Two Factor SSH with Google Authenticator
www.mnxsolutions.com/security/two-factor-ssh-with-google-authenticator.html
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
You can use this existing implementation and Google Authenticator application with SSH via an included PAM in the Google Authenticator open source application.