GB1595796A

GB1595796A - Security systems

Info

Publication number: GB1595796A
Application number: GB882677A
Authority: GB
Inventors: Hugh John Pushman
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-04-21
Filing date: 1978-04-21
Publication date: 1981-08-19

Description

(54) SECURITY SYSTEMS (71) I, HUGH JOHN PUSHMAN, of British Nationality, of 63 Wollaton Road, Ferndown, Dorset, do hereby declare the invention for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to security systems.

According to the invention, a security system comprises key and, separate from the key, a control means for actuation bv the key, the key comprising a first generator operative to generate a pseudo-random sequence of signals at a predetermined frequency, and the control means being capable of receiving said signals and comprising a second generator of the same pseudo-random sequence of signals as the first generator and comparison means operative to compare a received signal sequence with the signal sequence generated by the second generator and responsive to complete correlation between at least part of the two sequences to develop a control signal to effect a control function, the comparison means being operative to cause the second generator to generate its signal sequence at a frequency less than said predetermined frequency until complete correlation between at least part of the two sequences is detected.

The system may be such that the key has to be positioned in contact with the control means to actuate it. For example, the signals provided by the key could be electrical and the key and control means could be provided with cooperating contacts. The system may instead be such that the key can be disposed remote from the control rneans to actuate it, e.g. it may be connected thereto by a communication link, e.g. a telephone circuit, or by radiation, e.g. light (visible or invisible), other electro-magnetic radiation or acoustic radiation.

In a first system embodying the invention and described in more detail hereinbelow, the first generator comprises a shift register together with associated logic circuitry and operates at a first, predetermined frequency, continually repeating the sequence. The second generator can be like the first generator, save that it operates at a different frequency. The frequency of the second generator can be variable in accordance with the degree of correlation obtained, e.g. from 90% of that of the first generator for zero correlation to 100% of that of the first generator for 100% correlation. The correlator compares the two sequences of signals over part only of the sequence, the part being sufficiently long to ensure that complete coincidence of the sequences over said part will only occur if the sequences are in fact identical.When complete coincidence eventually occurs, the control signal is developed. Assuming that, as is desirable, the sequence is a long one, the correlator will in general have to carry out a large number of comparison operations before complete coincidence occurs. Even though high operating frequencies can be employed, it generally takes a substantial time (e.g. of the order of one second) before the control signal is developed. A result of this is that the chances of success of an unauthorised attempt to actuate the control means by trying all possible sequences of signals can be made vanishingly small. If, say, the time for the control means to respond is about one second, and the number of possible sequences is very high, the time required for trying every possible sequence would be prohibitively long.

Another system embodying the invention, also described in more detail below, is a modficiation of the system outlined above in that the control means comprises a simple, single-bit ' stop/go" correlater which compares the two signal sequences on a bit-by-bit basis. That is to say, the second generator is started and stopped in accordance with whether each received signal matches or does not match a signal porduced by the second generator, whereby "roll-over occurs until the two signals eventually "lock on".

As explained in more detail below, the two system outlined above are suited to use in an electrically noisy environment.

Although in many systems embodying the invention the key will, like the key of a conventional mechanical lock, be manually portable, it is not necessarily so.

For example, if a system in accordance with the invention is used for locking and/ or unlocking a garage door, the key could be embodied in a vehicle and be arranged to generate the signal sequence by flashing the headlights thereof, for instance by means of a mechanical timer switch, the control means being light-sensitive and being associated with the garage door lock.

As just mentioned, one application of systems in accordance with the invention is to the opening and closing of locks, in which case the control means is associated with the lock. For instance, the control means can be incorporated in a safe. However, other applications of the system are posssible.

Naturally, the control means of a system embodying the invention can only be actuated by a key having a generator providing the same sequence of signals as that provided by the generator of the control means. To provide additional security, the code of the sequence generated by the generator of the control means can be altered, for example at regular or irregular intervals or upon use. The code of the key generator sequence could be similarly variable, whereby the user would have to set the appropriate code on the key before it could actuate the control means.

A further advantage of a key having provision for varying the code of the sequence manually is that it can only be used to operate an associated control means by a person knowing the correct code setting.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a pseudo-random pulse generator that can be employed in both the key and control means of systems embodying the invention; Figure 2 is a block diagram of a first system embodying the invention; Figure 3 is a block diagram of the key of a second system embodying the invention; Figure 4 is a diagram showing waveforms present in the key of Figure 3; and Figure 5 is a block diagram of the control means of the second system.

