EP1152108B1 - Hand-free access system for a motor vehicle - Google Patents

Description

The present invention relates to an access system said hands free for a motor vehicle, ie a system of wireless communication to enter the vehicle without key. This system can also be applied to hands-free startup of the vehicle, ie when starting without a key.

Such a system generally comprises a device identification to be worn by a user and able to establish bidirectional remote and wireless communication with a unit central control unit on the vehicle, to authenticate the user and control means of condemnation / unlocking door locks when the user has been recognized authentic. Initialization of the communication protocol can be activated by operating the outside door handle, for hands-free access, or by pressing a start button, in the hands-free start mode. The system is capable of establishing bidirectional communication when the identification device is located at a distance less than a predetermined distance from the vehicle, usually of the order of a few meters, to avoid, on the one hand, interference with other sources of the environment and, on the other hand, to avoid the system at such a distance that the user is too far from the vehicle to be aware of the operations performed by the said system.

Some currently proposed systems use magnetic induction at very short range, both to supply energy and transport information from the central unit of the vehicle to the identification device that is in the field electromagnetic generated by the antennas of the vehicle. However, a Such a system allows communication only at a very short distance from the vehicle of the order of a few cm. Another common system proposed is to use low-frequency carrier waves, the order of 125 kHz for communication from the vehicle to the identification device, and ultra-high carrier waves frequency, for example of the order of 434 or 868 MHz, for the zone Europe, and 315 or 902 MHz for the USA zone. However, in this In this case, the identification device must include a battery to power its own electronic circuits. To minimize consumption electric device, it can be provided, for example, that the device identification is dormant for 9 ms and waking for 1 ms, while periods of 10 ms.

Of course, two-way communication between the vehicle and the identification device is encrypted, in order to avoid any inadvertent operation of the system and to secure it against malefactors. In Figure 1 of the accompanying drawings, there is shown a example of encryption system already known. In this figure 1, we have represented a vehicle V which comprises in its central unit a memory 1 containing a secret key K and a number generator random 2, the random numbers R generated having, for example, a length of 56 bits. This random number R is sent to the identification device I, as indicated by the arrow 3. Simultaneously, this same random number R is mixed with the key secret K according to a complex associative function f, in a mixer 4 which is connected to its input to the memory 1 and to the generator random numbers 2. The mixer 4 delivers, as output, a signal representative of the mixture of the secret key K and the random number R, namely the signal f (R, K). This signal is stored in a memory 5 connected to the output of the mixer 4. This signal is sent to the device identification number I, in the form of a signal of a length example of 28 bits, as indicated by the arrow 6. In the vehicle V, the signal f (R, K) is mixed again with the secret key K in a mixer 7, which is connected to its input to memories 1 and 5 above. This mixer 7 mixes the two signals according to a function associative complex g. The vehicle V then memorizes in a 8 connected to the output of the mixer 7 the signal representative of the mixture, namely the signal g (R, f, K).

On the identification device side, the same secret key K is stored in a memory 11 and a mixer 14 having the same associative function f receives as input the secret key provided by the memory 11 of the identification device I and the random number R received by the identification device from the vehicle. The identification device I stores the output signal of the mixer 14 in a memory 15 and compares this signal in a comparator 16 with the signal received according to arrow 6 from vehicle V. If the two signals are not identical, subject to the time of hardware and signal delay in the authorized transmission, the identification device interrupts the communication as being unauthorized. On the other hand, if both signals match, the signal is mixed in a mixer 17 with the secret key provided by the device memory 11 identification I, according to the same associative function g mentioned above. The output signal of the mixer 17 is stored in a memory 18 of the identification device to be then sent to the vehicle following the arrow 9 in the form of a signal having a length per example of 20 bits. Finally, the signal received by arrow 9 is compared with the signal received from the memory 8 of the vehicle in a comparator 10. If both signals match, subject to delays due to hardware response time and signal transmission in the authorized zone, the rest of the communication is authorized and the unit central office of the vehicle may, if necessary, order the condemnation or unlocking of locks vehicle. Of course, another encryption protocol can be used to secure the transmission of data.

