''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).
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See: MicroSim