EP1152108A2 - Hand-free access system for a motor vehicle - Google Patents

Abstract

The vehicle (V) emits a stream of security bits which are returned by the remote unit (I) after a delay. When a returned bit is received the vehicle emits the next bit. A counter records the number of bits emitted, and a maximum acceptable time for communication of this number of bits computed, and compared to the recorded time for a communication. An excessive time stops the communication.

Description

The present invention relates to a so-called hand access system free for motor vehicle, i.e. a system of wireless communication allowing entry into the vehicle without key. This system can also be applied to hands-free starting of the vehicle, i.e. when starting without a key.

Such a system generally includes a device identification intended to be worn by a user and capable of establishing wireless two-way remote communication with a unit control unit on board the vehicle to authenticate the user and order locking means / unlocking the door locks when the user has been recognized as authentic. Communication protocol initialization can be activated by operating the exterior door handle, to hands-free access, or by pressing a start button, in hands-free start mode. The system is able to establish said two-way communication when the identification device is located at a distance less than a predetermined limit distance from the vehicle, generally of the order of a few meters, to avoid, on the one hand, interference with other signal sources from the environment, and, on the other hand, to avoid the functioning of the system at a distance such that the user is too far from the vehicle to be aware of the operations carried out by said system.

Certain systems currently proposed use systems with Magnetic induction at very short range, to both supply energy and transport information from the central unit of the vehicle to the identification device in the field electromagnetic generated by the vehicle's antennas. However, a such a system allows communication only at very short distance from the vehicle in the order of a few cm. Another commonly used system proposed is to use low frequency carrier waves, around 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 area Europe, and 315 or 902 MHz for the USA zone. However, in this case, the identification device must include a battery to power its own electronic circuits. To minimize consumption electric, we can provide, for example, that the device identification is dormant for 9 ms and awake 1 ms, during periods of 10 ms.

Of course, two-way communication between the vehicle and the identification device is encrypted, in order to avoid any untimely operation of the system and to secure it from criminals. In Figure 1 of the accompanying drawings, there is shown a example of an already known encryption system. 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 2, the generated random numbers R having, for example, a length of 56 bits. This random number R is transmitted towards the identification device I, as indicated by arrow 3. Simultaneously, this same random number R is mixed with the key secret K following a complex associative function f, in a mixer 4 which is connected to its input to memory 1 and to the generator of random numbers 2. Mixer 4 outputs 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 mixer 4. This signal is sent to the device of identification I, in the form of a signal of length by 28-bit example, as indicated by 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 at its input to memories 1 and 5 above. This mixer 7 mixes the two signals according to a function complex associative g. Vehicle V then stores in a memory 8 connected to the mixer output 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 signal at the mixer output 14 in a memory 15 and compare 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 transmission delay in the area of authorized transmission, the identification device interrupts the communication as unauthorized. On the other hand, if the two signals match, the signal is mixed in a mixer 17 with the secret key provided by memory 11 of the device identification I, following the same associative function g above. The mixer 17 output signal is stored in memory 18 of the identification device to be then sent to the vehicle according to arrow 9 in the form of a signal having a length by 20 bit example. Finally, the signal received by arrow 9 is compared with the signal received from the vehicle memory 8 in a comparator 10. If these two 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 vehicle center may, if necessary, order the condemnation or unlocking of the door locks of the vehicle. Of course, another encryption protocol could be used to secure data transmission.

