Not sure if to use SSH keys? 
Or Yubikey?
Or Google Authenticator?
On a whole bunch of machines?<p>Why not have it all?<p>Using privacyIDEA [1] you can<p>* manage SSH Pub keys for several servers
* manage your Google Authenticator centrally
* manage Yubikeys centrally or whichever OTP token.<p>In addition you can combine it to use the Google Authenticator or any other OTP ++PLUS++ SSH keys [2].<p>[1] <a href="http:&#x2F;&#x2F;privacyidea.org" rel="nofollow">http:&#x2F;&#x2F;privacyidea.org</a>
[2] <a href="https:&#x2F;&#x2F;www.privacyidea.org&#x2F;ssh-keys-and-otp-really-strong-two-factor-authentication&#x2F;" rel="nofollow">https:&#x2F;&#x2F;www.privacyidea.org&#x2F;ssh-keys-and-otp-really-strong-t...</a>