GB2101849A - Encrypting digital signals for phase modulation on a carrier wave - Google Patents

Abstract

In order to overcome the problems of low frequency and d.c. signals when phase modulating a digitally encoded input signal which has been encrypted using a key signal of a pseudo-random binary sequence form, on to a carrier, the clock frequency of the key signal is a factor 1/N times that of the clock signal used in a digitally encoding the input signal, where N is greater than but not equal to one. Values of N between 2 and 4 have been found to be useful.

Description

SPECIFICATION Encrypting digital signals The present invention relates to encrypting digital signals particularly for transmission by phase modulating a carrier signal.

Various means are known for encrypting a digital signal. Communications International, February 1980, pages 22 and 23, discloses a digital encryption system in which an audio input is converted to digital form using a high quality adaptive delta modulator. The digital data is then combined with the output from a non-linear key generator system and synchronisation data to form an encrypted signal. If the encrypted signal is transmitted as an FM signal there are few difficulties because the modulator can operate satisfactorily at low frequencies and with d.c. signals.

Howeverthe generally available mobile and portable radio equipment phase modulate the information onto a carrier wave whether it be for transmission by radio or along a length of cable. If the delta modulator and the key generator have the same clock frequencies or the key generator clock frequency cy is higher than that of the delta modulator then it is difficult to send encrypted digitised speech over a normal phase modulated channel especially if the equipment is equipped with a frequency synthesizer.

The reason for this difficulty is that the lowfrequen- cy (LF) components in the encrypted signal cannot be phase modulated and in consequence appear as noise in the resultant signal.

Accordingly it is an object of the present invention to omit or reduce LF and d.c. signal components in an encrypted signal so that it can be phase modulated on a carrier frequency.

According to one aspect of the present invention there is provided a method of encrypting digital signals in which a digitally encoded input signal is modulo 2 added to a pseudo-random binary sequence (PRBS) signal, wherein the clock signal of the PRBS signal is 1/N times that of the clock signal used in the digital encoding of the input signal, where N is greater than but not equal to one.

By the value of N being greater than one, for example between 2 and 4, the LF components and d.c. in the encrypted signal are substantially reduced or eliminated. so that the encrypted signal can be phase modulated satisfactorily on a carrier. Furthermore the reduced LF component means it is possible to relax the group delay, that is the delay in transformers, filters and in a.c. couplings, requirements.

The actual value selected for N, which may sweep over a range of values, is a trade-off between the desired degree of security and the advantages gained in transmission.

According to another aspect of the present invention there is provided a digital signal encrypting device comprising a digital encoder for digitally encoding an input signal, a key generator for providing a pseudo-random binary sequence signal, a modulo 2 adder coupled to the outputs of the digital encoder and the key generator, and means for providing a clock signal to the key generator whose frequency is 1/N times that of a clock signal supplied to the digital encoder, where N is greater than but not equal to one.

In an embodiment of the present invention the digital encoder comprises a delta modulator.

The present invention also relates to a phase modulated signalling system including the digital signal encrypting device in accordance with the present invention.

The present invention will now be explained and described, by way of example, with reference to the accompanying drawings, wherein: Figure lisa block schematic circuit diagram of the digital signal encrypting device in accordance with the present invention, and Figures 2 to 4 show various curves for explaining the invention.

Referring now to Figure 1, the encryption device comprises a source of digital data in the form of a variable slope delta modulator 10 having an input 12 for an audio frequency (AF) signal and an input 14 for a clock signal fb. Typically, the clock signal fb is 16 kHz so that a reasonable quantisation of the audio frequency can be obtained. The output of the delta modulator 10 is connected to one input of an exclusive OR circuit 20 which acts as a modulo 2 adder or balanced modulator. A key generator 16 in the form of a pseudo-random bit sequence (PRBS) produces a key signal in response to a clock signal fc applied to its input 18. The output of the exclusive OR circuit 20 together with synchronizing signals is phase modulated on a carrier and transmitted either by radio or along a cable.At the receiving end, not shown, the encrypted signal is applied to another modulo 2 adder to which an identical key signal is also applied and the output of that modulo 2 adder comprises the delta modulated signal which, after demodulation in a delta demodulator, comprises the AFsignal having an intelligible form.