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 process of piracy is shown in Figure 2. According to this method, we assume that the user U who is wearing the identification device I is located at a distance from vehicle V greater than the authorized distance of communication, for example from 10 to 100 m 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 start of the communication, for example by pulling on the exterior door handle. This triggers the emission of low frequency signals by the vehicle to the relay box 20, as indicated by the zigzag arrow 21. This signal 21 sent by the vehicle is received by a coil 22 of the relay box 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, towards a second relay box 30, which is carried by another hacker who closely follows user U. The exchange of information between the two relay boxes 20 and 30 being carried out at very high frequency, it communication is possible from a long distance. The second relay box 30 includes an antenna 31 for receiving the signal 26 emitted by the relay box 20. The antenna 31 is connected to a broadband receiver at the same frequency as 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 34 to send a signal 35 to the device identification I which conforms to 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 in turn issue its response signal 36, said response signal 36 being sent at high frequency and received by an antenna 37 of the second relay box 30, by example at 434 MHz. The antenna 37 is connected to a receiver 38, which goes convert the signal at 434 MHz to a signal at a different frequency, for example at 315 MHz. The signal is then transmitted by a transmitter at broadband 39 via an antenna 40 to the first relay box 20, this difference in frequency being necessary for the different signals do not interfere with each other. Of course, the frequency of signal 41 sent back by the second relay box 30 is different both the frequency of signal 26 and signal 36. This signal 41 is received by an antenna 27 of the first relay box 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 which is sent via the antenna 42 of the first relay box 20 to the vehicle V, as represented by the zigzag arrow 43.

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

To detect such hacking and interrupt communication, one solution could be to measure the propagation time of UHF radio waves, comparing this measured time with a time predetermined corresponding to a communication in an area limited allowed around the vehicle. However, to detect the time delay due to distance, compared to communication time overall, it is necessary to have broad bandwidth, which corresponds to a rapid communication rate, for example of in the range of 20 to 40 Mb / s. With such a communication rate, 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 very high bandwidth, which comes up against applicable regulations that severely limit bandwidths allowed, to avoid saturation of the environment with waves electromagnetic.

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

To this end, the invention relates to an access system called hands free for motor vehicle, comprising a device identification intended to be worn by a user and capable of establishing wireless two-way remote communication with a unit control unit on board the vehicle to authenticate the user and order locking means / unlocking the door locks when the user has been recognized as authentic, said system being capable of establishing said two-way communication when the identification device is located at a distance less than a predetermined limit distance from the vehicle, characterized in that said central unit is able to generate, during communication, an anti-piracy bit stream, each bit of said train being transmitted to the identification device under the form of a radio frequency signal representative of said bit, so that when the identification device receives said signal, the latter is re-issued by the identification device with a delay time greater than or equal to the useful transmission time of the bit, and that reception by the central control unit of said re-emitted signal triggers the central unit transmitting the signal representative of the next bit in said train, the number of bits transmitted or received from said train, being counted by a counter of the central unit, so that the latter compare said number and the actual duration corresponding to the program or upon receipt of said number of bits, in order to determine, beyond a predetermined threshold value, a hacking attempt causing stopping said communication. "Transmission time" means useful "the transmission time of the binary information of said bit compared the total time of the cell with which said bit is associated, which includes generally said useful time and a time of silence to allow the reception of the signal re-emitted by the identification device during said time of silence.

Advantageously, said predetermined threshold value is given by a time window which corresponds to the total duration theoretical emission of the anti-piracy bit stream, when the identification device is at a distance less than or equal to the predetermined limit distance. This time window is a constant precise generated for example by an oscillator controlled by quartz associated with a counter controlled by the micro-controller 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 of the anti-piracy train was not issued or received by the central processing unit command in said window.

According to another characteristic, the central unit can include in memory a double entry correspondence table, namely the number of bits transmitted and the corresponding actual transmission time, for output the actual distance between the identification device and the central unit, the data from said table having been acquired beforehand by experimentation and which can be periodically updated, when the user wearing the identification device is seated 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 counting of the number of bits transmitted 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 retransmit the signal received from the vehicle to the vehicle with a predetermined delay time.

According to another characteristic, the central control unit includes a radio carrier wave oscillator frequency, connected to a phase modulator, which is controlled by the Anti-piracy bit stream generated by the central unit of the vehicle.

According to yet another characteristic, the anti-piracy bit stream is generated by the central processing unit after the data transmission authentication methods encrypted to the identification device. In this case, the identification device may include a phase inverter to selectively reverse 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 of the identification device, at a slower rate than that of the anti-piracy bit stream.

