METHOD FOR ESTIMATING DISTANCE AND ELECTRONIC UNIT FOR A VEHICLE

Description

TECHNICAL FIELD TO WHICH THE INVENTION RELATES

[0001] The present invention relates to estimating a distance between an identifier and a vehicle.

[0002] It relates more specifically to a method for estimating a distance and an electronic unit for vehicles.

[0003] The invention is particularly advantageously applicable in cases where it is desired to protect the vehicle from relay attacks. TECHNOLOGICAL BACKGROUND

[0004] We are familiar with PEPS-type systems (for "Passive Entry - Passive Start" ) in which the implementation of a feature (such as unlocking the doors of a vehicle or starting such a vehicle) is conditional upon the presence near the vehicle of an identifier (usually carried by the vehicle user).

[0005] Document WO 2007 / 128319 discloses a wireless entry system that locks / unlocks a lock (e.g., on a car) based on the proximity of an authorized wireless electronic device (e.g., a mobile phone) to a fixed detection system. The system determines whether the user is close enough to authorize access and locks or unlocks the device accordingly.

[0006] US document 2003 / 090 365 proposes to measure reception phases of electromagnetic signals exchanged between the identifier and the vehicle for two different frequencies of the electromagnetic signals.

[0007] The distance between the identifier and the vehicle can then be estimated based on the difference in measured phases. SUBJECT OF THE INVENTION

[0008] In this context, the present invention proposes a method for estimating the distance between a vehicle equipped with a first wireless communication module and an identifier equipped with a second wireless communication module, comprising the following steps: generation of a randomly ordered list; reception, by at least one of the first and second wireless communication modules, of electromagnetic signals having a frequency evolving successively among a plurality of frequencies in accordance with said list (in the order indicated in said list); for each frequency of the plurality of frequencies, measurement of a reception phase of the electromagnetic signal having the frequency concerned; estimation of said distance on the basis of the measured phases.

[0009] The frequency of the electromagnetic signals used to estimate the distance thus evolves unpredictably, and a relay attack such as the one described later with reference to the figure 3 It will therefore be in vain.

[0010] According to features of the invention: The method includes a step of transmitting descriptive data of the list between the first wireless communication module and the second wireless communication module; the method includes a step of encrypting the descriptive data of the list; the descriptive data of the list is transmitted encrypted between the first wireless communication module and the second wireless communication module;

[0011] Depending on optional and therefore non-limiting characteristics: The process includes a step of transmission, by the other of the first and second wireless communication modules, of said electromagnetic signals having a frequency evolving successively among the plurality of frequencies in accordance with said list; the step of estimating said distance includes a step of determining a slope of a regression line linked to points defined each by a frequency of the plurality of frequencies and the associated measured phase; the reception step is implemented by the first wireless communication module.

[0012] The estimation process may also include at least one of the following steps: emission by the first wireless communication module of electromagnetic signals having a frequency evolving successively among the plurality of frequencies; reception by the second wireless communication module of wireless communication signals to the first wireless communication module.

[0013] The second transmitted phases can then be used during the step of estimating said distance.

[0014] The aforementioned process may further include a step of possibly implementing a vehicle functionality based on the estimated distance.

[0015] The invention also proposes an electronic unit for vehicles comprising: an element designed to generate a randomly ordered list; an element designed to encrypt descriptive data of the ordered list, said descriptive data of the list representing the successive values ​​of the frequencies or frequency indices; a communication module controlled by the electronic unit to transmit the encrypted descriptive data; an element designed to control the reception (by a wireless communication module equipping the vehicle), from an identifier, of electromagnetic signals having a frequency evolving successively among a plurality of frequencies in accordance with said list; an element designed to measure, for each frequency of the plurality of frequencies, a reception phase of the electromagnetic signal having the frequency concerned; an element designed to estimate a distance separating the vehicle and the identifier on the basis of the measured phases.

[0016] Such an electronic unit may also exhibit at least one of the optional characteristics presented above in terms of process.

[0017] When the electronic unit is realized on the basis of a microprocessor and at least one memory (as described below), at least some of the aforementioned elements can be implemented by means of instructions stored in said memory and designed to perform the function of the element concerned when executed by the microprocessor. DETAILED DESCRIPTION OF A PROJECT EXAMPLE

[0018] The description that follows, with regard to the attached drawings, given by way of non-limiting examples, will make it clear what the invention consists of and how it can be carried out.

[0019] Regarding the attached drawings: there figure 1 schematically represents the main elements of a system in which the invention can be implemented; the figure 2 schematically represents the propagation of two signals of different frequencies; there figure 3 represents a possible relay attack on a system of the type of that of the figure 1 ; and the figure 4 is a flowchart representing a method for estimating the distance between an identifier and a vehicle.

