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A two-dimensional signal-scrambling method implemented by digital signal-processing techniques that eliminate the need for frame synchronization without impairing security is presented. Such techniques include short time Fourier analysis and the filter bank concept. The use of special digital finite-impulse-response filters, which make it possible to implement the system algorithm completely via commercial processor software, is discussed. As a result, the system can be configured with very little hardware. Methods for determining available keyspace and selecting keys are also presented.