Secure communication system

DETAILED DESCRIPTION The present invention is embodied in a secure communication system consisting of a message source of plain text, signal manipulation means, a transmission medium for the cipher text, inverse signal manipulation means and the ultimate receiver of the plain text.
In accordance with the invention both the signal manipulation means and the inverse means operate under control of a synchronously occuring timing reference derived from any publicly available signal, which can be obtained at both the sending and receiving stations in a reliable, continous, low-noise fashion.
For example, if the sending message originates at a fixed station telephone, and the designated receiver is a mobile cellular telephone, the signal from any strong, public broadcasting station can serve as the source of the signal manipulation timing reference provided that both sender and receiver have simultaneous, reliable access to the same broadcast station.
Federal radio station WWV, which transmits time signals, could also be used.
Having established that in accordance with the invention the identical, reliable reference signal exists at both the sending and receiving stations, one can proceed to construct a variety of feature extraction means, operating on this reference signal to obtain a timing reference signal for message security purposes.
Signal features which can be extracted are for example; instantaneous spectrum as determined by a comb filter, leading to discovery of momentary presence of a specific signal frequency; timing of signal zero crossing; momentary low amplitude (a pause); timing of zero crossing signals after frequency filtering (e.
g.
, low-pass filtering); and combinations subject to arbitrary logic rules of any of the above features.
Generally, the feature extraction means used for generating a timing reference signal can be any means that extracts the instantaneous presence of any preselected signal feature, and may include appropriate filtering, amplitude threshold discrimination, clipping and limiting, zero-crossing detection, or time interval discrimination