However, despite this encryption protocol, there is a way to hack the system, without knowing either the secret key or the different associative functions of the encryption protocol. This method of Piracy is shown in Figure 2. According to this method, it is assumed that the user U who carries the identification device I is located at a distance of the vehicle V greater than the authorized distance of communication, for example from 10 to 100 m distance from the vehicle. In this case, a pirate equipped with a first relay box 20 can approach vehicle V at a sufficient distance to communicate with it, for example at a distance of the order of 1 to 5 m. This hacker activates the beginning of the communication, for example by the outside door handle. This triggers the emission of low frequency signals from the vehicle to the relay housing 20, as indicated by the arrow in zig-zag 21. This signal 21 sent by the vehicle is received by a coil 22 of the relay housing 20, which is connected to a receiver 23 at 125 kHz. This receiver 23 is connected to a transmitter at broadband at high frequency, of the order of several MHz. The transmitter 24 transmits via its antenna 25, as represented by the arrow 26, to a second relay housing 30, which is worn by another hacker who closely follows the user U. The exchange of information between the two relay housings 20 and 30 being performed at very high frequency, it It is possible to perform this communication over a long distance. The second relay housing 30 includes an antenna 31 to receive the signal 26 emitted by the relay housing 20. The antenna 31 is connected to a broadband receiver at the same frequency as the transmitter 24 of the first relay box 20. The signal thus received is retransmitted at low frequency at 125 kHz by a transmitter 33 which is connected to a coil transmission 34 to send a signal 35 to the device identification I which complies with the signal 21 emitted by the vehicle. The signal 35 being the repetition of the authentic signal of the vehicle, the identification device I will recognize it and emit in turn its response signal 36, said response signal 36 being sent high frequency and received by an antenna 37 of the second relay housing 30, by example at 434 MHz. The antenna 37 is connected to a receiver 38, which will convert the 434 MHz signal into a signal at a different frequency, for example at 315 MHz. The signal is then emitted by a transmitter at broadband 39 via an antenna 40 to the first relay housing 20, this difference of frequency being necessary so that the different signals do not interfere with each other. Of course, the frequency of signal 41 sent back by the second relay housing 30 is different both the frequency of the signal 26 and the signal 36. This signal 41 is taken by an antenna 27 of the first relay housing 20, said antenna 27 being connected to a broadband receiver 28 of the same frequency as the transmitter 39. The receiver 28 is connected to a transmitter 29 which transforms the 315 MHz signal into a 434 MHz signal that is sent via the antenna 42 of the first relay housing 20 to the vehicle V, as represented by the zig-zag arrow 43.

It is sufficient for hackers to use relay boxes with broadband links, for example greater than 50 MHz, which is possible because pirated systems do not have to respect the regulations; additional transit time due to distance can then be of the order of a few nanoseconds, which is negligible in comparison with the necessary time constants for normal authorized transmission. For example, the total communication can be in the order of 20 to 40 ms, and the duration total operation of the system to trigger the unlocking or condemning electric locks can be of the order of 100 ms.

To detect such hacking and interrupt the communication, a solution could be to measure the propagation time of UHF radio waves, comparing this measured time with a time predetermined amount corresponding to a communication in a zone limited allowed around the vehicle. However, to detect the time delay due to distance, compared to a communication time overall, it is necessary to have a high bandwidth, which corresponds to a fast communication rate, for example of the order of 20 to 40 Mb / s. With such a rate of communication, It is necessary to operate at very high frequency, for example at 2.4 GHz. But to measure very short times, it is necessary to have a bandwidth very important, which comes up against the applicable regulations that severely restrict bandwidth allowed to avoid saturation of the environment by electromagnetic.

EP-A-0 992 408 discloses a system according to preamble of claim 1.

The object of the invention is to eliminate the aforementioned drawbacks and to propose a system of access said hands-free for motor vehicle, to detect hacking of the system, in particular by intermediates relay boxes, taking into account the time of signal propagation between the vehicle and the identification device.

For this purpose, the invention relates to an access system said hands free for motor vehicle, according to claim 1.

"Transmission time" useful "the transmission time of the bit information of said bit relative to the total time of the cell with which is associated said bit which comprises generally said useful time and a time of silence to allow the receiving the signal re-transmitted by the identification device during said silence time.

Advantageously, said predetermined threshold value is given by a window of time that corresponds to the total duration theoretical transmission of the anti-piracy bitstream, when the identification device is at a distance less than or equal to the predetermined limit distance. This window of time is a constant precise generated for example by a quartz controlled oscillator associated with a counter under control of the microcontroller which indicates the number of periods to count. The central control unit is suitable to determine said hacking attempt when the total number of bits the anti-piracy train was not issued or received by the Central Unit of command in said window.

According to another characteristic, the central unit may comprise in memory a double-entry correspondence table, namely the number of transmitted bits and the corresponding actual transmission duration, for output the actual distance between the identification device and the central unit, the data of said table being previously acquired by experimentation and which can be periodically updated, when the user wearing the identification device is sitting on the driver's seat of the vehicle, the distance from the identification device to the central unit of the vehicle then being substantially constant.

For example, the count of the number of bits sent is stopped when the central unit detects the reception of the last bit of the train anti-piracy.

Preferably, the identification device comprises a line to analog delay to re-transmit the signal received from the vehicle to the vehicle with a predetermined delay time.

