In a pulse code modulation system, a source of key signals comprising a source of random pulses, multisection delay device, means for applying said pulses to said multisection delay device, apparatus for combining the outputs of predetermined sections of said delay device and apparatus for enciphering pulse code modulation signals by combining said signals with said combined output from said sections of said multisection delay device.
A signal transmitted from a first device to a second device is encrypted using an encryption key generated from a preceding part of the signal itself. The signal is decrypted in the second device using a decryption key generated from a preceding part of the received and decrypted signal. This encryption method provides effective privacy protection, because the encryption and decryption keys are constantly changing. Since the transmitted signal provides its own encryption and decryption keys, the method is inexpensive to implement, and can be used in systems such as packet transmission systems that transmit signals intermittently.
A signal transmitted from a first device to a second device is encrypted using an encryption key generated from a preceding part of the signal itself. The signal is decrypted in the second device using a decryption key generated from a preceding part of the received and decrypted signal. This encryption method provides effective privacy protection, because the encryption and decryption keys are constantly changing. Since the transmitted signal provides its own encryption and decryption keys, the method is inexpensive to implement, and can be used in systems such as packet transmission systems that transmit signals intermittently.
A secure communication system, which may be safely used even in the prese of an enemy interceptor, includes a transmitter and a receiver. The transmitter comprises a first modulo-2 adder, having as one input a sequence of N-bit binary numbers. A first random read-only memory (ROM), comprises a plurality of storage cells. The input of the ROM is connected to the output of the modulo-2 adder. Each of the cells of the ROM, which have distinct addresses, contain a random number, with no two cells containing the same random number. The input to the ROM is a binary number representing a specific address, whereas the output of the ROM is a signal representing a random binary number. A delay line has its input connected to the output of the random read-only memory, its output constituting the second input to the modulo-2 adder. A second random ROM, having the same type of hardware but different random content, has its input connected to the output of the delay line. A second modulo-2 adder has its two inputs connected to the outputs of the first and second random read-only memories, the sequence of binary bits to be transmitted appearing at its output. Another component of the transmitter is a buffer memory, whose input and output are connected to the output of the second modulo-2 adder, the sequence of binary words to be transmitted appearing at the output of the second modulo-2 adder.
A transmit control system using in-band tones for activating and deactivating one or more remote sites. The transmit control system detecting energy in two or more spectral regions to determine if the signal received is a harmonic signal or a signalling tone to prevent accidental deactivation of the transmitter system. In one application, a modified Goertzel algorithm is employed by a digital signal processor to provide tone detection and a two stage detection provides controlled switching in noisy environments.
The present link delay calculation system using a common reference signal and a pulsed or gated tone signal to determine the link delay. The system using a discrete-time Fourier transform to measure phase information and obtain the link delay time. One such system employing a global positioning satellite signal to synchronize signal generation at a control site and demodulation at a remote site.