[0020] There figure 1 schematically represents the main elements of a system in which the invention can be implemented.

[0021] Such a system comprises a vehicle 10, here a motor vehicle, and an identifier 20, for example a key or access badge for the vehicle 10 (or, alternatively, a user terminal, such as a mobile phone or smartphone - or "smartphone" according to the commonly used English application, equipped with vehicle access rights 10).

[0022] Vehicle 10 is equipped with an electronic control unit 11 and a communication module 12.

[0023] The electronic control unit 11 includes, for example, a microprocessor and at least one memory, for example, a non-volatile rewritable memory. The memory stores, in particular, program instructions which, when executed by the microprocessor, enable the electronic control unit 11 to implement the processes described below. The memory also stores values ​​or parameters used during these processes, for example, measured phase values ​​Φi (as explained later).

[0024] The memory of the electronic control unit 11 also stores a cryptographic key K (which was, for example, written into the electronic control unit 11 during its manufacture).

[0025] Alternatively, the electronic control unit 11 could be implemented as an application-specific integrated circuit (or ASIC for "Application Specific Integrated Circuit " .

[0026] The communication module 12 is designed to establish a wireless connection with other electronic devices, in this case a type of connection "Bluetooth Low Energy" (Or "BLE " . The communication module 12 is therefore specifically designed to transmit and receive electromagnetic signals (typically with a frequency greater than 1 MHz, or even 500 MHz), here in the 2.4 GHz band.

[0027] The identifier 20 is generally worn by a user of the vehicle 10 and allows the control of certain functions of the vehicle 10 (for example unlocking the doors of the vehicle 10), in particular when it is approached the vehicle 10. The identifier 20 may also possibly include control buttons, with the help of which the user can control at least some of the aforementioned functions or other functions of the vehicle 10.

[0028] Identifier 20 includes a control unit 21 and a communication module 22.

[0029] The control unit 21 is, for example, implemented using a microprocessor and at least one memory, for example, rewritable non-volatile memory. This memory stores, in particular, program instructions which, when executed by the microprocessor, enable the control unit 21 to implement the processes described below. The memory also stores values ​​or parameters used during these processes.

[0030] The memory of the control unit 21 also stores the cryptographic key K. In the case where the identifier 20 is a badge (or key) for access to the vehicle, the cryptographic key K was for example written into the memory of the control unit 21 during the manufacture of the identifier 20. In the variant mentioned above where the identifier 20 is a user terminal, the cryptographic key K was for example received from a remote server and stored during a subscription phase to a service for ordering the vehicle's functions using the user terminal.

[0031] Alternatively, the control unit 21 could be implemented as an application-specific integrated circuit.

[0032] The communication module 22 is designed to establish a wireless link (here of type "Bluetooth Low Energy" Or "BLE ") with other electronic devices, in particular with the vehicle's electronic control unit 11 via the aforementioned communication module 12. The communication module 22 is therefore also designed to transmit and receive electromagnetic signals (typically with a frequency above 1 MHz, or even 500 MHz), here in the 2.4 GHz band.

[0033] Thanks to the wireless link thus established between the communication module 12 of vehicle 10 and the communication module 22 of identifier 20, data can be exchanged between the electronic control unit 11 of vehicle 10 and the control unit 21 of identifier 20, as explained later.

[0034] The electromagnetic signals exchanged between the communication modules 12, 22 can also be used to evaluate the distance d separating the identifier 20 and the vehicle 10, based on the principle explained now with reference to the figure 2 .

[0035] We have indeed schematically represented in figure 2 the propagation of two signals of respective frequencies f 1 , f 2 (different from each other) between a transmitter module TX and a receiver module RX (and along an Ox axis passing through these two modules).

[0036] Measuring the phase Φ i of each of these signals at the RX receiver allows us to deduce the distance d separating the TX transmitter module and the RX receiver module according to the formula: d = c . ϕ 2 − ϕ 1 / 2 π . f 2 − f 1 , where c is the speed of electromagnetic waves.

[0037] It is possible to perform such phase measurements Φ i for more than two signals of distinct frequencies fi, which makes it possible to overcome the phenomena of reflection or refraction that may occur at certain frequencies.

[0038] There figure 3 illustrates how a system operating on such a principle could be the target of a sophisticated relay attack.

[0039] A first attacker A is located near vehicle 10 (precisely at a distance d 1 from vehicle 10) and carries a first electronic module 30 which receives the signal emitted by vehicle 10 at a frequency fi (it is assumed here that the transmitter module TX is the communication module 12 of vehicle 10).

