GB2597658A - Signal authentication system and method - Google Patents

Abstract

A transmitter and/or receiver of the invention comprises of a system for encoding or decoding respectively an authentication signal comprising of an open CDMA signal in which a portion of the chips forming it are replaced with chips at cryptographically derived positions in the CDMA sequence. Thus, a suitably authorised receiver receiving such an encoded authorisation signal, and adapted to decode the cryptographically derived positions is able to generate a correlation with its own internal modified reference signal that proves the identity of the transmitted signal, whilst unauthorised receiver is able to correlate any unmodified chips with its open internal reference. The proportion of chips altered is preferably chosen to allow an unauthenticated receiver to successfully recover the receive signal. An authorised receiver may be adapted to correlate using both a modified reference code, and an open reference code, and compare the results.

Description

Signal Authentication System and Method The present invention relates to authentication schemes for signals. More particularly, it relates to the authentication of coded signals, such as digital spread spectrum signals, code division multiple access (COMA) signals, including those typically used in Global Navigation Satellite Signal (GNSS) systems, communications systems, and radar and imaging systems.

Many GNSS systems, such as the US Global Positioning System (GPS), the European Galileo system, and the Chinese Beidou system transmit their navigation messages using a COMA signal that comprises of a sequence of ranging code chips, where the ranging code is unique to a particular satellite, or a particular service used within a satellite.

Each of these systems includes, in the various services they provide, an Open Service (OS) signal. Open services (OS) signals are typically available to all users and are produced using a published CDMA ranging code (for example a systematic code generated with a logic network) or pre-defined, so-called, memory codes. Many of these systems also provide a Protected service, where the ranging code is encrypted. This is typically generated with a keyed cryptographic function where the key is only known to Authorised users. Protected signals can be received and recovered through correlation and integration by Authorised users who have been provided with access to the key being utilised. Protected signals provide protection to attacks such as spoofing (an attacker generating a signal that looks authentic but seeks to mislead or create a denial of service).

The signals transmitted generally comprise of a sequence of chips called a primary ranging code sequence, which occurs in a time period called the primary ranging code period. The sequence, which is typically hundreds or thousands of chips in length, is generally repeated, with data being imposed on the sequence in known ways, such as by inversion of the complete sequence, or multiple transmissions of the complete sequence, to represent information or data bits.

The transmission of both an open and a separate protected signal places extra demands for resources (such as power and bandwidth) on the transmitting station infrastructure.

The object of the invention is to provide an alternative system that ameliorates or overcomes the disadvantages of the prior art.

According to a first aspect of the present invention there is provided a transmitter, comprising at least of a signal generator and an encryption module, wherein the signal generator comprises of an open service CDMA code generator for the generation of an open service signal comprising of a sequence of chips, the transmitter being adapted to have an authentication mode, in which the encryption module is arranged to replace a proportion of the open service signal chips with chips at cryptographically derived positions in the sequence.

The proportion of chips that are replaced at the cryptographically derived positions may range from a minimum of a single chip in a sequence or block, up to 100% of the chips in the sequence or block.

The sequence of chips of the open service signal may in some embodiments be a repeated sequence of chips, where the sequence may be tens, hundreds, or thousands of chips in length.

Embodiments of the invention therefore provide a way to include authentication for a transmitted open signal as an overlay that is available to Authorised users who have been provided with appropriate means for decrypting the proportion of chips that have been modified at the cryptographically derived positions. The authentication may be verified, in some embodiments, by decoding a signal as transmitted by the transmitter when in the authentication mode, and comparing a decoded signal amplitude (e.g. a correlation amplitude) against a threshold, as detailed later.

Advantageously, in some embodiments of the invention the encryption module is arranged to invert a chip value as generated by the open service CDMA code generator at the cryptographically derived positions. Thus, a code encrypted in this manner would comprise of the regular open service sequence, but with certain bits inverted. The positions of the inverted bits are accessible only to those with the ability to decrypt the encryption used. The encryption and decryption may be performed using for example a pre-shared key approach, or alternatively a public/private key dissemination arrangement, or any other appropriate means.

