Abstract: An IFF system produces a challenge in which some or all of the anti-interference pulses that are inserted between adjacent bits of an encrypted message are replaced with "information pulses" that convey information which a decryption sub-system uses to decipher the encrypted message. To produce an information pulse that replaces a given anti-interference pulse, the system combines an information bit with the anti-interference pulse. The decryption sub-system recovers the information bit from the information pulse by comparing the state of that pulse with the state of the anti-interference pulse that it replaced. If the states of the two pulses are the same, the information bit is a zero and if the states differ, the information bit is a one. The information included in the information pulses may be, for example, an address that the decryption sub-system uses to select a cryptographic key from one or more stored tables of keys.

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: Method and apparatus for secure transmission of analog signals, particularly facsimile signals. The analog signal is first converted to a frequency-modulated signal, if not already one, and is then converted to a sequence of digital words representing the instantaneous frequency thereof, by sampling at a fixed rate. Next the digital words representing the sample values are written in sequence into a buffer memory, at the constant sampling rate. The digital words are then read out of the memory at a different, pseudo-randomly variable rate and converted back to an analog FM signal for transmission. The transmitted signal is, thus, a time-delay-modulated version of the original FM signal. Optionally, the digital word values may be transformed before being converted back to analog frequency form, so that the modulation content of the transmitted signal is scrambled, as well. Further security may be obtained by reading out from the memory in a pattern which differs from the write-in address pattern.

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.