[0040] The first electronic module 30 performs a modulation of the received signal by means of a frequency carrier f P and transmits the modulated signal obtained to a second electronic module 40 carried by an attacker B located near the identifier 20.

[0041] As clearly visible in figure 3 , the electronic modules 30, 40 are separated by a distance d 2 , while the second electronic module 40 is at a distance d 3 from the identifier 20.

[0042] The second electronic module 40 demodulates the signal it receives and thus obtains the signal of frequency fi, which it transmits to the identifier 20.

[0043] The phase φ corresponding to the propagation of electromagnetic signals from vehicle 10 to identifier 20 via attackers A, B is: φ = 2 π . f P − f i . d 2 / c + 2 π . f i . d 1 + d 3 / c

[0044] Therefore, if the attackers know the successive emission frequencies fi and design the electronic modules 30, 40 so that the quantity (f P - fi ) is constant (i.e., the carrier frequency f P is modified for each emission frequency fi so that: f P - fi = cte), the attack will go undetected since the distance estimate proposed above gives in this case: d = c . φ f 2 − φ f 1 / 2 π . f 2 − f 1 = d 1 + d 3 .

[0045] In other words, since the quantity (f P - fi ) is constant, the term 2π.(f P - fi ).d 2 / c is also constant from one measurement to another and becomes zero when we take the difference of the measured phases φ(f 2 ), φ(f 1 ).

[0046] We are now describing with reference to the figure 4 a method for estimating the distance d that is not subject to such an attack.

[0047] This process begins at step E2 in which the electronic control unit 11 of the vehicle 10 generates a list of randomly ordered frequencies fi.

[0048] In practice, the electronic control unit 11 stores for example a set of predefined frequencies f 1 , f 2 , ..., f N (where N is for example between 50 and 100) and generates by random draw an ordered list of indices i(1), i(2), ..., i(N): the list of randomly ordered frequencies is then fi(1) , fi(2) , ..., fi(N) .

[0049] The electronic control unit 11 then proceeds to a step E4 of encryption of descriptive data D of the ordered list, by means of a cryptographic encryption algorithm using the cryptographic key K.

[0050] The descriptive data D of the ordered list represent, for example, the successive values ​​of the frequencies fi, in the order given by the ordered list generated in step E2. In the example mentioned above, the descriptive data D of the ordered list can, alternatively, represent the list of indices i(1), i(2), ..., i(N).

[0051] The electronic control unit 11 then commands the communication module 12 to transmit the encrypted descriptive data [D] K< (step E6).

[0052] The communication module 12 thus transmits (step E8) the encrypted descriptive data [D] K< via the wireless link established between the communication module 12 of the vehicle 10 and the communication module 22 of the identifier 20.

[0053] It is noted that it is possible to plan, during or after the establishment of the wireless link, for an authentication process of the identifier 20 by the electronic control unit 11 of the vehicle 10, for example by verifying (possibly by means of a challenge-response type protocol) that the identifier 20 does indeed have access rights to the vehicle 10 (that is to say in practice that the identifier 20 stores a given cryptographic key, for example the aforementioned cryptographic key K).

[0054] The communication module 22 of identifier 20 receives the encrypted descriptive data [D] K< at step E10 and transmits this data [D] K< to the control unit 21 (step E12).

[0055] The control unit 21 can thus proceed at step E14 to decrypt the descriptive encrypted data [D] K<, by means of a decryption algorithm using the cryptographic key K (stored as already indicated in the control unit 21).

[0056] As just mentioned, the example described here proposes using a symmetric-key encryption system. However, an alternative encryption system could be used in which the encryption key and the decryption key are different, for example, a public-key (used for encryption) and private-key (used for decryption) system.

[0057] The control unit 21 can thus store at step E15 the list of randomly ordered frequencies indicated by the descriptive data D. In the implementation example described above, it can be foreseen that the control unit 21 stores the set of predefined frequencies f 1 , f 2 , ..., f N (identical to that stored in the electronic control unit 11) and that the control unit 21 then stores in practice at step E15 the ordered list of indices i(1), i(2), ..., i(N) defined by the descriptive data.

[0058] In the example described here, the ordered list of frequencies is randomly generated at the vehicle 10 level (specifically by the electronic control unit 11) and transmitted to identifier 20 in such a way that knowledge of this list is shared by both entities. Alternatively, the ordered list of frequencies could be generated at identifier 20 and transmitted, for example in encrypted form, to vehicle 10, which also allows knowledge of the list to be shared.