Alternatively, in some embodiments, the values at the cryptographically derived positions may also be cryptographically derived. Thus, in such embodiments, in general not all of the chips at the cryptographically derived positions will have their values changed compared to that of the open service signal, and typically only approximately half will be changed. However, this can be sufficient to provide a verification authentication, as described herein.

Preferably, in some embodiments, the encryption module is arranged to select chips to be replaced that vary in position within the sequence between successive repetitions of the primary ranging code sequence or in distinct time periods. This makes it more difficult for an attacker trying to decode the encrypted chip positions or to corrupt or interfere with the authentication chips, such as through transmitting targeted interference signals. Also, in some embodiments, the portion of chips to be cryptographically replaced may vary between successive sequences. Again, this makes any attacker's task more complex.

In some embodiments the encryption module may be synchronised to a real-time clock independent from a clock used to produce the open-service chip sequence. In practice however, it is much more convenient for the encryption module to be synchronised to a clock derived from the open service signal itself. In this way the clock may be subject to similar delays and transmission issues as those experienced by the open service (and composite) signals.

In some embodiments the encryption module is arranged to invert a proportion of the chips present in a block of N successive chips, where N need not coincide with the number of chips in the open service chip period (i.e. the primary ranging code period). Thus, in such embodiments, the encryption blocks do not bear any relation in size to the open service primary ranging code period. This helps to provide variations in the proportion of chips within an open service CDMA sequence length that are encrypted, between successive open service CDMA sequences.

In some embodiments the encryption used varies the chips chosen for inversion in successive blocks.

In some embodiments the transmitter is adapted to vary, according to a chosen mode of operation, the number of chips in the open service chip sequence that are chosen to be altered. In the limit, this may extend to altering sufficient chips that the open service signal is unrecoverable, effectively switching off the open service signal for ordinary users, whilst still leaving it available for Authorised users. In some embodiments the mode of operation may extend to varying the number of chips altered, or considered for alteration, in the open-service signal in a range from 1 chip to 100% of the chips in a primary ranging code sequence. It will be appreciated that if almost all of the chips are altered by inverting the bits (as opposed to choosing values cryptographically), then the primary ranging code will be effectively an inversion of the original open-service code, and so be relatively easy to decode. Therefore, in those embodiments where inversion of the cryptographically chosen bits is performed then typically, an average of 50% of the bits in a primary ranging code sequence will be chosen when it is desired to prevent the open service code from being decoded. This may advantageously be done in a manner that may invert a higher percentage in one sequence or block, and then a lower percentage in the next, to provide a greater uncertainty to those attempting to decode the signal. For example, 70% may be inverted in one sequence, then 30% in the next, then 60% etc., where the average over many sequences may fall between say 40% and 60%.

It will be appreciated that the purpose of the invention is to provide authentication, to Authorised users, that the signal received was that transmitted by an Authorised transmitter (i.e. a transmitter used by an Authorised user, rather than, for example, a transmitter that is attempting to spoof the receiver into accepting its false signals as being genuine) while still, in general, allowing an ordinary user to decode the non-encrypted parts of the signal and to use that for their purposes. Note here that an "ordinary user' is one who can decode the open service signal but does not have the ability to decode the encrypted parts of the transmitted code, whilst Authorised users are those who can decode the open service, but who have the appropriate means (such as decryption keys etc.) to decode the encrypted parts. It will be apparent to a normally skilled person that the recovered authenticated signal (as recovered using a reference code which includes the encrypted modifications) should have a similar amplitude to that of an unaltered open service CDMA signal that has been recovered using the unaltered reference signal.

It will also be apparent to a normally skilled person that if a user of the open service CDMA signal who did not have the ability to decode the authenticated signal using the authenticated reference code were to just use the unaltered open-service CDMA reference code, then they would still be able to correlate those parts of the signal that have not been altered, and would produce a correlation result. As the correlation would not be successful with respect to the encrypted parts of the authenticated signal, the correlation peak would be lower than it otherwise would if the CDMA signal had not been altered to produce the authenticated signal. However, provided that not too much of the open service CDMA signal is altered, it will still generally produce a useable output, albeit with a decreased signal to noise level of the correlated signal. Thus, the invention allows an authentication means to be added to the transmitted signal whilst still allowing the signal to be decoded by a general user.

