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The subject authentication scheme encompasses a large family of authentication systems which may be built over existing transmission systems. By superimposing a carefully designed secret modulation on the waveforms, authentication is added to the signal without requiring additional bandwidth. The authentication information (tag signal) is sent concurrently with data (message signal). The authentication is designed to be stealthy to the uninformed user, robust to interference, and secure for identity verification. The tradeoffs between these three goals are identified and analyzed. The use of the authentication for channel estimation is also considered, and improved bit errors are demonstrated for time-varying channels. With a long enough authentication code word an authentication system is achieved with very slight data degradation. Additionally, by treating the authentication tag as a sequence of pilot symbols, the data recovery may be improved by the aware receiver.