It can then be foreseen that the central unit of the vehicle comprising a phase demodulator for demodulating the signal received by the vehicle from the identification device, a logic gate Exclusive OR whose inputs are respectively linked to said audit phase demodulator and a digital signal retarder, said retarder being able to delay by bit useful time each bit of the train of anti-piracy generated by the central unit, said logic gate being able to output a digital signal representative of the signal encrypted response from 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 authentication signal generated by the central unit, in order to authenticate the identification device.

In another embodiment, the exit from said door exclusive OR logic is connected to an input of a second door exclusive OR logic whose other input receives a digital signal encrypted authentication generated by the central unit, in order to deliver at output a signal representative of the successive time offsets of each bit of the anti-piracy train, this last signal being received at the input of an integrator to sum the response times linked 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 limit value, beyond which a hacking attempt is detected.

Advantageously, it can be expected that the signal from of the identification device is received by the central unit and transmitted to the input of an envelope detector to detect the rising edge of said signal, the detection of this rising edge being able to trigger, of a hand, incrementing the number of bits by one 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 additional time to avoid any interference between the transmission and reception of the signal by the unit central.

In this case, it is possible to provide for the central unit to include a modulator says all or nothing to switch the central unit in the transmission or reception mode of signals to or from identification device, said all-or-nothing modulator being controlled by the detection of the rising edge of the signal received in origin of the identification device.

Advantageously, the identification device comprises a receiver for receiving low cadence wake-up signals, receiver successively comprising a static envelope detector low threshold radio frequency and low frequency amplifier.

For 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, characteristics and advantages thereof will appear more clearly during the detailed explanatory description which follows of a particular embodiment of the invention, given only to Illustrative and non-limiting title, with reference to the attached schematic drawing, in which:

• FIG. 1 is a functional block diagram representing the encryption protocol for securing the bidirectional transmission of data between a vehicle and an identification device;
• Figure 2 is a block diagram illustrating a means of hacking the encryption system through two relay boxes;
• FIG. 3 is a simplified functional block diagram of a hands-free access system according to the invention;
• Figure 4 is a more detailed block diagram corresponding to the diagram of Figure 3;
• FIG. 5 represents several timing diagrams illustrating the complete interrogation frames sent 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 arrow VI, corresponding to the start of the anti-piracy sequence;
• Figure 7 shows the first two timing diagrams of Figure 5;
• FIG. 8 is a partial and enlarged view of a portion of the timing diagrams of FIG. 7, indicated by the 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 and more detailed form.

The motor vehicle V has in its central unit a microcontroller 50, which is usually in a semi-sleep state or waiting for an alarm clock. When the user activates the exterior door handle, an activation signal is sent to the microcontroller 50, as indicated by arrow 51. In response, the microcontroller sends a general power signal, such as represented by arrow 52, to supply the various components central unit electronics. Then, the microcontroller 50 generates a series of signals at low cadence sk, for example of the order of 2 to 100 Kb / s on line V5. The data carried on line V5 is successively an awakening signal, a signal representative of a number random R with a length for example of 56 bits, a signal representative of the function f (R, K) of a length for example of 28 bits, and of a signal representative of service data s, for example with a length of 100 to 5,000 bits, for example data on the maintenance, vehicle adjustment, etc. (see Figure 5). Line V5 is connected to a transmitter 53 to transmit signals via an antenna 54 towards the identification device I as represented by the arrow E. The transmitter 53 includes, as best seen in FIG. 4, a oscillator 55 to generate an ultra high carrier wave frequency, said oscillator being supplied by line 52. The oscillator 55 is connected to a phase modulator 56, which in turn is connected to a all-or-nothing modulator 57, the latter having an input connected to the line V5 and its output connected to the above-mentioned antenna 54. When issuing data at low rate sk on line V5, the phase modulator is inactive. The all or nothing modulator 57 is intended for alternately allow the transmission of a signal and the reception of a signal by the unit center of the vehicle, as explained below. For exemple, the amplitude of the transmitted signal is of the order of 2 V rms.