Thus, an ordinary user will want to be able to decode the transmitted signal using just the open-service CDMA reference signal. To allow comparison of relative decoded signal strengths, Authorised users will also wish to decode transmitted signals in a similar manner. Thus, in some embodiments the receiver is further arranged to correlate the received signal with a reference code comprising an unmodified version of the open service CDMA reference code as stored in the reference code generator to produce a recovered open service correlated signal.

In GNSS receivers that are arranged to receive a CDMA signal (as well as other CDMA receivers) an important first step is to acquire each signal, which means determining its exact arrival time and its exact frequency offset, due to Doppler frequency shifts in the received signal etc. If its arrival time were known sufficiently accurately then a reference code which includes the encrypted modifications (as explained herein) could be used in the acquisition process. However in most cases, before acquisition has been completed the exact arrival time is not known so it is not possible to know the positions of the cryptographically modified chips. Therefore even the Authorised user is forced to use the unmodified version of the open service CDMA reference code during this acquisition period.

Advantageously therefore, in some embodiments of the invention, typically only a minority of the chips will be cryptographically On position and/or value) modified, where that minority is chosen to allow the correlation peak achievable by an ordinary user to be sufficiently large to permit successful acquisition of the signal by the ordinary user, with an acceptably small loss in signal detection performance in the acquisition process.

Once each signal is acquired the receiver will, as in a normal GNSS receiver, or other CDMA correlation receiver, start to track the signal, to follow its variations in arrival delay and Doppler frequency. Again the receiver may not initially have sufficiently accurate information about the timing of the arriving signal to determine the positions of the cryptographically modified chips, so it will have to use the unmodified version of the open service CDMA reference code. Because only a minority of chips are modified, tracking performance will, for the Authorised user as much as ordinary user, be only slightly degraded.

Using the example of a navigation receiver, once sufficient signals are being tracked the receiver will, in the same way as any other GNSS receiver, be able to determine the receiver's position, velocity and the time in the usual manner.

Once time has been determined the Authorised receiver (i.e. a receiver equipped to decode an open service signal modified as explained herein) is arranged to synchronise the cryptographically derived positions and values of the alternative bits to coincide with those used in the generation of the signal in the transmitter. This may be done using any suitable search method, and methods are known for doing this. They include, for example applying different time and frequency offsets and looking for a best match at a correlation output.

The Authorised receiver can then start to correlate each received signal with the corresponding authenticated signal, that is, the reference code including the encrypted modifications. As a result the correlation peaks are of full height and the tracking performance is unimpaired.

The next task for the Authorised receiver is to determine whether the signals being received are the authenticated signals.

Conveniently, in some embodiments, the invention is arranged to compare amplitudes (which may for example be root-mean-square amplitudes, mean-square amplitudes, or mean-absolute amplitudes of the recovered authenticated signal and the recovered open service correlated signal, and to use the comparison to determine an authentication measure relating to the recovered authenticated signal.

The authentication measure may, in some embodiments relate to the expected relative amplitudes of the received authentication signal as correlated with an authentication reference code, and as correlated with a standard open service code. The authentication measure may relate to some function of these amplitudes.

In some embodiments, where a "real-world" receiver is subject to noise etc., the authentication measure may be obtained over any given integration time interval as follows: Authentication Measure = Recovered Authenticated Signal -Recovered OS Signal From this an authentication confidence can be calculated such as: Authentication Confidence -Recovered Authentication Signall This measure confirms the authentication of the received signal if it is sufficiently close to the expected proportion of authentication chips over the measurement interval.

Appropriate, well known techniques such as a likelihood ratio test may be applied to achieve a statistical measure and an appropriate threshold may be used.