Referring first to Figure 1, the pseudorandom generator shown therein comprises an n-stage shift register 10. A clock pulse generator (not shown) supplies pulses to the shift register 10 to shift its contents from left to right as shown in the drawing. The rth and sth states of the shift register are connected to inputs of a logic circuit 12, which may be an exclusive-OR gate or a "2's complement" circuit. The output from the logic circuit 12 is connected back to the first stage 1 of the shift register.

When clock pulses are supplied to the shift register 10, by virtue of the logic circuit 12 a pseudo-random output sequence of a predetermined number of bits will be generated at the output of the logic circuit 12. The output is not truly random in that if all relevant factors are known its form can be accurately predicted. However, each bit in the sequence bears little or no correlation to adjacent ones so that for practical purposes the output can be considered random.

The nature of the sequence is dependent upon the initial state of the shift register 10, the number of stages of the shift register 10, the logical operation carried out by the logic circuit 12 (other operations than an exclusive-OR operation can be carried out), and the stage tappings employed (the logic circuit 12 can be connected to different stages and not necessarily to two stages).

A further relevant factor is that the code produced may or may not be " optimum ".

An optimum code is defined as one where the code adopts all possible states before repeating itself. For example a 16 stage register can produce a code 216 bits long before repeating itself. A non-optimum code is defined as one where the sequence stops short of the maximum possible figure.

For example, a 16 bit register may be arranged to produce codes of 48 K bits and 16 K bits or of 16 K, 32 K and 16 K bits. Whether the code is optimum or not is determined by the stages connected to the logic circuit 12 and the nature of the logical operation it carries out. If the code is not optimum, the code cycle is determined by the initial state of the register.

The mathematical theory of pseudorandom generators as described above is well-developed, and the characteristics of the sequences they produce can be analysed and forecast.

Referring now to Figure 2, the illustrated security system comprises a key 20 and a control means 30.

The key 20 comprises a pseudo-random generator 22 of the form described above with references to Figure 1 and a clock pulse generator 24 operating at 1 Mbits/s.

The pseudo-random output from the generator 22 is connected to a transducer 26 which converts the electrical output signal sequence into, for example, electromagnetic radiation for transmission to the control means 30.

The control means 30 comprises a transducer 32 responsive to receiving the radiation from the key 20 to convert it back into electrical signals and to apply such signals to a first input of a correlator 34.

The control means 30 further comprises a pseudo-random generator 36 of identical construction to the generator 22. The pseudo-random output from the generator 36 is connected to a second input of the correlator 34. The generator is driven by a clock pulse generator 38 of variable frequency, the frequency of which is controlled by the correlator 34.

Each of the pseudo-random generators 22 and 36 comprises a 16 stage shift register and produces a pseudo-random sequence of 64 K bits.

The illustrated system operates as follows. The key is energised and the radiation therefrom is directed towards the control means 30. The correlator 34 employs simple "and" logic and compares the sequence received from the key 22 with that generated localy by the generator 36 over a short part only of the 64 K bit sequence, for example about 16 consecutive bits. It is assumed that initially, as is overwhelmingly likely, the two coded sequences applied to the correlator are not in synchronism so that the correlator detects little or no correlation between the two signals, and that the clock pulse generator 38 operates at about 10 percent below the frequency of that of the clock generator 24, for example about 0.9 Mbits/s.As the code signal sequences supplied to the correlator 34 are at slightly different frequencies, the codes will " roll over" until, in due course, the sequences will be in synchronism and complete correlation over the part of the sequences examined is obtained. The frequency of the clock pulse generator 38 is varied in accordance with the amount of correlation obtained: as the correlation rises from zero to unity (i.e. 100%) the frequency of the generator 38 will rise from 0.9 Mbits/s per sec to 1 Mbits/s per sec. The average "roll over" differential speed will be 5 percent. Therefore the time for carrying out a complete search of the received code by comparing part thereof step by step with the locally generated code will be about 24 x 64 K microseconds, giving an average of 10 x 64 microseconds, i.e.

0.64 seconds. Looking at matters another way, the time for carrying out all the necessary comparisons to determine whether the codes is identical is proportional to the number of comparisons to be made divided by the comparison rate, namely the average difference between the frequencies of the two clock pulse generator 24, 38.

Thus, provided the codes generated by the two pseudo-random generators 22, 36 are identical, after a delay of at most about 0.64 seconds complete correlation will be established: the control means 30 will lock on to the key 20. Once this occurs, the correlator 34 will provide an output signal on an output terminal 40 to actuate, for example, a lock.

