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Based on the concept of an access matrix, a new protection system that achieves access control is proposed. In this system, associated with each accessor is only a key, and associated with each resource is only a lock, and through simple operations on the keys and locks, privacy decisions of the protection system can be revealed. Amechanism that realizes this system is also described. Noticing the importance of the role hierarchies play in access control, another mechanism is devised such that each node in the hierarchy is assigned a key and a simple operation on any two keys reveals the relationship of the two nodes corresponding to the two keys.