It will be appreciated by the normally skilled person that the transmitter according to embodiments of the present invention may also typically comprise of further elements or functional blocks, including one or more oscillators, amplifiers, filters, modulators, power supplies etc. depending on the application in which the invention is being used, and such elements would typically be required to make a functioning, practical transmitter.

However, they do not form part of the key inventive elements, and their use and configuration would be clearly understood by the normally skilled person, and so haven't been described in detail.

According to a second aspect of the present invention there is provided a receiver for receiving Code Division Multiple Access (CDMA) signals, comprising at least of a decoder, a reference code generator and a decryption module, wherein the receiver is adapted to decode, in the decoder, the received signal by correlating the received signal with data from the reference code generator, wherein the reference code generator has stored therein a local copy of an open service CDMA reference code used in the generation of the signal in an associated transmitter as described herein, and further Authentication Measure wherein the decryption module is arranged to replace a proportion of the chips from the reference code generator with alternative chips at cryptographically derived positions in the sequence, to produce an authentication reference code used for the correlation, wherein the correlation between the received signal and the authentication reference code produces a recovered authenticated signal.

Thus, a receiver according to some embodiments may be arranged to process signals produced by a transmitter as described herein, and to produce an authentication reference code for use in decoding the received signal.

As is the case with at least some embodiments of the transmitter, advantageously the receiver may be arranged to replace a proportion of the chips from the open-service reference code generator by inverting the chips of the open service CDMA reference code at the cryptographically derived positions. The decryption module may be used to determine which chips are to be inverted.

It will be appreciated by the normally skilled person that embodiments of the invention would typically have further functional components concerned with the reception and demodulation of a signal, to make a working receiver, depending upon the particular application to which it is to be put, such as amplifiers, filters, demodulators etc. The configuration of such elements would be known to the normally skilled person, and so are not discussed in detail further.

It will further be appreciated by the normally skilled person that embodiments of the invention may be used on, in GNSS signals, a pilot signal and/or on a data signal (where both signals comprise of CDMA data streams). Conveniently, it may just be applied to the pilot signal, leaving the data signal unaltered.

When applied to a signal, as previously explained, there is a reduction in the signal available for correlation by an ordinary user according to the proportion of bits given over for alteration. Where, say 30% of the bits are altered, then there will be a 1.5dB (approx.) loss over an unaltered open service signal, which may be compensated for by having longer integration times, as would be appreciated by the normally skilled person. The loss over the unaltered open-service signal drops to approx. 0.7dB when 15% of the bits are altered.

The invention will now be described, by way of example only, with reference to the following Figures of which: Figure 1 diagrammatically illustrates a high level block diagram of a CDMA correlation-s based receiver; Figure 2 diagrammatically illustrates a high level block diagram of a composite reference signal generator as may be used in a transmitter or receiver according to an embodiment of the present invention; Figure 3 diagrammatically illustrates a high level block diagram of a composite reference signal generator suitable for use in a receiver according to an embodiment of the present invention; and Figure 4 diagrammatically illustrates a high level block diagram of a correlation decoder according to an embodiment of the present invention.

Shown in Figure 1 is a system 10 for decoding CDMA signals, such systems being commonplace in GNSS systems, positioning, radar and imaging systems, as well as in telephony and other communications protocols. A received signal, such as a radio signal is typically amplified and mixed down to become a baseband signal before being provided as an input 12 to a complex mixer 14. A second input to mixer 14 is from a locally derived reference code 16 comprising a sequence of chips, that is intended to match an expected code used in the generation of the received signal. These inputs to the mixer are multiplied together and the result integrated in integrator 18, to produce a recovered signal 20. A known synchronisation process to temporally align the locally derived reference code with the incoming received signal is done, and when so aligned, the integrator produces a correlation peak of a size dependent upon the length of the input correlation sequence, and the degree of similarity between the two inputs to the mixer.

