ruby script/plugin install svn://rubyforge.org//var/svn/auth-typekey

In short, add the following line to either your ApplicationController or the specific controller you want to protect:

authenticates_via_typekey :key=>"Your key", :exclude => ["actions","to","exclude"], :logout_url => {:controller=>'home',:action=>'index'}

You can get the typekey key from http://www.typekey.com. Once you sign up for an ID, the key will be listed at the bottom of your preferences. You will also need to set up your rails application in your typekey preferences. Just add the full url for the controller which will handle authentication (either specified by the :controller parameter or the one that include the authenticates_via_typekey)

There is currently no advanced documentation for this plugin.