By varying the length of the code and the differential search speed, the average time taken for lock-on can be adjusted over a wide range, for example from milliseconds to tens of seconds, depending on the type of application. As mentioned above, the use of a substantial comparison time has the advantage that the control means 30 is highly immune to unauthorised operation. For example, if someone attempted to actuate the control means 30 by trying all possible sequences that could be obtained with the pseudo-random generator, then since each sequence would have to be generated for the time taken for lock-on to occur the time taken to run through all possible sequences would be prohibitively long.

As mentioned above, the operative connection between the key 20 and control means 30 can be effected in a variety of ways. For instance, if light is used, the transducer 26 could be a light emitting diode (LED) and the light could either be shone through space on to the control means 30 or conducted thereto by means of an optical fibre. Naturally, if the connection between the key 20 and control means is electrical, the transducer 26, 32 are not needed.

A system which is a modification of that described above with reference to Figure 2 will now be described with reference to Figures 3 to 5, Figures 3 and 5 showing, respectively, the key and control means of the system.

Referring first to Figure 3, the key comprises a speudo-random generator 22' constituted by a 12-stage shift register 50 and an exclusive-OR gate 52 which has, for example, two inputs connected, for example, to the 11th and 12th stages of the shift register, and an output connected back to the first stage of the shift register.

The output of the generator 22', namely the output of the exclusive-OR gate 52, is connected to an input of an encoder 54. A clock pulse generator 56 supplies clock pulses to the shift register 50 and to the encoder 54. An output of the encoder 54 is connected via a transducer 26' to an output 58, though if an output signal in electrical form is desired the transducer 26' is not needed.

The encoder 54 carries out so-called biphase-mark encoding of the output signal sequence of the pseudo-random generator 22' before the signal sequence is passed to the transducer 26'. The encoder 54 operates by phase-modulating the clock signal from the clock pulse generator 56 with the pseudo-random signal sequence from the generator 22'. More specifically, the output signal of the encoder 54 comprises a signal which reproduces the clock signal except that it is either in or out of phase (0 or 1800) with the clock signal in accordance with the state of the output of the pseudo-random generator 22'. Thus, for example, Figure 4 shows the clock signal at (a), the signal sequence to be encoded at (b), and the encoded signal at (c).

Every time the trailing edge of a clock pulse coincides with the signal sequence to be encoded being at, for example, the low level, the phase of the encoded signal is changed, whereby the encoded signal is either in or out of phase with the clock signal, as represented in Figure 4(c) by the symbols I and 0, respectively. Encoding the pseudo-random signal sequence in this manner leads to the clock rate being "embedded" in the output signal provided by the key, whereby the clock signal can be recovered and used at the control means to avoid the need to synchronise separatelygenerated clock signals.Another advantage of this encoding technique is that the information content of the encoded or modulated pseudo-random signal sequence is contained only in the phase relationship of the pulse edges of the encoded signals, i.e. not in the polarity of the signals, so that the signal sequence has no "DC" content.

The control means shown in Figure 5, in contrast to the control means 30 of the system of Figure 2, comprises a relatively simple, single bit "stop/go" correlator which, if the received and internallygenerated pseudo-random signal sequences match for one bit, goes on to compare the next bits of the sequences. If the compared bits do not match, the internal pseudorandom sequence is stopped, i.e. the control means "hesitates" until a continuing match over a complete cycle of the code is obtained.

In more detail, the control means of Figure 5 comprises a differentiator 60 connected via transducer 32' to an input 62 for receiving the signal from the key of Figure 3, though if the signal is in electrical form the transducer 32' is not needed.

From the differentiator 60 the incoming signal is passed to a detector 64 that demodulates or decodes the signal and supplies the recreated pseudo-random key signal sequence (Figure 4(b)) to a first of two inputs of an exclusive-OR gate 66 and supplies the recovered clock signal (Figure 4(a)) to an integrator 68 and to a first of two inputs of an AND-gate 70.

The control means of Figure 5 further comprises a pseudo-random pulse generator 36' which is of identical construction to the pseudo-random pulse generator 22' of the key of Figure 3 and comprises a 12stage shift register 50' and an exclusive-OR date 52' having a palr of inputs connected to the 11th and 12th stages of the shift register, the output of the gate 52' being connected both to the first stage of the shift register and also being connected to the second input of the exclusive-OR gate 66.