According to another characteristic, the central control unit has an oscillator generating radio carrier waves frequency, connected to a phase modulator, which is controlled by the anti-piracy bitstream generated by the central unit of the vehicle.

According to one characteristic of the invention, the anti-piracy bit stream is generated by the CPU after the transmission of data encrypted authentication to the identification device. In this In this case, the identification device may comprise a phase inverter to selectively invert the phase of the signal representative of the bits anti-piracy received by the identification device from the vehicle, said inverter being controlled, for each bit of anti-piracy, by an encrypted digital authentication response signal identification device, at a slower rate than that of the anti-piracy bitstream.

One can then foresee that the central unit of the vehicle comprises a phase demodulator to demodulate the signal received by the vehicle from the identification device, a logic gate Exclusive OR whose entries are respectively linked to audit phase demodulator and a digital signal retarder, said self-timer being able to delay a useful bit every bit of the train anti-piracy generated by the central unit, said logic gate being capable of outputting a digital signal representative of the signal encrypted response of the identification device.

In a first embodiment, the signal delivered in output of the aforementioned exclusive OR logic gate is compared by the unit central to an encrypted digital signal of authentication generated by the central unit, in order to authenticate the identification device.

In another embodiment, the output of said door OR logic is connected to an input of a second gate Exclusive OR logic whose other input receives a digital signal encrypted authentication generated by the central unit, in order to deliver output a signal representative of successive time offsets of each bit of the anti-piracy train, the latter being received at the input of an integrator to sum the related response times at each anti-piracy bit shift slot, the output of said integrator being connected to a comparator to compare said sum response time with a predetermined time-out value, beyond which an attempted hacking is detected.

Advantageously, it can be expected that the signal coming from the identification device is received by the Central Unit and transmitted to the input of an envelope detector for detecting the rising edge of said signal, the detection of this rising edge being able to trigger, a part, the incrementation of a unit of the number of bits counter and, on the other hand, the transmission of the next bit in the anti-piracy bit stream, after a predetermined duration which corresponds to a bit time useful possibly increased by extra time to avoid any interference between the transmission and the reception of the signal by the unit Central.

In this case, it can be provided that the central unit comprises a modulator says all or nothing to switch the CPU in the transmission or reception of signals to or from identification device, said all-or-nothing modulator being controlled by detecting the rising edge of the signal received in from the identification device.

Advantageously, the identification device comprises a receiver to receive low-speed wakefulness signals, this receiver comprising successively a static envelope detector low-threshold radio frequency and a low frequency amplifier.

By way of example, the useful bit time can be between 50 and 200 ns.

• la figure 1 est un schéma synoptique fonctionnel représentant le protocole d'encryptage pour sécuriser la transmission bidirectionnelle de données entre un véhicule et un dispositif d'identification ;
• la figure 2 est un schéma synoptique fonctionnel illustrant un moyen de piratage du système d'encryptage par l'intermédiaire de deux boítiers relais ;
• la figure 3 est un schéma synoptique fonctionnel simplifié d'un système d'accès mains libres conforme à l'invention ;
• la figure 4 est un schéma synoptique fonctionnel plus détaillé correspondant au schéma de la figure 3 ;
• la figure 5 représente plusieurs chronogrammes illustrant les trames complètes d'interrogation émises par le véhicule et reçues en réponse par le véhicule ;
• la figure 6 est une vue partielle et agrandie d'une portion des chronogrammes de la figure 5, indiquée par la flèche VI, correspondant au début de la séquence d'anti-piratage ;
• la figure 7 reprend les deux premiers chronogrammes de la figure 5;
• la figure 8 est une vue partielle et agrandie d'une portion des chronogrammes de la figure 7, indiquée par la flèche VIII, au cours de la séquence d'anti-piratage ; et
• la figure 9 est une vue partielle et agrandie d'une portion des chronogrammes de la figure 5, indiquée par la flèche IX, et correspondant à la fin de la procédure d'anti-piratage.

The invention will be better understood, and other objects, details, features and advantages thereof will appear more clearly in the following detailed explanatory description of a particular embodiment of the invention, given only to illustrative and nonlimiting, with reference to the appended schematic drawing, in which:

• FIG. 1 is a functional block diagram illustrating the encryption protocol for securing the bidirectional transmission of data between a vehicle and an identification device;
• FIG. 2 is a functional block diagram illustrating a means of piracy of the encryption system by means of two relay boxes;
• Figure 3 is a simplified block diagram of a hands-free access system according to the invention;
• FIG. 4 is a more detailed functional block diagram corresponding to the diagram of FIG. 3;
• FIG. 5 represents several timing diagrams illustrating the complete interrogation frames transmitted by the vehicle and received in response by the vehicle;
• Figure 6 is a partial and enlarged view of a portion of the timing diagrams of Figure 5, indicated by the arrow VI, corresponding to the beginning of the anti-piracy sequence;
• Figure 7 shows the first two chronograms of Figure 5;
• Fig. 8 is a partial and enlarged view of a portion of the timing diagrams of Fig. 7, indicated by arrow VIII, during the anti-piracy sequence; and
• Figure 9 is a partial and enlarged view of a portion of the timing diagrams of Figure 5, indicated by the arrow IX, and corresponding to the end of the anti-piracy procedure.

