Designing An Authentication System

Designing An Authentication System (http://web.mit.edu/kerberos/www/dialogue.html) tells the story of the creation of the SystemMetaphor for Kerberos (ThreeLineSystem). Some other authentication system metaphors are:

Coke machine: There's a very secure place that collects access tokens, and a less secure place that distributes goodies once the TokenCollector? gives the OK.
LockoutBoard? in an industrial plant: All the padlocks have to be removed before the switch can be thrown.
CreditCardTerminal?: A token is sent to a central system and validated before you can finish the transaction.
MarxBrothers movie: You have to say "swordfish" to get into the room (from HorseFeathers).