The signal emitted E is received by an antenna 100 of the device identification I with an attenuation of the order of -40 dBm, which represents an attenuation coefficient of 100 times, i.e. the signal received by the identification device has an amplitude of around 20 mV. The antenna 100 is connected to a radio frequency filter 101 (only shown in Figure 4) to eliminate the spurious frequencies. The output of filter 101 is connected to a branching, on the one hand, to a receiver at low cadence and at low consumption 102 and, on the other hand, to a delay line analog 103. Low speed signals sk are not transmitted via delay line 103, but mainly via the receiver 102. As best seen in FIG. 4, the receiver 102 includes successively a radio frequency envelope detector 104 for reconstitute the signals at low cadence on line I5, which correspond to those of line V5. Detector output envelope 104 is connected to a low frequency amplifier 105, of which the output is connected in parallel, on the one hand, to a sequence decoder awakening 106 and, on the other hand, to a microcontroller 107 of the device identification I via line 108. When the communication with the vehicle, only the decoder 106 is supplied with permanently by the battery of the identification device. In other words, the awakening data are decoded by the decoder 106, in order to send a wake-up command to the microcontroller 107, at the output of the decoder 106. The microcontroller 107 then awakens all the other components of the identification device. So the data following, namely the signals R, f and s, are transmitted directly to the microcontroller 107 via the parallel line 108.

After transmission of the service data s, the microcontroller 50 of vehicle V generates anti-piracy bits h at the slow rate sk of the microprocessor, said anti-piracy bits being received by a 58 random binary sequence generator buffer output the high-speed anti-piracy bits fk, on a line V7. The output of buffer memory 58 is connected to a branch, on the one hand, with the phase modulator 56 for modulate in phase the carrier wave generated by the oscillator 55, and on the other hand, with a digital timer with 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 global transmission frame V2 of the signals by the antenna 54 is illustrated in FIGS. 5 and 7. In referring more particularly to FIG. 6, the beginning has been represented of 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 of, for example, between 50 and 200 ns. The show anti-piracy bits h1, h2 ... hn is authorized by the modulator while or nothing 57, as a function of a control signal V1 which is emitted by a time base management unit 60 of vehicle V, which unit 60 is capable of delivering the clock signals at low cadence sk, at medium cadence mk and high cadence fk. Of course, unit 60 is connected to the microcontroller 50, as indicated by the double arrow 61. The generation of signal V1 will be explained later.

The identification device I receives via its antenna 100 on the line I1 (see figure 6) a signal which corresponds to the anti-piracy bit h1 emitted by vehicle V, with a delay time δ which corresponds to signal propagation time between vehicle and device identification. The signal then passes through the analog delay line 103, which can for example be constituted by a copper winding, which reduces 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 the virtual signal at the output of the analog delay line 103, it is tell the envelope of said analog signal, said signal being delayed with a predetermined duration Dℓ. The delay line 103 output 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, the frame of which is shown on line I6. As visible on line I6 shown in FIG. 8, the bit rate of the signals g is at medium rate mk, lower than the high rate fk of the anti-piracy bits h. In this indeed, it is expected that the microcontroller 107 of the device identification transmits signal g at low speed sk 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 of said signal. The output of the phase inverter 109 is connected to a amplifier 111 having a gain of + 20 dB to transmit the signal with an amplitude of the order of 200 mV. This amplified signal is represented on line I4 and corresponds to transmission bit h1 with the delay δ due to signal propagation and analog delay Dℓ due to the delay line 103. This signal 14 is retransmitted by an antenna 112 towards vehicle V, as represented by the arrow RE. When receiving of the first bit of anti-piracy h1 by the identification device, this first bit of anti-piracy will trigger the generation of the encrypted signal authentication g by the identification device with a view to control the phase inverter 109. For this purpose, the output of amplifier 111 is also connected to a trigger line 113 which comprises in series a diode 114 and a trigger 115 for activate a basic time management unit 116 which delivers clock signals at low cadence sk and at medium cadence mk, said unit 116 being connected to microcontroller 107, as indicated by the double arrow 117.