In systems such as GNSS navigation systems, the length of the code is typically many hundreds of chips in length, or longer. This allows a large correlation gain to be achieved, and so allow the recovery of weak signals. Such long sequences also allow room for error in one or more bits, which will inevitably occur in practical (e.g. noise-prone) systems. Thus, there is a level of redundancy built in to allow parts of the signal to be corrupted whilst still allowing signal recovery through the correlation process. The level of redundancy may be defined at the system specification stage of the design.

The redundancy that is built into such systems allows some of the chips to be altered during generation and transmission of an encoded signal, whilst still allowing the signal to be successfully received and decoded using the local copy of the unaltered reference signal, provided that not too many of the chips have been altered. This allows additional information to be imparted on the transmitted signal by altering one or more of the chip values.

Where the system of Figure 1 is used with a locally generated reference code that is identical to an open-service reference code, then the system is a prior art system. However, where it is used with a locally generated reference code that has been altered as described herein, then the system of Figure 1 may implement an embodiment of the present invention.

Figure 2 shows parts of a composite ranging code generator 22 of a transmitter as may be used in an embodiment of the invention. The transmitter may be a radio transmitter, or some other type of signal transmitter or signal generator, and will be described here in the context of a transmitter of a GNSS system It comprises of an open service ranging code generator 24, for generating a (publicly known) reference code; an authentication puncturing sequence generator (APSG) 26, for cryptographically selecting particular chips of the open service ranging code to be altered; a key 28 used in the generation of a cryptographic sequence; an inverter 30 that provides an inverted version of a chip currently being provided by the ranging code generator 24 as part of a chip sequence, and a switch 32 for selecting for further processing and transmission either (at any given instant) a chip from the open service ranging code generator, or the inverted version thereof (it thus being a composite code 36). It further comprises of a mode selector 34, that determines a mode of operation of the system, as explained further below. A practical system will likely have amplification, filtering etc. and probably frequency up-conversion also, but these have not been shown for clarity purposes, and because their purpose is well known, and not directly relevant to the present invention.

In operation the open service ranging code generator generates a ranging code for use by anyone (hence it being a publicly known code). The APSG is a cryptographic unit that, together with its key 28 as supplied to it generates a switching signal that, according to a cryptographically determined pattern, operates the switch at selected instances so that, when so operated, it directs for transmission an inversion of the open service ranging code chip currently being generated. The selected instances (i.e. the choice of chips to be inverted) are clearly not predictable to anyone without knowledge of the cryptographic algorithm and key used in their generation. In one mode of operation, as determined by the mode selector 34, approximately 15%, on average, of the chips in a repeated sequence from the open service ranging code generator are chosen from the inverted sequence, as provided by inverter 30, for transmission. The positions of these inverted chips within the sequence are of course not known or detectable by anyone without the knowledge of the cryptographic unit and key.

Other embodiments may have, e.g. 5%, 10%, 20%, 30% 40% or 50% or more of the chips altered (or selected for consideration for alteration). In most instances, it will be the aim of the system to provide minimal disruption to the open-service signal whilst still providing a means for an Authorised user to authenticate the received signal. Therefore, the lower end of the range is likely to be preferred in such instances.

The key may be pre-shared between the transmitter and the receiver, or alternatively, keys may be distributed to or between the transmitter and receiver using known means, such as public/private key dissemination, or any other form, as would be understood by the person normally skilled in the art.

The mode selector is able to select other modes of operation. For example, in another mode the mode selector 34 may be arranged to set the APSG to select a different proportion of the chips for inversion before being transmitted. Such proportion may range from zero (i.e. wherein the open service ranging code is transmitted in full, with no alterations being made to it), to a value whereby the open service code is unrecoverable. This may, in some embodiments be around 50% -60% of the chips.

An embodiment of a receiver may have an identical composite ranging code generator to that shown in Figure 2, for operation when the receiver has been synchronised in time with the incoming signal, as described above. Such a generator in a receiver would then be used to supply the locally generated reference code signal 16 in Figure 1 Synchronisation of the timing of the APSG in the receiver with that of the transmitter is important to allow the receiver to process the correct chips, and so allow recovery of the signal through the integration process 18 of Figure 1. Such synchronisation is achieved with reference to a clock signal common to both the receiver and transmitter, such as a GNSS derived clock signal, as described above.

