sure both the phone and the computer dates and time match.

8. Using a separate terminal, ssh to remote system with just the password, this should fail. Then try using OTP/password for your password and you should get authenticated.