The output of the exclusive-OR gate 66 is connected to an input of a retriggered time delay circuit 72 and via an inverter 74, to the second input of the AND-gate 70. An output of the integrator 68 and an output of the time delay circuit 72 are connected to respective ones of a pair of inputs of an AND-gate 76, the output of which is connected to an output 78 of the control means.

The control means of Figure 5 operates as follows. Assume that a pseudo-random signal sequence is supplied thereto by the key of Figure 3. The first bit of the decoded sequence is applied to the first input of the exclusive-OR gate 66 and the bit currently being produced by the pseudorandom generator 36' is applied to the second input of the gate 66. These bits are compared by the gate 66. If the bits do not match, the output signal from the gate 66, inverted by the inverter 74, is operative on the AND-gate 70 to inhibit supply of clock pulses from the detector 64 to the shift register 50'. Consequently, the current bit produced by the generator 36' remains unchanged and is thus compared with the next bit of the received signal sequence.

This process is repeated until two compared bits match. The output from the gate 66 is then operative via the inverter 74 and the AND-gate 70 to allow clock pulses to reach the shift register 50'. Consequently, the pseudo-random generator 36' is started up, whereby the gate compares the next bit of the received signal sequence with the next bit locally generated by the generator 36'. This process continues, i.e. the generator 36' keeps being started and stopped in accordance with whether a match or a mismatch is detected by the exclusive-OR gate 66. The pseudo-random signal sequence generated by the generator 36' is thus generated at an average rate less than that at which the identical sequence is received from the key. The two sequences thus " roll over" until, eventually, they "lock on", i.e. they become synchronised. When this occurs, the gate 66 continually indicates a match. The retriggered time delay circuit 72 is responsive to the gate 66 continually indicating a match for a period equal to about twice the length of the speudorandom sequence to provide an enabling level to the associated input of the ANDgate 76. The other input of the AND-gate 76 is meanwhile being provided with an enabling level by the integrator 68, such level being provided whenever a received signal is present by virtue of the integrating action of the integrator. Consequently, after a delay of about twice the length of the pseudo-random sequence after the received and locally generated sequences have been synchronised, a control signal indicating same is developed at the output 78 to effect a control function.

To provide additional security, it is possible to replace the pseudo-random generators 22, 36 or 22', 36' in the systems of Figure 2 and Figures 3 to 5 by more complex pseudo-random generators each comprising a plurality of individual generators as described with reference to Figure 1, the individual generators being compounded in parallel and interacting. For instance, the outputs of the logic circuits 12 of two or more generators as shown in Figure 1 could be connected to a combining circuit which is responsive to the individual sequences or codes from the individual generators to produce a more complex pseudo-random signal sequence. The compounding of individual code generators can lead to the possibility of astronomic numbers of possible codes.The use of parallel code configurations is particularly convenient in the case of transmission from the key to the control means by means of fibre optics.

As another example of how individual generators can be compounded, one speudorandom generator output can be switched at a fixed rate by another pseudo-random generator. VariabIe parameters of the composite output sequence in this case are the characteristics of the two individual generators plus the rate and phase of the switching. As an extension of this example, the switching rate can itself be made random in that it can be controlled by the output of a third pseudo-random generator.

Care must be taken that the net result of compounding of generators does not produce such a code or sequence that the time taken to lock on is unacceptable. The compounding should therefore be done in such a manner that sophisticated correlator techniques working in parallel and on a statistical basis can be designed using relevant pseudo-random sequence theory.

Note that the theory of pseudo-random sequences must be carefully applied, since by way of a simple example, the logical combination of the outputs of 2 identical generators albeit phase shifted, can resulted in an identical code sequence merely further phase shifted.

An advantage of the various systems described above is that they have a certain amount of resistance to the introduction of errors into the pseudo-random sequence supplied by the key to the control means.

That is to say, if some of the bits in the sequence are in error when applied to the correlator, this will not prevent the occurrence of locking on. (For example in the cotnrol means of Figure 5, if after synchronisation the exclusive-OR gate 66 occasionally detects a mismatch due to corruption of the received signal, this will not cause the locked-on state to be lost, i.e. the control signal indicating lock-on will not disappear from the output 78). Consequently, these systems are especially suited to use in an electrically noisy environment and/or in applications where the transmission link between the key and control means (e.g.

a telephone or radio circuit) is subject to phenomena that could cause errors to be generated in the transmitted signals.

