Two-factor authentication for SSH

What this post added

This post introduces and details the technical steps for implementing two-factor authentication for SSH using `libpam-google-authenticator`. It covers package installation, user-level configuration of the authenticator (including time-based tokens, scratch codes, and rate-limiting), modification of `/etc/pam.d/sshd` to include `pam_google_authenticator.so`, and updates to `/etc/ssh/sshd_config` to enable `ChallengeResponseAuthentication` and specify `AuthenticationMethods publickey,keyboard-interactive`. It also provides command-line options for automating the authenticator setup.