We will now refer to Figures 3 and 4, which represent the hands-free access system according to the invention respectively under Simplified form and more detailed.

The motor vehicle V comprises in its central unit a micro-controller 50, which is usually in a state of semi-sleep or waiting for an alarm clock. When the user operates the outside door handle, an activation signal is sent to the micro-controller 50, as indicated by the arrow 51. In response, the micro-controller sends a general power signal, as represented by the arrow 52, to supply the different components electronic units of the central unit. Then, the microcontroller 50 generates a series of low speed signals sk, for example of the order of 2 to 100 Kb / s on line V5. The data conveyed on line V5 are successively an awakening signal e, a signal representative of a number random R of a length for example of 56 bits, a signal representative of the function f (R, K) with a length of, for example, 28 bits, and a signal representative of service data s, for example from 100 to 5000 bits, for example data on the maintenance, vehicle tuning, etc. (see Figure 5). Line V5 is connected to a transmitter 53 for transmitting via an antenna 54 the signals to the identification device I as represented by the arrow E. The transmitter 53 comprises, as best seen in FIG. oscillator 55 to generate an ultra high carrier wave frequency, said oscillator being powered by line 52. The oscillator 55 is connected to a phase modulator 56, which is in turn connected to a all-or-nothing modulator 57, the latter having an input connected to the line V5 and its output connected to the antenna 54 above. When issuing low-rate data sk on line V5, the phase modulator is inactive. The all-or-nothing modulator 57 is intended alternatively allow the transmission of a signal and the reception of a signal by the unit center of the vehicle, as explained later. For exemple, the amplitude of the transmitted signal is of the order of 2 V effective.

The emitted signal E is received by an antenna 100 of the device with an attenuation of the order of -40 dBm, which represents an attenuation coefficient of 100 times, ie the signal received by the identification device has an amplitude of the order of 20 mV. The antenna 100 is connected to a radio frequency filter 101 (only shown in Figure 4) to eliminate the parasitic frequencies. The output of the filter 101 is connected to a branch, on the one hand, to a low rate receiver and low consumption 102 and, on the other hand, to a delay line 103. Low-speed sk signals are not transmitted by the delay line 103, but essentially pass through the receiver 102. As best seen in FIG. 4, the receiver 102 comprises successively a radio frequency envelope detector 104 for reconstruct the low-speed signals on line I5, which correspond to those of line V5. The detector output envelope 104 is connected to a low frequency amplifier 105, which the output is connected in parallel, on the one hand, to a sequence decoder 106 and on the other hand to a microcontroller 107 of the device identification I via a line 108. At the start of the communication with the vehicle, only the decoder 106 is powered by permanence by the battery of the identification device. In other words, waking data are decoded by the decoder 106, in order to send a wakefulness command at the microcontroller 107, at the output of the decoder 106. micro-controller 107 then wakes up all the other components electronic identification device. So the data following, namely the signals R, f and s, are transmitted directly to the micro-controller 107 via the parallel line 108.

After transmission of the service data, the microcontroller 50 of vehicle V generates bits of anti-piracy h at slow rate sk of the microprocessor, said anti-piracy bits being received by a buffer with random binary sequence generator 58, for outputting the anti-piracy bits at a high rate fk, on a line V7. The output of the buffer 58 is connected to a branching, on the one hand, with the phase modulator 56 for phase modulating the carrier wave generated by the oscillator 55, and on the other hand, with a digital timer of a useful bit time 59, whose function will be explained later. Every bit of anti-piracy is transmitted in the form of a radio frequency signal via the antenna 54 in direction of the identification device I. The overall transmission frame V2 signals by the antenna 54 is illustrated in Figures 5 and 7. In Referring more particularly to Figure 6, the beginning is shown the interrogation frame of the anti-piracy bits h on the line V2. On line V2 of FIG. 6, it can be seen that each bit of anti-piracy is carried by a very high frequency oscillating wave having a bit time Tb for example between 50 and 200 ns. The show anti-piracy bits h1, h2 ... hn is allowed by the modulator while or nothing 57, depending on a control signal V1 that is emitted by a time base management unit 60 of the vehicle V, which unit 60 is able to deliver the low clock rate signals sk, average cadence mk and high rate fk. Of course, unit 60 is connected to the microcontroller 50, as indicated by the double arrow 61. The generation of the V1 signal will be explained later.

