This is an ancient idea. It began when the first person drew a line across his cave entrance, proclaimed that what was on one side of the line was his, and then proceeded to defend his cave against all who disagreed with him. The notion covers a lot of different things: computer rooms behind locked doors and armed guards, tamper-resistant set-top boxes for pay-TV, secure tokens for access control, smart card chips for electronic commerce applications, and a bomb that blows up if you try to defuse it. The physical instantiation of the secure perimeter is different in each of these cases, but the fundamental benefit of the idea is the same: “It's a whole lot easier to design a computer security system if we can leverage the innate physical security of a device, and assume that parts of the system cannot be accessed by large classes of people.”
And that's true. It's easier to design a secure pay-for-parking system if you assume that crooks can't empty the parking meters into their pockets. It's easier to design a secure library if you assume that people can't sneak books out of the building inside their overcoats. And it's easier to design an electronic wallet if you assume that people can't arbitrarily modify the amount of money they have.
Here's a perfect cashless monetary system: Everyone carries around a piece of paper with a number on it representing the number of ducats in his wallet. When someone spends money, he crosses out the number and writes the lower ...