Method for the secure transmission of real-time messages between a transmitter and a receiver located in a motor vehicle

Abstract

The invention relates to a method for the secure transmission of real-time messages between a transmitter (12) and a receiver (22) located in a motor vehicle (10), characterized in that it comprises at least a first step, performed during a normal driving cycle of the motor vehicle (10), consisting of the receiver (22) comparing a first transmitter authentication code (MAC1-E) received with a first receiver authentication code (MAC1-R). The first message (M1) is rejected if the first transmitter authentication code (MAC1-E) differs from the first receiver authentication code (MAC1-R), and the first message (M1) is accepted if the first transmitter authentication code (MAC1-E) is identical to the first receiver authentication code (MAC1-R). Figure 3

Description

Title of the invention: Method for the secure transmission of messages in real time between a transmitter and a receiver which are on board a motor vehicle Technical field

[0001] The present patent application relates to a method for the secure transmission of messages in real time between a transmitter and a receiver which are on board a motor vehicle, the method being particularly suitable for securing transmissions in a system for monitoring the pressure of the tires of a motor vehicle. Prior art

[0002] It is known to equip a motor vehicle with a monitoring system known under the name "TPMS" (English acronym for "Tyre Pressure Monitoring System").

[0003] Such a monitoring system generally comprises a central computing unit and electronic devices called “wheel units”, each wheel unit equipping an associated wheel of the vehicle.

[0004] A set of radio frequency communications ensures communication between each wheel unit and the central computing unit.

[0005] Each wheel unit comprises sensors in order in particular to detect an anomaly of the wheel. These sensors can, for example, be a tire inflation pressure sensor, a temperature sensor and a wheel acceleration sensor.

[0006] In addition, each wheel unit has a battery and associated memory.

[0007] Two types of monitoring systems are known: a direct monitoring system which measures the tire pressure directly and in real time by means of the associated pressure sensor, and an indirect monitoring system which evaluates the rotation speed of the wheels of the motor vehicle, for example using the anti-lock braking system.

[0008] In the context of the direct monitoring system, it is possible for a malicious person to falsify the content of a radio frequency communication frame from a wheel unit and to transmit this erroneous data to the central computer of the vehicle.

[0009] Upon receipt of a frame comprising the known communication protocol, the central computer performs a simple check of the identifier of the sender of this frame.

[0010] Thus, if this identifier is known to the vehicle, the data contained in this frame is then accepted and processed.

[0011] For example, in the event of low pressure, a false pressure alert may then be raised and displayed on the vehicle's dashboard.

[0012] This type of falsification can cause dissatisfaction among the vehicle driver, vehicle returns to garages and ultimately distrust of the direct monitoring system.

[0013] Furthermore, such falsification may allow a malicious person to cause the driver to stop the vehicle so that the driver can verify the veracity of this alert. During this stop, the malicious person will thus have the opportunity to steal the vehicle.

[0014] With the emergence of autonomous vehicles, this scenario becomes critical since in the event of a pressure alert on one of the wheels, the vehicle will stop immediately, thus causing an interruption of service.

[0015] Document EP-A1-3050251 is known, which describes and represents a method for authenticating message frames transmitted via a communication network of a motor vehicle, by means of a message authentication code, known by the English acronym MAC for "Message Authentication Code".

[0016] The message authentication code is a code which accompanies data in order to ensure the integrity of the latter, by making it possible to verify that they have not undergone any modification, after a transmission for example.

[0017] In addition, the authentication code makes it possible to ensure that the transmitting wheel unit has not been impersonated, thus legitimizing the content of the transmitted message. [Technical problem remaining]

[0018] The type of secure transmission method described in document EP-Al-3050251 is effective but is not optimized for the transmission of data in a monitoring system of the “TPMS” type.

[0019] Indeed, the method described in document EP-A1-3050251 is based on two-way wired communication between two actors, which guarantees the infallibility of the exchanges.

[0020] This method authenticates a second message based on the assumption that the parameters which construct this authentication and which are transmitted in a first message will have been received by the receiver.