Assuming that the transmitted electromagnetic signals propagate at the speed of light, namely 3.10 ⁸ m / s, we can consider that the duration of signal transmission 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 around 30 ns. To 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, depending on the bandwidth allocated to the carrier.

The signal re-emitted RE by the identification device is received by a receiver 62 via an antenna 63, with an 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 around 2 mV. As best shown in Figure 4, the receiver 62 includes a radio frequency filter 64 connected to the antenna 63, the 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 ranging up to 10,000 times, and in particular to issue at the output of said amplifier 65 a signal of the order of 2V effective. The amplifier 65 is connected at the outlet to a branch between, on the one hand, a amplitude demodulator 66, which detects the envelope of the signal received, and, on the other hand, a phase demodulator 67. The envelope detector 66 outputs a digital signal shown on line V4, said signal being delivered to the input of the unit of the aforementioned time base management 60. On reception of the anti-piracy bit received in return, the time base management unit 60 triggers the transmission of the following anti-piracy bit h2 with an offset corresponding to the useful bit time Tb + ε, ε corresponding to a low safety delay in order to avoid any overlap between the transmission and reception of signals by the vehicle. Thus, the reception of the anti-piracy bit in return by the vehicle triggers the next clock stroke at the rate fk as well as the switching of the all-or-nothing modulator 57, via the line V1. Of course, one could also predict that the small delay ε is close to 0s.

Each new clock stroke at the rate fk triggered by the reception of 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 reception of a bit triggers the emission of a next anti-piracy bit, we can also directly count the number of anti-piracy bits issued as a function of time. When the actual end of the train anti-piracy bits is detected by the vehicle central unit, the unit 60 sends a signal Fr to stop counting 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 arrow 69. If the real end Fr is included in the window maximum reception Fm, as shown in Figure 5, this means that the transmission was not hacked. Depending on the number of bits, the microcontroller 50 can calculate the real distance between the vehicle V and the identification device I, according to a table of correspondence previously memorized. If this distance exceeds the authorized distance, the communication is interrupted as being result of a hacking attempt or simply as being communication too far from the vehicle.

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

The system according to the invention also allows, thanks to the counter 68, to determine whether the total number of anti-piracy bits has received successfully during a maximum reception window Fm, shown in Figure 5. The timing diagrams of Figures 5 to 9 correspond to a non-pirated signal transmission. The real end En of the reception of the anti-piracy bit stream 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 + 2 δ). Thus, we see that with a train of n anti-piracy bits, we multiply by the variable n the duration of 2δ propagation which is a direct function of the distance between the device identification and vehicle. So, if it is difficult to detect with low cost electronic circuits with 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, i.e. durations of the order of ms. However, if the hacker anticipates on the re-emission 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, one can provide, as an alternative or supplement, another way to calculate 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 input of a door exclusive OR logic 70. This logic gate 70 receives on its other input a signal V8 representative of the function h, delivered at the output of the digital self-timer 59. Logic gate 70 could be replaced by another type of mixer, as shown in Figure 3. The signal V8 corresponds to signal V7 with a delay corresponding to offset Dℓ of the analog retarder 103 of the identification device I. The logic gate 70 outputs a signal V10, which is representative of the offset between the V8 and V9 signals, i.e. a signal representative of the encrypted authentication signal g emitted 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 cadence mk to cadence sk before sending it to the microcontroller 50 which will compare the signal g received from the authentication device with the g signal generated at vehicle level, for authentication of communication, which corresponds to the last step 10 illustrated on the diagram of figure 1.