[0059] The control unit 21 of identifier 20 then commands (step E16) the communication module 22 to emit electromagnetic signals having successively the frequencies listed in the ordered list of frequencies.

[0060] For example, we represented at step E18 on the figure 4 the emission of an electromagnetic signal of frequency fi(1) by the communication module 22 of identifier 20.

[0061] This electromagnetic signal of frequency fi(1) is received by the communication module 12 of the vehicle 10 at step E20, which allows for a phase measurement Φ i(1) of the received electromagnetic signal. Reference may be made to US document 5,220,332 for further details.

[0062] The measured phase Φ i(1) is received by the electronic control unit 11 and stored at step E22.

[0063] Steps similar to steps E18 to E22 are performed for each of the frequencies in the ordered frequency list, in the order established by that list.

[0064] We also represented on the figure 4 the emission by the communication module 22 of an electromagnetic signal of frequency fi(N) (last frequency of the ordered list in the example described) at step E24.

[0065] This electromagnetic signal of frequency fi(N) is received by the communication module 12 of the vehicle 10 at stage E26, which allows a phase measurement Φ i(N) of the received electromagnetic signal to be obtained.

[0066] The measured phase Φ i(N) is received by the electronic control unit 11 and stored at step E28.

[0067] The electronic control unit 11 thus stores the measured phases Φ i for a plurality of frequencies fi (regardless of the order in which these measurements were carried out) and can deduce at step E30 an estimate of the distance d separating the identifier 20 and the vehicle 10.

[0068] For example, taking into account that the different points with coordinates (fi, Φi) are theoretically located on a straight line with slope c / (2π.d) according to the distance estimation principle recalled with reference to the figure 2 , step E30 includes for example the determination of the slope of a regression line linked to the points of coordinates (fi , Φ i ) and the determination of the distance estimated as a function of this slope.

[0069] The electronic control unit 11 of the vehicle 10 can then optionally control a function of the vehicle 10 at step E32 based on the estimated distance. For example, the electronic control unit 11 can control the unlocking of the vehicle 10's doors if the estimated distance is less than a predetermined threshold.

[0070] In the example just described, the electromagnetic signals (having successively the frequencies fi in the order indicated in the ordered list of frequencies) are emitted by the communication module 22 of the identifier 20.

[0071] Alternatively, these electromagnetic signals could be emitted by the communication module 12 of the vehicle 10; the reception phases Φ i would then be measured at the level of the communication module 22. The measured phases Φ i in association with each frequency fi could then be transmitted (via the wireless link established between the communication modules 12, 22, possibly in encrypted form) to the electronic control unit 11 for estimation of the distance d (as in step E30 described above); the measured phases Φ i could also be used within the identifier 20 to estimate the distance d (according to the principle recalled above), in which case the distance d estimated within the identifier 20 is transmitted to the electronic control unit 11 via the wireless link established between the communication modules 12, 22 (possibly in encrypted form).

[0072] According to yet another variant, electromagnetic signals having successive frequencies fi (in the order indicated by the randomly ordered list) are emitted by the communication module 22 of identifier 20, and a phase measurement Φ i is performed at the vehicle 10 as described above with reference to the figure 4 . In addition, electromagnetic signals having successive frequencies fi (in the order indicated by the randomly ordered list, or in another randomly defined order as explained above) are emitted by the communication module 12 of the vehicle 10 and a phase measurement Φ' i is carried out at the identifier 20.

[0073] The phase values ​​Φ' i measured at identifier 20 are transmitted to the electronic control unit 11 via the wireless link established between the communication modules 12, 22.

[0074] According to this variant, the electronic control unit 11 determines, for each frequency fi, the sum of the corresponding phase Φ i measured at the level of the vehicle 10 and the corresponding phase Φ' i measured at the level of the identifier 20, and estimates the distance d separating the identifier 20 and the vehicle 10 on the basis of these sums (each associated with a frequency fi).

[0075] Such sums correspond in effect to a round trip of the electromagnetic signal, that is to say to a distance equal to 2.d, and make it possible to overcome the phase reference difference that may exist between the two communication modules 12, 22, as explained for example in document US 5 220 332.

[0076] Indeed, the process of signal exchange and phase measurement for reception is then as follows for a given frequency f: The first module (for example, communication module 12) sends an electromagnetic signal having a given frequency f at a reference phase Φref; the second module (here, communication module 22) receives the signal with an (absolute) phase Φ'abs = Φref + 2*π*d*f / c; the second module measures this phase with its own reference phase Φ'ref, the measured phase therefore being: Φ ′ = Φ ′ abs − Φ ′ ref = Φ ref + 2 * π * d * f / c − Φ ′ ref ; The second module sends an electromagnetic signal having the same given frequency f at its own reference phase Φ' ref; the first module receives this electromagnetic signal with an (absolute) phase Φ abs = Φ' ref + 2*π*d*f / c; the first module measures this phase with its phase reference Φ ref, resulting in a measured phase: Φ = Φ abs − Φ ref = Φ ′ ref + 2 * π * d * f / c − Φ ref .