To provide additional security, the systems described above can be elaborated in a variety of ways. For instance, the coding of the sequence generated by the pseudo-random generator of the control means could be varied from time to time as described hereinabove with reference to Figure 1, for instance by providing a multiposition switch between the shift register 10 and the logic circuit 12 to change the stage tappings i.e. to change the number of tappings and/or the particular tappings connected to the logic circuit. The code could be changed at fixed or random intervals (e.g. by means of an internal timer) or could be changed automatically after each occasion of use. The setting of the code generated by the pseudo-random generator of the key will be similarly variable, and the user would have to- know the correct setting to be able to actuate the control means.In this way, additional security similar to that provided by a combination lock could be obtained.

It is possible to manually vary the key code, for example by means of a keyboard provided on the key. Manual code variations can be superimposed on the output of the code generator and/or superimposed on the phasing of compounded parallel generators.

As will be appreciated, provision on the key of means for manually setting an appropriate code has the advantage that loss or theft of the key does not in itself mean that the finder or thief can operate the control means without hindrance. The possibility of the code being changed from time to time or by use means that the keyholder himself may not be able to use the key until he has obtained information as to the correct setting from an alternative or remote source. This facility could help prevent a key being used by the holder under duress, for instance under threat to himself or to hostages.

It is possible for the key to be fabricated in two or more parts, whereby two or more persons will be needed to actuate the control means. It is further possible to arrange that variations of the set code from an appropriate setting will allow actuation of the control means, but will cause a hidden or silent alarms to be actuated.

Operation of the control means from a completely remote location, e.g. via a telephone line, is feasible.

A system in accordance with the invention could be used to safeguard the transporation of valuables. For instance, a container and/or vehicle provided with a control means in accordance with the invention and with the key not carried on the vehicle could not be unlocked until its destination has been confirmed and cleared.

In the systems described above with reference to the drawings, no signal for synchronising the codes of the key and control means is needed and a high degree of security is provided.

The various circuits disclosed above are suitable for embodiment in LSI form, e.g.

one chip each for the key and control means. Each chip may comprise, for example, a programmable ROM, whereby a desired coding for the pseudo-random generator can be "burnt-in".

WHAT I CLAIM IS: - 1. A security system comprising a key and, separate from the key, a control means for actuation by the key, the key comprising a first generator operative to generate a pseudo-random sequence of signals at a predetermined frequency, and the control means being capable of receiving said signals and comprising a second generator of the same pseudo-random sequence of signals as the first generator and comparison means operative to compare a received signal sequence with the signal sequence generated by the second generator and responsive to complete correlation between at least part of the two sequences to develop a control signal to effect a control function, the comparison means being operative to cause the second generator to generate its signal sequence at a frequency less than said predetermined frequency until complete correlation between at least part of the two sequences is detected.

2. A security system according to claim 1, wherein the comparison means comprises a correlator having a first input connected to receive a received pseudo-random signal sequence and a second input connected to receive the pseudo-random signal sequence -from the second generator.

3. A security system according to claim 2, wherein the second generator is continuously operable and the frequency at which the second generator is operable is variable in accordance with the degree of correlation between the two signal sequences detected by the correlator in such a way that the operating frequency of the second generator approaches that of the first generator as the degree of correlation approaches 100%.

4. A security system according to claim 1, wherein the control means comprises means to recover the frequency of a received pseudo-random signal sequence, generator control means to control application of the received frequency to the second generator, and comparison means to compare each signal in the received signal sequence with a signal produced by the second generator and operative on the generator control means in response to each said comparison to prevent the second generator running if the compared signals do not match and to allow the generator to run if the compared signals do match, whereby the second generator is started and stopped until the two signal sequences, if identical, are synchronised.

5. A security system according to claim 4, including a retriggered time delay circuit responsive to the comparison means indicating matching during a predetermined period to develop said control signal.

6. A security system according to claim 5, including an integrator operative to detect whether a signal sequence is being received, and gate means to inhibit said control signal if the integrator does not detect receipt of a signal sequence.

7. A security system according to any one of the preceding claims, wherein the first and second generators each comprise a shift register and a logic circuit connected to receive the contents of at least one stage of the shift register, the logic circuit having an output which is connected back to a stage of the shift register and at which, in use, a pseudo-random signal sequence is developed.