The microcontroller 50 delivers the signal g specific to the vehicle to a buffer memory 74 at the rate sk, so that it sends it back to the cadence 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 issued on the line I6, as shown in FIG. 8. The logic gate 73 mixes the V10 and V11 signals in order to output only the offsets due at the signal propagation time between vehicle V and the device identification I, as shown on line V12. Leaving the logic gate 73 is connected to the input of an integrator 75, which goes output a signal V13 which will go up by stairs in function of time, for each time slot C representative of the time of signal propagation. Line V13 is connected to the input of a unit 76 receiving on another input a threshold limit value 77 which corresponds to a maximum acceptable total propagation time n x 2δ ≤ T max, with δ corresponding to the propagation time over a authorized distance. Depending on whether the amplitude of the output signal on the line V13 is higher or lower than this limit value 77, a signal of hacking attempt will be sent or not via line 78 to the microcontroller 50.

The integrator 75 makes it possible to detect a hacking attempt, even if the hacker adds a heel of anticipation to the signal re-issued by the identification device. Indeed, if this heel signal can trap the counter 68, however the logic gate 73 will interpret this signal-stub as a wrong signal at least with a chance on two, and therefore will output a slot equivalent to this heel signal, which niche will be added by the integrator 75 of the same so that for delays due to propagation time.

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

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

Alternatively, one could replace the antennas 54 and 63 of the vehicle V 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 Figure 4, to switch between the reception mode and the transmission mode as appropriate. So analogous, we could replace the antennas 100 and 112 of the device identification by a single antenna 121 connected to a diplexer 120, shown 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 at several points in vehicle V. Distance measurements are then carried out sequentially between the identification device I and each of the vehicle's transmit / receive antennas V. These measurements distance are achieved via the propagation time δ anti-piracy bits using the correspondence table mentioned 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 triangulation calculation performed by the central unit. This allows in besides taking into account the position of the identification device I relative to vehicle V, for example, which door it is on closest, when making decisions in the unit central to command the condemnation or unlocking of vehicle door locks. This variant can obviously also be carried out with transmitting and receiving antennas separated.

In all cases, the delay line 103 of the device of identification I allows to postpone the retransmission of each anti-piracy bit compared to its reception in order to use the same frequency radio for transmitting the corresponding signals in the direction from vehicle V to 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 receipt of the hog signal, the vehicle V receives from the device identification 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 the anti-piracy bits h being a binary sequence random or pseudo-random, it results in a spread of the spectrum of radio frequency signals used to transmit it from 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 anti-piracy bit sequence h is high. Such spread spectrum is known to be advantageous from the point of view of signal robustness against interference and echoes multiple, which facilitates multiple signal transmissions, for example example the round trip of the signal between the vehicle and the device identification. On the other hand, the demodulation of the encrypted signal authentication level at the central unit of the vehicle is rendered all the more difficult as the spectrum of the signal RE is spread out.

The arrangement of the delay line 103 for retransmitting the bits anti-piracy successive h delayed by a bit time useful as high rate modulation fk of the encrypted authentication signal g, itself being transmitted at the average rate mk, ensures as the corresponding demodulation key, i.e. the same random sequence of anti-piracy bits h, is always available at level of the central control unit upon reception of the signal RE, which allows the demodulation of the received RE signal 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 unit is received by the identification device I in the form of a signal transmitted directly and from an echo having undergone a reflection on an obstacle, is ensured by the fact that the identification device I always reacts to the first signal received, which can only be the signal transmitted directly.

Although the invention has been described in connection with an example particular achievement, it is obvious that it is by no means limited and that it includes all the technical equivalents of the means described as well as their combinations if these fall within the scope of the invention.

