Abstract: Synchronization information for synchronizing encrypting and decrypting key generators (12 and 16) in a secure communications link (10) is transmitted without exacting a bandwidth penalty. Pointer comparators (17 and 18) at the transmission and reception ends of the link monitor the cipher text transmitted between the generators to determine whether it includes a predetermined naturally occurring sequence of bits, referred to as a "pointer" sequence. Upon the occurrence of the pointer sequence, the pointer comparators (17 and 18) trigger synchronization circuits (20 and 22) to read a sequence in the cipher text that occurs a predetermined period of time after the occurrence of the pointer. Accordingly, both synchronization circuits (20 and 22) read the same cipher-text sequence, which is a naturally occurring part of the cipher text.

Abstract: A privacy communication system digitizes a voice signal and divides the signal into different frequency bands or time segments and shifts the bands or segments in frequency and/or time under control of a continually changing pseudo-random key word to develop an encrypted transmitted signal having the same time/bandwidth product as the voice signal.

Abstract: A signal scrambling system of the type in which the frequencies of the information signal band are inverted by heterodyning the information signal with a varying beat frequency. In accordance with the invention the beat frequency varies in a sequence of small increments which, in their totality, provide large overall frequency excursions and which, individually, are sufficiently small to avoid the generation of unduly strong spurious signals within the information signal band.

Abstract: A voice privacy system enhances the privacy of a transmission by disguising the amplitude characteristics and cadence content of transmitted voice signals. Encoding apparatus first divides a voice signal to be transmitted into two or more frequency bands. One or more of the frequency bands is frequency inverted, delayed in time relative to the other frequency bands and then recombined with the other frequency bands to produce a composite signal for transmission to a remote receiver. By selecting the magnitude of the delay to approximate the time constants of the cadence, or intersyllabic and phoneme generation rates, of the speech to which the voice signal corresponds, the amplitude fluctuations of the composite signal are substantially lessened and the cadence content of the signal is effectively disguised. It thus becomes extremely more difficult for unauthorized listeners to extract cadence information from the signal as a means of extracting intelligence from the signal.