8. A security system according to claim 7, wherein said logic circuit of each generator is an exclusive-OR gate.

9. A security system according to claim 7 or claim 8, including, in each generator, switch means connected between the shift register and the logic circuit and operable to enable variation of the number of stages and/or the stages of the shift register to which the logic circuit is connected to cause variation of the pseudo-random signal sequence developed at the output of the logic circuit.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the key until he has obtained information as to the correct setting from an alternative or remote source. This facility could help prevent a key being used by the holder under duress, for instance under threat to himself or to hostages. It is possible for the key to be fabricated in two or more parts, whereby two or more persons will be needed to actuate the control means. It is further possible to arrange that variations of the set code from an appropriate setting will allow actuation of the control means, but will cause a hidden or silent alarms to be actuated. Operation of the control means from a completely remote location, e.g. via a telephone line, is feasible. A system in accordance with the invention could be used to safeguard the transporation of valuables. For instance, a container and/or vehicle provided with a control means in accordance with the invention and with the key not carried on the vehicle could not be unlocked until its destination has been confirmed and cleared. In the systems described above with reference to the drawings, no signal for synchronising the codes of the key and control means is needed and a high degree of security is provided. The various circuits disclosed above are suitable for embodiment in LSI form, e.g. one chip each for the key and control means. Each chip may comprise, for example, a programmable ROM, whereby a desired coding for the pseudo-random generator can be "burnt-in". WHAT I CLAIM IS: -

1. A security system comprising a key and, separate from the key, a control means for actuation by the key, the key comprising a first generator operative to generate a pseudo-random sequence of signals at a predetermined frequency, and the control means being capable of receiving said signals and comprising a second generator of the same pseudo-random sequence of signals as the first generator and comparison means operative to compare a received signal sequence with the signal sequence generated by the second generator and responsive to complete correlation between at least part of the two sequences to develop a control signal to effect a control function, the comparison means being operative to cause the second generator to generate its signal sequence at a frequency less than said predetermined frequency until complete correlation between at least part of the two sequences is detected.

10. A security system according to claim

9, wherein the swtich means of the first generator is manually operable and the switch means of the second generator is automatically operable.

11. A security system according to claim 10, wherein the control means includes a timer operative on the switch means thereof to periodicaly after the pseudo-random signal sequence.

12. A security system according to any one of claims 7 to 11, wherein, in each generator, the pseudo-random signal sequence developed at the output of the logic circuit constitutes the pseudo-random signal sequence generated by the generator.

13. A security system according to any one of claims 7 to 11, wherein each of the first and second generators comprises at least one further shift register and logic circuit connected as set forth for the first-mentioned shift register and logic circuit, and means to combine the individual pseudo-random signal sequences developed, in use, at the outputs of the logic circuits to produce a composite pseudo-random signal sequence that constitutes the pseudorandom signal sequence generated by the generator.

14. A security system according to any one of the preceding claims, wherein the key includes means to modulate a further signal with the pseudo-random signal sequence of the first generator and the control means includes means to demodulate the received modulated signal prior to applying it to the comparison means.

15. A security system according to claim 14, wherein the means to modulate and the means to demodulate provide angle modulation and demodulation, respectively, of the further signal.

16. A security system according to claim 15, wherein the means to modulate and the means to demodulate provide phase modulation and demodulation, respectively, of the further signal.

17. A security system according to any one of claims 14, 15 and 16, wherein said further signal is a digital signal.

18. A security system according to any one of claims 1 to 17, wherein the pseudorandom signal sequences are in electrical form, the key comprises a transducer to convert the pseudo-random signal sequence into non-electrical form for transmission to the control means, and the control means comprises a transducer to reconvert a received pseudo-random signal sequence to electrical form prior to its application to the comparison means.

19. A security system according to claim 18, wherein the key transducer is operative to convert the Dseudo-random signal sequence into a luminous (visible or invisible) form.

20. A security system according to claim 19, wherein the key transducer is a light emitting diode.

21. A security system according to claim 19 or claim 20, including light guide means for transmitting the luminous signal sequence from the key to the control means.

22. A security system according to claim 21, wherein the light guide means comprises fibre optics light guide means.

23. A security system according to any one of claims 1 to 17, wherein the pseudorandom signal sequences are in electrical form and the key and control means are so cosntructed that the key can supply its signal sequence to the control means by direct electrical connection of the key and control means.

24. A security system according to any one of claims 1 to 17, wherein the pseudorandom signal sequences are in electrical form, the system comprising a communications link to transmit the pseudo-random signal sequence of the first generator from the key to the control means.

25. A security system according to claim 24, wherein the communications link is a telephone circuit.

26. A security system according to claim 24, wherein the communications link is a radio link.

27. A security system substantially as herein described with reference to Figure 2 of the accompanying drawings

28. A security system substantially as herein described with reference to Figures 3 to 5 of the accompanying drawings.