Claims (14)

Hands-free access system for a motor vehicle (V), comprising an identification device (I) intended to be worn by a user (U) and capable of establishing bidirectional remote and wireless communication with a central processing unit on-board command to authenticate the user and command means for locking / unlocking the locks of the opening elements when the user has been recognized as authentic, said system being capable of establishing said two-way communication when the identification device is located at a distance less than a predetermined limit distance from the vehicle, characterized in that said central unit is capable of generating, during communication, a train of anti-piracy bits (h), each bit of said train being transmitted to the identification device (I) in the form of a radio-frequency signal representative of said bit, so that when the device receives the said signal, the latter is re-transmitted by the identification device with a delay time (Dℓ) greater than or equal to the useful transmission time (Tb) of the bit, and that the reception by the central unit of control of said re-transmitted signal triggers the transmission by the central unit of the signal representative of the next bit in said train, the number of bits transmitted or received from said train being counted by a counter (68) of the central unit, so that this compares said number and the actual duration (Tt) corresponding to the transmission or reception of said number of bits, in order to determine, beyond a predetermined threshold value, a hacking attempt causing the stop of said communication. System according to Claim 1, characterized in that the said predetermined threshold value is given by a time window (Fm) which corresponds to the total theoretical duration of transmission of the anti-piracy bit stream, when the device d the identification (I) is at a distance less than or equal to the predetermined limit distance, and the central control unit is able to determine said hacking attempt when the total number of bits of the anti-hacking train has not has been sent or received by the central control unit in said window. System according to claim 1 or 2, characterized in that the central unit includes in memory a double input correspondence table, namely the number of bits transmitted and the corresponding actual transmission duration, to output the distance between the identification device and the central unit, the data from said table being acquired beforehand by experimentation and being able to be periodically updated. System according to one of claims 1 to 3, characterized in that the counting of the number of bits transmitted is stopped when the central unit detects the reception of the last bit (hn) of the anti-piracy train. System according to one of claims 1 to 4, characterized in that the identification device (I) comprises an analog delay line (103) for retransmitting the signal received from the vehicle (V) to the vehicle with a time of predetermined delay (D ℓ). System according to one of claims 1 to 5, characterized in that the central control unit comprises an oscillator (55) generating carrier waves in radio frequency, connected to a phase modulator (56), which is controlled by the anti-piracy bit stream (h) generated by the vehicle central unit (V). System according to one of Claims 1 to 6, characterized in that the anti-piracy bit stream (h) is generated by the central unit after the transmission of the encrypted authentication data (R, f) to the identification device (I). System according to claim 7, characterized in that the identification device I comprises a phase inverter (109) for selectively reversing 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 an encrypted digital authentication signal (g) of response from the identification device, at a slower rate (mk) than that (fk ) of the anti-piracy bit stream. System according to claim 8, characterized in that the central unit of the vehicle (V) comprises a phase demodulator (67) for demodulating the signal received by the vehicle from the identification device (I), a logic gate Exclusive OR (70), the inputs of which are respectively connected to said phase demodulator and to a digital signal retarder (59), said retarder being capable of delaying each bit of the anti-piracy train by a useful bit time (Tb) (h) generated by the central unit, said logic gate being able to output a digital signal (V10) representative of the encrypted response signal from the identification device. System according to Claim 9, characterized in that the signal (V10) delivered at the output of the above-mentioned 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). System according to claim 9 or 10, characterized in that the output of said exclusive OR logic gate (70) is connected to an input of a second exclusive OR logic gate (73) whose other input receives an encrypted digital signal authentication (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 train, this last signal being received at the input of an integrator (75) for summing the response times associated with each anti-piracy bit shift slot (C), the output of said integrator being connected to a comparator (76) for comparing said sum of response times with a predetermined time limit value (77), beyond which a hacking attempt is detected. System according to one of claims 1 to 11, characterized in 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 able to trigger, on the one hand, the incrementation of a unit of the bit number counter (68) 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 useful bit time possibly increased by an additional time (ε) to avoid any interference between the transmission and reception of the signal by central unity. System according to claim 12, characterized in that the central unit comprises an all-or-nothing modulator (57) for switching the central unit into the transmission or reception mode of the signals to or from the identification device ( I), said all-or-nothing modulator being controlled by the detection of the rising edge of the signal received from the identification device. System according to one of claims 1 to 13, characterized in that the identification device comprises a receiver (102) for receiving wake-up signals at low cadence (sk), this receiver successively comprising a detector low threshold radio frequency envelope (104) and a low frequency amplifier (105).

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