The identification device I receives via its antenna 100 on the line I1 (see FIG. 6) a signal corresponding to the anti-piracy bit h1 emitted by the vehicle V, with a delay time δ which corresponds to the signal propagation time between the vehicle and the device Identification. The signal then passes through the analog delay line 103, which may for example be constituted by a copper winding, which makes it possible to reduce the energy consumption by the device identification because this delay line does not need to be powered in energy. FIG. 8 shows the signal I2v which corresponds to to the virtual signal at the output of the analog delay line 103, it is say to the envelope of said analog signal, said signal being delayed of a predetermined duration Dℓ. The output of the delay line 103 is connected to the input of a phase inverter 109 to reverse the phase of the analog signal transmitted by the delay line 103, according to a encrypted authentication control signal g, whose frame is represented on line I6. As visible on line I6 shown in FIG. 8, the bit rate of the signal g is at medium rate mk, weaker than the high rate fk anti-piracy bits h. In this Indeed, it is expected that the microcontroller 107 of the device of identification transmits at low speed sk the signal g to a memory buffer 110 which outputs the signal g at the average rate mk to the phase inverter 109.

The phase inverter 109 outputs an analog signal, whose virtual signal is represented in I7v which represents the envelope said signal. The output of the phase inverter 109 is connected to a amplifier 111 having a gain of + 20 dB to emit the signal with an amplitude of the order of 200 mV. This amplified signal is represented on the line I4 and corresponds to the transmission bit h1 with the delay δ due to the propagation of the signal and to the analog delay Dℓ due to the delay line 103. This signal 14 is reemitted by an antenna 112 to the vehicle V, as represented by the arrow RE. When receiving of the first anti-piracy bit h1 by the identification device, this first bit of anti-piracy will trigger the generation of the encrypted signal authentication device g by the identification device in order to control the phase inverter 109. For this purpose, the output of the amplifier 111 is also connected to a trigger line 113 which has in series a diode 114 and a trigger 115 for activate a basic time management unit 116 which delivers low rate clock signals sk and medium rate mk, said unit 116 being connected to the microcontroller 107, as indicated by the double arrow 117.

Assuming that the transmitted electromagnetic signals propagate at the speed of light, ie 3.10 ⁸ m / s, we can consider that the duration of transmission of the signals is of the order of 3 ns per meter of distance between the vehicle and the identification device I. In other words, for a round trip between the vehicle V and the identification device I, spaced a distance of about 5 meters, the propagation time δ would be the order of 30 ns. At this propagation time δ, one could add the response time of the electronic circuits, which could be of the order of a few ns or tens of ns, according to the bandwidth allocated to the carrier.

The signal reissued RE by the identification device is received by a receiver 62 via an antenna 63, with attenuation of the order of - 40 dBm, which represents an attenuation coefficient of 100 times, that is, the signal received by the vehicle has an amplitude of the order of 2 mV. As best shown in FIG. 4, the receiver 62 has a radio frequency filter 64 connected to the antenna 63, whose output signal is represented by line V3, this signal being delivered to the input of a logarithmic amplifier 65 which has a gain of 80 dB, which makes it possible to reach a multiplication coefficient up to 10,000 times, and in particular to output this amplifier 65 a signal of the order of 2V effective. The amplifier 65 is connected at the output to a junction between, on the one hand, a amplitude demodulator 66, which makes it possible to detect the envelope of the received signal, and on the other hand, a phase demodulator 67. envelope detector 66 outputs a digital signal represented on line V4, said signal being delivered at the input of the unit of time base management 60 above. Upon receipt of the anti-piracy bit received in return, the time base management unit 60 triggers the emission of the anti-piracy bit following h2 with an offset corresponding to the useful bit time Tb + ε, ε corresponding to a weak delay in order to avoid any overlap between transmission and the reception of the signals by the vehicle. Thus, the reception of the bit anti-piracy back by the vehicle triggers the clock stroke following at the rate fk as well as the tilting of the modulator all or nothing 57, via line V1. Of course, we could also foresee that the small delay ε is close to 0 s.