In order to enable the encrypted signal to be phase modulated on a carrier, it is necessary to reduce or eliminate the low frequency component and d.c.

component by making the frequency spectrum go to zero. This is done by making the frequency of the clock signal fc of the key generator 16 a factor N less than that of the clock signal fb of the delta modulator 10. A value N = 2 has been found to be particularly convenient because the encrypted signal still has a high degree of security but the signal is capable of being phase modulated on a carrier. The encrypted signal essentially sounds like a noise signal which is broken up by the syllabic rate, in other words it sounds like bursts of noise of variable duration. If N has a value greater than 8, it has been found that the transmission advantages are retained but the security diminishes and accordingly the actual selection of the value of N is a trade-off between these two requirements.

Figure 2 illustrates, by way of comparison, the key signal which is at a clock rate equal to that of the delta modulator, that is N = 1 and fc = fb, and also a curve for N = 2 where fc = fb/2. For example, if fb = 16 kHz with N = 1 the first zero crossing is at 16 kHz (that is fb), whereas in the case of N = 2 the first zero crossing is at 8 kHz (that is fb/2).

Figure 3 illustrates the behaviour of the delta modulator in the case of having a small signal 22 and in the case of having a large signal 24. With the small signal the envelope is very narrow and is centred on fb/2 kHz but as the signal increases in amplitude then the envelope begins to fill out as illustrated but is still centred on fib12 kHz Figure 4 illustrates by a full line 26 the effect of the modulo 2 addition of the key signal with the delta modulator output when the key signal generator clock frequency is fb, that is 16 kHz. At zero frequency the encrypted signal amplitude is still quite high and this has the effect that the modulation envelope is folded back as shown in broken lines 28 with the effect that the white noise remains high.With such a high level of white noise, the security of the information is intact but as explained previously, the signal cannot be satisfactorily phase modulated on to a carrier. In contrast, the other curve 30 in Figure 4 illustrates what happens with the modulo 2 addition of the delta modulated signal with a key signal having a clock frequency of fb/2 kHz. As shown, the envelope centred at fb/2 kHz falls to zero at 0 and 16 kHz. With an increase in the amplitude of the signal, the envelope under the curve 30 fills itself out, that is tends to square itself off, but the zero points remain the same. As illustrated, the low frequency components 32 are negligible. Consequently the encrypted signal is suitable for phase modulation on a carrier wave.

Although in the described embodiment reference has been made to delta modulation, it is to be understood that any source of digital data which has a suitable spectrum (spectrum can be shaped by digital precoding techniques, for example Manchester coding, before encryption) can be used.

The security of the illustrated and described system can be improved by varying the value of N according to any predetermined algorithm thus providing an additional refinement to the encryption provided by the key generator.

Claims (10)

1. A method of encrypting digital signals in which a digitally encoded input signal is modulo 2 added to a pseudo-random binary sequence (PRBS) signal, wherein the clock signal of the PRBS signal is 1/N times that of the clock signal used in the digital encoding of the input signal, where N is greater than but not equal to one.

2. A method as claimed in Claim 1, wherein N is a fixed value between 2 and 4.

3. A method as claimed in Claim 1, wherein N varies in value between 2 and 4.

4. A method as claimed in Claim 1, 2 or 3, wherein the input signal is digitally precoded in a delta modulator.

5. A method of encyrpting digital signals substantially as hereinbefore described with reference to the accompanying drawings.

6. A digital signal encrypting device comprising a digital encoder for digitally encoding an input signal, a key generator for providing a pseudorandom binary sequence signal, a modulo 2 adder coupled to the outputs of the digital encoder and the key generator, and means for providing a clock signal to the key generator whose frequency is 1/N times that of a clock signal supplied to the digital encoder, where N is greater than but not equal to one.

7. A device as claimed in Claim 6, further comprising means for varying the value of N.

8. A device as claimed in Claim 6 or 7, wherein the digital encoder is a delta modulator.

9. A digital signal encrypting device substantially as hereinbefore described with reference to the accompanying drawings.

10. A phase modulated signalling system including a digital signal encrypting device as claimed in any one of Claims 6 to 9.