In an embodiment of the invention the chips chosen for alteration (e.g. inversion) are selected by using a keyed algorithm that cryptographically generates a sequence of random numbers, one for each chip. If the value of a given number generated as a percentage of some maximum value of the range exceeds a chosen threshold then the corresponding chip is inverted. Selection of the threshold therefore gives a direct relationship to the average percentage of chips to be altered in a sequence or block. Of course, other embodiments may use different algorithms to select the chips to be altered.

Although Figure 2 shows an embodiment wherein cryptographically chosen chips are inverted, in other embodiments the cryptographically chosen chips may be instead set to a cryptographically chosen value, where that value may, in some instances, be the same as that of the open service chip sequence. Of course, although some chips may remain the same in such embodiments, others would have their values changed, to enable an authentication reference code that is different to the open service code to be produced.

Figure 3 shows a further composite reference code generator of the type that may be used in a receiver. It is very similar to that shown in Figure 2, and like reference numerals refer to identical components. The difference between it and the embodiment of Figure 2 is that, as well as composite output 36 being provided as an output, a second output 38 is provided that comprises the standard open service reference code. This output may be used in a correlation decoder when a composite signal is not being transmitted by an associated transmitter (or by an ordinary user when a composite code is being transmitted), or may be used simultaneously with a composite code, as explained below in relation to Figure 4.

Shown in Figure 4 is a further embodiment of the invention, as implemented in a receiver. This comprises of a dual integrator approach, with one integrator arranged to process an input signal using an open service reference code, and another integrator arranged to process a composite reference code. The embodiment 40 therefore comprises of a first integration path comprising of a complex multiplier/mixer 42 that feeds an integrator 44, in integration path 46. An input signal 48 is equivalent to that of input 12 in Figure 1 and feeds one input to the multiplier 42. The multiplier 42 has another input from an open service reference code generator 50, and so the multiplier 42 and integrator 44 are acting in the same way as a prior art system that is decoding an open service signal.

A second integration path 52 again comprises of a complex multiplier/mixer 54 that feeds an integrator 56. A first input to the mixer is again from the input signal 48, while the second input is from a composite reference code generator 58 of the type shown in Figure 2. Each integrator produces an output. The output from the first integration path is the correlation of the input signal as multiplied by the unmodified open service reference code, and so is the same as would be obtained in a prior art system. The output from the second integration path is the correlation of the input signal as multiplied by the composite reference code.

When a signal is being transmitted that has been modified in a fashion as described herein, and assuming that the synchronisation between the transmit and receive-side APSGs has been completed, then the output from second integration path should be a recovered signal with its full signal-to-noise ratio, whilst the output from the first integration path will be a recovered signal having a signal-to-noise ratio that is reduced by an amount proportional to the number of chips in the open-service sequence that have been given over to alteration by the APSG process.

The receiver of this embodiment is adapted to compare the levels at the outputs of each of the integrators to indicate the authenticity of the signals transmitted. It does this using subtraction block 60 that is arranged to provide the difference between the two integrator path outputs. If the transmitter that is transmitting the received signals is the authentic transmitter, then the recovered signal from the second integration path will be larger than that from the first integration path, with a difference On a noiseless system) that is determined by the amount of the open-service code that has been given over to alteration.

The recovered signals from the two integration paths may be compared, e.g. as explained above (or in any other appropriate manner) to determine whether the received signal is an authentic signal.

It will be appreciated that data authentication may also be used alongside any signal-level authentication provided by embodiments of the present invention. Data authentication techniques typically provide for authentication of the data that is modulated onto the CDMA signal stream by, for example, including a digital signature. Data authentication techniques are well understood by the normally skilled person, and so will not be described in detail further. In satellite navigation systems these techniques are often known as Navigation Message Authentication (NMA). However, the present invention provides a benefit over NMA in that the authentication of the underlying CDMA signal is provided, which does not happen in pure NMA methods, and also the time taken to confirm the authenticity of the signal can be lower than in NMA.