Each new clock tick at the rate fk triggered by the receiving the previous anti-piracy bit, causes the incrementation of a unit in a counter 68, which counter 68 counts the number of bits received as a function of time. However, as the receiving a bit triggers the emission of a next anti-piracy bit, we can also count directly the number of anti-piracy bits issued as a function of time. When the actual end of the train bits of anti-piracy is detected by the vehicle's central unit, the unit 60 sends a signal Fr to stop counting of anti-piracy bits and to cause the counter 68 to send a representative signal the number of bits counted at the microcontroller 50, as indicated by the arrow 69. If the actual end Fr is included in the window of maximum reception Fm, as shown in Figure 5, this means that the transmission was not hacked. Depending on the number of bits, micro-controller 50 can calculate the actual distance between the vehicle V and the identification device I, according to a table of previously memorized correspondence. If this distance exceeds authorized distance, the call is interrupted as the result of a hacking attempt or simply as being communication at too great distance from the vehicle.

This counter thus makes it possible to detect piracy using relay boxes between the vehicle and the identification device. However, an attacker could try to trap this system by anticipating on the signal to be transmitted by the identification device. As for example, as soon as the pirate detects with a relay box the emission by the vehicle of a bit of anti-piracy, it can cause the emission anticipation of a signal-heel, in the preamble to the reissue of the signal delayed by the delay line of the identification device. Advantageously, this signal-heel would have a duration that would correspond to the signal propagation time over the extra distance between the identification device and the vehicle, in relation to the distance maximum allowed. Thus, the envelope detector 66 of the vehicle V detect, in the first place, the reception of this signal-heel, causing in turn the anticipated emission of the second bit of anti-piracy.

The system according to the invention also makes it possible, thanks counter 68, to determine whether the total number of anti-piracy bits has have been received during a maximum reception window Fm, shown in Figure 5. The timing diagrams of Figures 5 to 9 correspond to a non-pirated transmission of the signal. The real end Fr of the reception of the anti-piracy bitstream is, in these figures, well located in the maximum reception window Fm, as represented in FIG. 5. The total duration Tt of reception of the bits anti-piracy is calculated according to the following formula: Tt = n (Dℓ + Tb + ε + 2δ). Thus, we see that with a train of n bits of anti-piracy, we multiply by the variable n the duration of 2d propagation which is a direct function of the distance between the device identification and the vehicle. So, if it is difficult to detect with electronic circuits at low cost durations of the order of a few tens of ns, it is much easier to detect durations of the order of n x a few tens of ns, ie durations of the order of ms. However, if the hacker anticipates on the retransmission of the signal by the identification device, the variable 2n δ will remain within the acceptable limit and so the system will not be able to detect piracy.

For this purpose, it is possible to provide, alternatively or in addition, another way of calculating the delay time due to the propagation of the signal.

The phase demodulator 67 delivers a signal representative of the goh function on line V9 which is connected to an entrance of a door logical OR exclusive 70. This logic gate 70 receives on its other input a signal V8 representative of the function h, delivered at the output of digital retarder 59. The logic gate 70 could be replaced by another type of mixer, as shown in FIG. signal V8 corresponds to the signal V7 with a delay corresponding to Dℓ offset of the analog timer 103 of the identification device I. The logic gate 70 outputs a signal V10, which is representative of the offset between the signals V8 and V9, ie a signal representative of the encrypted authentication signal g transmitted by the identification device I, with a small offset corresponding to the signal propagation time which is equal to 2 δ, as visible on the figure 8.

The output of logic gate 70 is connected to a branch between, on the one hand, a low-pass filter 71 at the average rate mk which is the cadence corresponding to the signal g and, on the other hand, another Exclusive OR logic gate 73. The low-pass filter 71 is connected to a buffer 72, which transforms said signal from the rate mk to the cadence sk before sending it to the micro-controller 50 that will compare the signal g received from the authentication device with the signal g generated at the level of the vehicle, for the purpose of communication, which corresponds to the last step 10 illustrated in the diagram of Figure 1.

The microcontroller 50 delivers the signal g specific to the vehicle to a buffer 74 at the rate sk, so that it returns to the rate mk at the other input of the aforementioned logic gate 73. We have represented on line V11 the signal g which corresponds exactly to the signal g generated by the identification device and transmitted on the line I6, as shown in FIG. 8. The logic gate 73 mixes the signals V10 and V11 in order to output only the offsets due the propagation time of the signal between the vehicle V and the device I, as shown on line V12. The exit of the logic gate 73 is connected to the input of an integrator 75, which will to output a signal V13 which will climb up stairs according to of time, for each slot C representative of the time of signal propagation. Line V13 is connected to the entrance of a unit 76 receiving on another input a threshold limit value 77 which corresponds to a maximum acceptable total propagation delay n × 2δ ≤ T max, with δ corresponding to the propagation time on a allowed distance. Depending on whether the amplitude of the output signal on the line V13 is greater than or not this limit value 77, a signal of hacking attempt will be sent or not via line 78 to the micro-controller 50.

