Patent GB-1279224-A (GB)
Title: Method of and apparatus for enciphering and deciphering signals
IPC: H04K1/02
Filed: 1970-02-06
Abstract: 1279224 Cryptographic systems NOKIA AB OY 6 Feb 1970 [6 Feb 1969] 5878/70 Heading H4P A method of encyphering a signal to be transmitted in which successive values of an information insignificant variable or a signal derived therefrom are each represented by the interval between at least two successive cypher pulses, relationship between different intervals and corresponding values of the variable being determined according to a predetermined code sequence, signals indicating said intervals being transmitted and on reception are reconverted to the original form. At the transmitter, a signal to be encyphered e.g. an audio signal is fed to the input of a converter 1 also fed with pulses from a random pulse generator 2 causing converter 1 to give a continuous quantized output (D), Fig. 3 (not shown), closest to one of a set of four fixed values representing the change in amplitude of input signal since reception of a previous pulse. Signal (D) is passed through a coincidence detector 4 receiving in addition a signal from a code generator 3 producing a determined sequence of the four fixed values (G). Detector 4 is activated on receipt of a random pulse from generator 2 and begins to compare signals from converter 1 with generator 3 until coincidence is reached when it gives an output pulse which provides an encyphered signal which may be transmitted by any type of coding system or transmission media. When the output pulse ceases, detector 4 is de-activated and the code generator recommences its sequence on receipt of the next pulse from the generator 2. Generator 2 and code generator 3 are controlled by a pulse generator 8. The mean frequency of fs of pulses from the random pulse generator over a long period is lower than the frequency fk of generator 8. The time interval between pulses from theencyphering arrangement can be shown to be random. At the receiver, encyphered pulses (F) are passed to a code generator 5, similar to generator 3 operated as at the same rate by a pulse generator 9, generator 5 being reset to commence its pulse sequence each time an encyphered pulse is received. The output from generator 5 is passed to a store 6 but this does not forward its output to a converter 7 until the next pulse (F) is received, converter 7 performing the reverse function of converter 1 i.e. each time the output of store 6 changes the new amplitude value is added algebraically to the previous value to provide a waveform similar to (B), Fig. 3, this being the waveform resulting if the output from store 6 corresponded to (D). From waveform (B) an audio signal corresponding to that shown at (A) is derived. Since the pulse frequency of the code generators will be much greater than the mean frequency fs the output signal 206 will correspond closely to curve (D) thus the reconstructed signal should correspond closely to the original. A pseudorandom generator may be substituted for the pulse generator 2 which would enable a number of encyphered and time multiplex channels to be connected to the same transmission channel. This arrangement would also require similar pseudorandom generators at the receivers to be synchronized. Random pulse generator 2 may also be replaced by a random generator modulated by a suitable camouflage signal or alternatively the random pulse generator may be omitted and the pulse intervals determined solely by the code generator and a numerical value to be encyphered. In a further modification instead of transmitting encyphered pulses, numbers corresponding to the intervals between pulses may be transmitted which numbers may be pulse code modulated on a suitable carrier.