Computer security is addressed from the perspective of the spies, who attempt to build covert channels and then exploit the ones of maximum capacity. The close connection between classical noninterference and the existence of positive-capacity covert channels is demonstrated. It is shown how to characterize the space of covert channels associated with a particular system. The capacity of the system is defined as the maximum of the capacities of all its channels, and optimal algorithms for covert communication are given.
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