The integrator 75 can detect an attempted hacking, even in case the hacker adds an anticipation stub to the signal reissued by the identification device. Indeed, if this signal-heel can trap the counter 68, however the logic gate 73 will interpret this signal-heel as a wrong signal at least with a chance on two, and therefore will output a slot equivalent to this signal-heel, which slot will be added by the integrator 75 of the same only for delay due to propagation delay.

Finally, the microcontroller 50 can deliver different signals of output to the other components of the vehicle by the track 80 shown on Figures 3 and 4.

For example, the intermediate clock rate mk is between 20 and 60 times less than the rate fk.

Alternatively, the antennas 54 and 63 of the V vehicle by a single antenna 82 shown in broken lines in FIG. 4, which antenna 82 would be connected to a diplexer 81 shown in broken lines in FIG. 4, in order to switch between the reception mode and the transmission mode as appropriate. So analogous, one could replace the antennas 100 and 112 of the device identification by a single antenna 121 connected to a diplexer 120, represented in broken lines in Figure 4. The diplexers 81 and 120 could thus communicate with each other, as indicated by the double arrow T.

In a variant of the embodiment shown in fig ure 4, the central unit is connected to at least two, for example three, transmit / receive antennas such as the single antenna 82, arranged in several points of the vehicle V. Distance measurements are then performed sequentially between the identification device I and each of the transmitting / receiving antennas of the vehicle V. These measurements of distance are achieved via the propagation time δ anti-piracy bits using the correspondence table above. In this variant, it is possible to locate precisely, for example a few centimeters or tens of centimeters, the position of the identification device I by a calculation of triangulation carried out by the central unit. This allows in addition to taking into account the position of the identification device I relative to the vehicle V, for example, which door it is the closest when making decisions in the unit central to order the conviction or the decon- locks of the opening of the vehicle. This variant can obviously also be carried out with transmitting and receiving antennas separated.

In all cases, the delay line 103 of the device identification I allows to postpone the retransmission of each anti-piracy bit compared to receiving it in order to use the same frequency radio for the transmission of corresponding signals in the direction from the vehicle V to the identification device I (arrow E or T) and in the direction from the identification device I to the vehicle V (arrow RE or T).

As shown in Figure 5, on line V3, after the reception of the hog signal, the vehicle V receives from the device identification a signal hos where s represents service data which modulate the signal h by phase inversion, like this was the case for the signal g whose length is less than that of signal h.

The sequence of anti-piracy bits h being a binary sequence random or pseudo-random, it entails a spread of the spectrum of radiofrequency signals used to transmit the vehicle to the identification device and vice versa. This spread spectrum is all the more important as the modulation rate fk used for transmit the sequence of anti-piracy bits h is high. Such Spread spectrum is known to be advantageous from the point of view of robustness of signal against interference and echoes multiple, which facilitates multiple transmissions of the signal, example the round trip signal between the vehicle and the device Identification. On the other hand, the demodulation of the encrypted signal of authentication g at the level of the central unit of the vehicle is made all the more difficult as the spectrum of the signal RE is spread out.

The arrangement of the delay line 103 to re-transmit the bits anti-hacking successive h delayed a useful bit time as a high rate modulation fk of the encrypted authentication signal g, itself being transmitted at the average rate mk, ensures the corresponding demodulation key, that is, the same random sequence of anti-piracy bits h, is always available at level of the central control unit when receiving the signal RE, which makes it possible to demodulate the received signal RE regardless of the degree of spread of the spectrum of this signal.

In addition, the robustness of the access system with respect to echoes multiples, for example in the case where the signal E emitted by the unit central station is received by the identification device I in the form of a signal transmitted directly and an echo that has been reflected on an obstacle, is ensured by the fact that the identification device I always responds to the first received signal, which can only be the signal transmitted directly.

Although the invention has been described in connection with an example particular realization, it is obvious that it is not at all limited and that it includes all the technical equivalents of the means described and their combinations if these fall within the scope of the invention as defined by the claims.

Claims (12)