[0077] The sum (Φ + Φ') of the measured phases is: 2*(2*π)*d*f / c and we thus eliminate the phase shift between the two modules 12, 22.

[0078] In another conceivable embodiment, the reception of measured phases, the calculation of the aforementioned sums and the estimation of the distance on the basis of these sums could be carried out by the control unit 21 of the identifier 20 (the estimated distance could then possibly be transmitted from the control unit 21 to the electronic control unit 11 of the vehicle 10 via the established wireless link).

[0079] In all cases, the order in which the different frequencies fi are used for the emitted signals is random, such that an attacker cannot predict this order and adapt the carrier frequency fp to the frequency fi of the emitted signal (as explained above with reference to the figure 3 ). Attackers will therefore not be able to implement the attack described above with reference to the figure 3 .

Claims

1. A method for estimating a distance (d) between a vehicle (10) equipped with a first wireless communication module (12) and an identifier (20) equipped with a second wireless communication module (22), comprising the following steps: - generating (E2) by one of the first and second wireless communication modules (12, 22) a randomly ordered list of frequencies; - encrypting (E4) descriptive data (D) of the list, said descriptive data (D) of the list representing the successive values of the frequencies or the frequency indices, - encrypted transmission between the first wireless communication module (12) and the second wireless communication module (22) of the descriptive data (D) of the list between the first wireless communication module (12) and the second wireless communication module (22); - reception (E20; E26), by at least one of the first and second wireless communication modules (12, 22), of electromagnetic signals having a frequency (fi(1) ; fi(N) ) changing successively among a plurality of frequencies in accordance with said list; - for each frequency (fi(1) ; fi(N) ) of the plurality of frequencies, measuring a phase (Φi(1) ; Φi(N) ) of reception of the electromagnetic signal having the frequency concerned (fi(1) ; fi(N) ); - estimating (E30) said distance (d) based on the measured phases (Φ(i(1)) ; Φi(N) ).

2. Estimation method according to claim 1, comprising a transmission step (E18; E24), by the other of the first and second wireless communication modules (12, 22), of said electromagnetic signals having a frequency (f(i() (1)) ; f(i(N)) )changing successively among the plurality of frequencies in accordance with said list.

3. An estimation method according to one of claims 1 or 2, wherein the step of estimating (E30) said distance (d) comprises a step of determining a slope of a regression line related to points each defined by a frequency (fi(1) ; f(i(N)) )of the plurality of frequencies and the associated measured phase (Φ(i(1)) ; Φ(i(N)) ).

4. Estimation method according to one of claims 1 to 3, wherein the reception step (E20; E26) is implemented by the first wireless communication module (12).

5. Estimation method according to claim 4, further comprising the following steps: - transmission by the first communication module of a plurality of electromagnetic signals having successive frequencies fi in the order indicated by the randomly ordered list; - for each frequency of the plurality of frequencies, measuring, at the second wireless communication module, a second phase of a received electromagnetic signal having the relevant frequency; - transmission of the measured second phases from the second wireless communication module to the first wireless communication module.

6. Estimation method according to claim 5, wherein the transmitted second phases are used during the step of estimating said distance by determining, for each frequency fi , the sum of the corresponding phase Φi measured at vehicle 10 and the corresponding phase Φ'i measured at identifier 20, and estimating the distance d separating identifier 20 and vehicle 10 on the basis of these sums.

7. Estimation method according to one of claims 1 to 6, comprising a step of optionally implementing a vehicle function (E32) based on the estimated distance (d).

8. Electronic unit (11) for a vehicle (10) comprising: - a component designed to randomly generate an ordered list of frequencies ; - an element designed to encrypt descriptive data of the ordered list, said descriptive data (D) of the list representing the successive values of the frequencies or the frequency indices; - a communication module controlled by the electronic unit (11) for transmitting the encrypted descriptive data; - an element designed to control the reception, from an identifier, of electromagnetic signals having a frequency that changes successively among a plurality of frequencies in accordance with said list; - an element designed to measure, for each frequency of the plurality of frequencies, a reception phase of the electromagnetic signal having the frequency concerned; - an element designed to estimate a distance separating the vehicle and the identifier on the basis of the measured phases.