Yubikey Concepts, Configuration and Use
Logging in to internet sites (and private servers) with just a password is really not acceptable these days, at least for someone (like me) claiming to be interested in IT security. I therefore recently bought a Yubikey-4 authentication token.
Sadly, the documentation available from the manufacturer, and the internet in general, was not very helpful. I have therefore created some extensive notes on the Yubikey-4 which may be useful if you are also considering buying one (or have already done so).
UPDATE: The Yubikey-5 is available (since late September 2018). Note that the above article also covers Yubikey-5 features.