Embodiments of the invention may be applied to communications and other fields including radar, LIDAR etc., as well as navigation signals. Although this specification describes the embodiments of the invention generally in terms of use with a navigation signal, it should be understood that it has wider applicability, including to those fields mentioned above. The invention has been described in terms of binary signals, where the CDMA primary ranging code or its equivalent (depending on the type of system) comprises of a binary open service chip sequence. The invention extends to multi-valued chip sequences (where the number of values is greater than 2), as well as to analogue sequences.

Claims (16)

1. Claims 1. A transmitter, comprising at least of a signal generator and an encryption module, wherein the signal generator comprises of an open service CDMA code generator for the generation of an open service signal comprising of a sequence of chips, the transmitter being adapted to have an authentication mode, in which the encryption module is arranged to replace a proportion of the open service signal chips with chips at cryptographically derived positions in the sequence.
2. 2. A transmitter as claimed in claim 1 wherein the encryption module is arranged to invert a chip value as generated by the open service CDMA code generator at the cryptographically derived positions.
3. 3. A transmitter as claimed in claim 1 or claim 2 wherein the encryption module is arranged to select, in general, a portion of the chips to be replaced that varies in position within the sequence between successive sequences.
4. 4. A transmitter as claimed in any of the above claims wherein the portion of chips to be cryptographically replaced may vary between successive sequences.
5. 5. A transmitter as claimed in any of the above claims wherein the encryption module is synchronised to a real-time clock independent from a clock used to produce the open-service chip sequence.
6. 6. A transmitter as claimed in claim 2 and those dependent therefrom, wherein the encryption module is arranged to invert a proportion of chips in a sequence block of N successive chips, where N need not coincide with the number of chips in the open source sequence of chips.
7. 7. A transmitter as claimed in claim 6 wherein the chips chosen for inversion in successive blocks varies from one block to another.
8. 8. A transmitter as claimed in any of the above claims wherein the transmitter is adapted to vary, according to a chosen mode of operation, the number of chips in the open service chip sequence that are chosen to be altered.
9. 9. A transmitter as claimed in claim 8 wherein the number of chips varied may range from 1% to 50% of the chips
10. 10. A transmitter as claimed in claim 1 wherein the chip values that replace the open service chips at the cryptographically derived positions are cryptographically derived.
11. 11. A receiver for receiving Code Division Multiple Access (CDMA) signals, comprising at least of a decoder, a reference code generator and a decryption module, wherein the receiver is adapted to decode, in the decoder, the received signal by correlating the received signal with data from the reference code generator, wherein the reference code generator has stored therein a local copy of an open service CDMA reference code used in the generation of the signal in an associated transmitter as claimed in any of claims 1 to 10, and further wherein the decryption module is arranged to replace a proportion of the chips from the reference code generator with alternative chips at cryptographically derived positions in the sequence, to produce an authentication reference code used for the correlation, wherein the correlation between the received signal and the authentication reference code produces a recovered authenticated signal.
12. 12. A receiver as claimed in claim 11 wherein the replacement of a proportion of the chips from the open-service reference code generator is done by inverting the chips of the open service CDMA reference code at the cryptographically derived positions.
13. 13. A receiver as claimed in claim 11 or claim 12 wherein the decryption module is arranged to synchronise the cryptographically derived positions and values of the alternative bits to coincide with those used in the generation of the signal in the transmitter.
14. 14. A receiver as claimed in any of claims 11 to 13 wherein the receiver is further arranged to correlate the received signal with a reference code comprising an unmodified version of the open service CDMA reference code as stored in the reference code generator to produce a recovered open service correlated signal.
15. 15. A receiver as claimed in claim 14 wherein the receiver is arranged to compare amplitudes of the recovered authenticated signal and the recovered open service correlated signal, and to use the comparison to determine an authentication measure relating to the recovered authenticated signal.
16. 16. A transmitter-receiver pair comprising a transmitter as claimed in any of claims 1 to 10, and a receiver as claimed in any of claims 11 to 15.