How To Enable Two-factor SSH authentication via Google Authenticator

In this tutorial I'll show you how to set up two-factor authentication for SSH using Google Authenticator. Two-factor authentication is where you authenticate to a service (SSH in our case) with two pieces of information: one you know, and one you don't. The information you know is your password (which can be stolen) while the information you don't know is a randomly-generated PIN number that changes every few seconds. So even if your password is stolen, unless an attacker has the means to get the right PIN (tied to a hardware device), they cannot log into the protected service.