Today, when I switched from Google's SMS-based 2-step verification to the Android-based version, I discovered that there exists an open-source version of Google's HMAC-based One Time Password (HOTP) implementation. Better yet, it includes a PAM module that can be incorporated into PAM and thus SSH.

Because I sometimes log into my server from untrusted sources (university computer pool), I decided to give it a try. It turned out to be quite straightforward, and now I have to enter a One Time Password (OTP) in addition to my user password each time I log into my server. This is however not required when I use public key authentication from home.

Installation

As said, the set up is straightforward. First, besides the usual compiler toolchain, the following dependencies are required:

apt-get install libpam-dev libqrencode3

Next, grab google-authenticator from upstream:

git clone https://code.google.com/p/google-authenticator/

And install it:

cd google-authenticator/libpam
make && make install

Setup

Edit /etc/ssh/sshd_config and set:

ChallengeResponseAuthentication yes

Now, activate the authenticator by adding the pam module to the first line in /etc/pam.d/sshd:

auth required pam_google_authenticator.so nullok

Activation

You can now activate 2-step verification by executing google-authenticator, which should yield something like this (this is not the key I'm actually using):

command result

You can now scan the QR-Code produced by google-authenticator using the Android App. When you now try to log into your server, the prompt will look something like this:

root@t420:/home/fabian/ > ssh fabian@0x0b.de
Verification code:
Password:

At which point you can bring up the app that will look something like this:

android app


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