Shostack + Friends Blog Archive


Star Wars and Separation of Privilege

As we continue the series, illustrating Saltzer and Schroeder’s classic paper, “The Protection of Information in Computer Systems,” we come to the principle of separation of privilege.

Separation of privilege: Where feasible, a protection mechanism that requires two keys to unlock it is more robust and flexible than one that allows access to the presenter of only a single key. The relevance of this observation to computer systems was pointed out by R. Needham in 1973. The reason is that, once the mechanism is locked, the two keys can be physically separated and distinct programs, organizations, or individuals made responsible for them. From then on, no single accident, deception, or breach of trust is sufficient to compromise the protected information. This principle is often used in bank safe-deposit boxes. It is also at work in the defense system that fires a nuclear weapon only if two different people both give the correct command. In a computer system, separated keys apply to any situation in which two or more conditions must be met before access should be permitted. For example, systems providing user-extendible protected data types usually depend on separation of privilege for their implementation.

This principle is hard to find examples of in the three Star Wars movies. There are lots of illustrations of delegation of powers, but few of requiring multiple independent actions. Perhaps the epic nature of the movies and the need for heroism is at odds with separation of privileges. I think there’s a strong case to be made that heroic efforts in computer security are usually the result of important failures, and the need to clean up. Nevertheless, I committed to a series, and I’m pleased to be able to illustrate all eight principles.

This week, we turn our attention to the Ewoks capturing our heroes. When C3-P0 first tries to get them freed, despite being a god, he has to negotiate with the tribal chief. This is good security. 3P0 is insufficiently powerful to cancel dinner on his own, and the spit-roasting plan proceeds. From a feeding the tribe perspective, the separation of privileges is working great. Gods are tending to spiritual matters and holidays, and the chief is making sure everyone is fed.


It is only with the addition of Luke’s use of the force that it becomes clear that C3-PO is all-powerful, and must be obeyed. While convenient for our heroes, a great many Ewoks die as a result. It’s a poor security choice for the tribe.

Last week, Nikita Borisov, in a comment, called “Least Common Mechanism” the ‘Least Intuitive Principle.’ I think he’s probably right, and I’ll nominate Separation of Privilege as most ignored. Over thirty years after its publication, every major operating system still contains a “root,” “administrator” or “DBA” account which is all-powerful, and nearly always targeted by attackers. It’s very hard to design computer systems in accordance with this principle, and have them be usable.

Next week, we’ll discuss the principle of open design, and then close on that question of psychological acceptability.