1. Hands-free access system for motor vehicle (V), comprising an onboard central control unit on the vehicle and an identification device (I) designed to be carried by a user (U) and for setting up a remote and wireless bidirectional communication with said central unit, to authenticate the user and to control the door lock disabling/enabling means when the user has been authenticated, said system being designed to set up said bidirectional communication when the identification device is located at a distance less than a predetermined limit distance from the vehicle, the central unit being designed to send encrypted authentication data (R, f) to the identification device (I), said identification device being designed to generate an encrypted digital authentication response signal (g), characterized by the fact that said central unit is designed to generate, in the communication after the transmission of said encrypted authentication data (R, f), an anti-piracy bit stream (h), each bit of said stream being sent to the identification device (I) in the form of a radiofrequency signal representative of said bit, the identification device (I) including a phase inverter (109) for selectively inverting the phase of the signal representative of the anti-piracy bits received by the identification device from the vehicle (V), said inverter being controlled, for each anti-piracy bit, by said encrypted digital authentication response signal (g) from the identification device, at a rate slower (mk) than that (fk) of the anti-piracy bit stream, such that when the identification device receives said signal representative of an anti-piracy bit, the latter is retransmitted by the identification device with a delay time (Dl) greater than or equal to the useful transmission time (Tb) of the bit in a form modulated by said encrypted digital authentication response signal (g), and that the reception by the central control unit of said retransmitted signal triggers the transmission by the central unit of the signal representative of the next bit in said stream, the number of bits of said stream sent or received being counted by a counter (68) of the central unit, such that the latter compares said number and the actual time (Tt) corresponding to the transmission or reception of said number of bits, in order to determine, beyond a predetermined threshold value, a piracy attempt provoking the termination of said communication.
2. System according to Claim 1, characterized by the fact that said predetermined threshold value is given by a time window (Fm) which corresponds to the total theoretical transmission time of the anti-piracy bit stream, when the identification device (I) is at a distance less than or equal to the predetermined limiting distance, and the central control unit is designed to determine said piracy attempt when the total number of bits of the anti-piracy bit stream has not been sent or received by the central control unit within said window.
3. System according to Claim 1 or 2, characterized by the fact that the central unit includes in memory a double-input mapping table, giving the number of bits sent and the corresponding actual transmission time, to provide at the output the actual distance between the identification device and the central unit, the data of said table being acquired previously by experimentation and able to be re-updated at periodic intervals.
4. System according to one of Claims 1 to 3, characterized by the fact that the counting of the number of bits sent is stopped when the central unit detects the receipt of the last bit (hn) of the anti-piracy stream.
5. System according to one of Claims 1 to 4, characterized by the fact that the identification device (I) includes an analogue delay line (103) for retransmitting the signal received from the vehicle (V) to the vehicle with a predetermined delay time (Dl).
6. System according to one of Claims 1 to 5, characterized by the fact that the central control unit includes an oscillator (55) generating radiofrequency carrier waves, linked to a phase modulator (56), which is controlled by the anti-piracy bit stream (h) generated by the central unit of the vehicle (V).
7. System according to one of Claims 1 to 6, characterized by the fact that the central unit of the vehicle (V) includes a phase demodulator (67) for demodulating the signal received by the vehicle from the identification device (I), an exclusive OR logic gate (70), the inputs of which are respectively linked to said phase demodulator and to a digital signal delay unit (59), said delay unit being designed to delay by one useful bit time (Tb) each bit of the anti-piracy stream (h) generated by the central unit, said logic gate being designed to output a digital signal (V10) representative of the encrypted response signal from the identification device.
8. System according to Claim 7, characterized by the fact that the signal (V10) output by the abovementioned exclusive OR logic gate (70) is compared by the central unit to an encrypted digital authentication signal (g) generated by the central unit, in order to authenticate the identification device (I).
9. System according to Claim 7 or 8, characterized by the fact that the output of said exclusive OR logic gate (70) is linked to an input of a second exclusive OR logic gate (73), the other input of which receives an encrypted digital authentication signal (g) generated by the central unit, in order to output a signal (V12) representative of the successive time offsets of each bit of the anti-piracy stream, the latter signal being received at the input of an integrator (75) to obtain the sum of the response times associated with each anti-piracy bit offset pulse (C), the output of said integrator being linked to a comparator (76) to compare said sum of response times with a predetermined limit time value (77), beyond which a piracy attempt is detected.
10. System according to one of Claims 1 to 9, characterized by the fact that the signal from the identification device (I) is received by the central unit and transmitted to the input of an envelope detector (66) to detect the rising edge of said signal, the detection of this rising edge being designed to trigger, on the one hand, the incrementing by one unit of the number-of-bits counter (68) and, on the other hand, the transmission of the next bit in the anti-piracy bit stream, after a predetermined time which corresponds to a useful bit time, possibly increased by an additional time (ε) to avoid any interference between the transmission and the reception of the signal by the central unit.
11. System according to Claim 10, characterized by the fact that the central unit includes an on-off modulator (57) to switch the central unit to the mode for transmitting or receiving signals to or from the identification device (I), said on-off modulator being controlled by the detection of the rising edge of the signal received from the identification device.
12. System according to one of Claims 1 to 11, characterized by the fact that the identification device includes a receiver (102) for receiving low rate (sk) alert signals (e), this receiver including in succession a low-threshold, static radiofrequency envelope detector (104) and a low-frequency amplifier (105).

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