[0021] However, in the case of wireless communication of the radiofrequency type, message loss can occur frequently, for example due to an electromagnetic disturbance.

[0022] Thus, by applying the method described in document EP-A 1-3050251 to a wireless communication system, such as a TPMS system, some messages would not be durably authenticated, which would result in a failure, as if the wheel units that had not managed to be authenticated were out of service. Statement of the invention

[0023] The present invention aims in particular to resolve these drawbacks and to propose an alternative transmission method.

[0024] These objectives, as well as others which will appear on reading the following description, are achieved with a method for the secure transmission of messages in real time between a transmitter and a receiver which are on board a motor vehicle, characterized in that it comprises at least a first step which is executed during a current driving cycle of the motor vehicle and which consists of: - generate a first arbitrary running number, - establishing, by the transmitter, a first transmitter authentication code produced by an authentication algorithm with a first authentication parameter as input to said algorithm, the first authentication parameter comprising at least: i. a second previous arbitrary number stored by the transmitter during a previous driving cycle of the motor vehicle, and ii. a first encryption key, - transmit, by the transmitter, at least a first message which includes: i. said first encryption key and the first running arbitrary number which are encrypted by a second encryption key, and ii. said first issuer authentication code, - receive, by the receiver, said first message, - establishing, by the receiver, a first receiver authentication code produced by an authentication algorithm with the first authentication parameter as input to said algorithm, the first authentication parameter comprising at least: i. the second previous arbitrary number received and stored by the receiver during said previous rolling cycle, and ii. the first encryption key received in said first previously received message, - comparing, by the receiver, the first transmitter authentication code received with the first receiver authentication code, the first message being rejected if the first transmitter authentication code differs from the first receiver authentication code, and the first message being accepted if the first transmitter authentication code is identical to the first receiver authentication code.

[0025] The method according to the invention makes it possible to limit the validity of a message in time to a driving cycle of the motor vehicle, so that a message recorded by a attacking a previous rolling cycle and returned to a current rolling cycle will be rejected due to the obsolescence of the returned message, which in particular helps prevent a message replication attack.

[0026] In practice, during this first step of the method, a plurality of first messages are sent to compensate for the possible loss of one or more first messages during their transmission.

[0027] According to other optional characteristics of the method according to the invention, taken alone or in combination:

[0028] the method comprises at least one second step which is carried out following the first step during said current rolling cycle and which consists of: - establishing, by the transmitter, a second transmitter authentication code produced by the authentication algorithm with a second authentication parameter as input to said algorithm, the second authentication parameter comprising at least: i. the first running arbitrary number stored by the transmitter during the first step, and ii. the first encryption key, - transmit, by the transmitter, at least one second message which includes said second transmitter authentication code, - receive, by the receiver, said second message, - establishing, by the receiver, a second receiver authentication code produced by the authentication algorithm with the second authentication parameter as input to said algorithm, the second authentication parameter comprising at least: i. the first current arbitrary number received and stored by the receiver during said first step, and ii. the first encryption key received and stored by the receiver during said first step, - comparing, by the receiver, the second transmitter authentication code received with the second receiver authentication code, the second message being rejected if the second transmitter authentication code differs from the second receiver authentication code, and the second message being accepted if the second transmitter authentication code is identical to the second receiver authentication code;

[0029] the first step constitutes a learning phase during which the transmitter, which is associated with an identifier, transmits a plurality of successive first messages to the receiver to enable the transmitter to be identified by the receiver if the transmitter is unknown to the receiver, the transmitter identifier being capable of being validated upon receipt of the first message by the receiver if the first transmitter authentication code transmitted is identical to the first authentication code received receiver. This feature allows you to limit the duration of learning an unknown transmitter;

[0030] the learning phase is capable of being executed in the event of loss of memory of the transmitter;

[0031] each authentication parameter comprises functional data of the motor vehicle which participate in the establishment of said authentication codes. The changing and unpredictable nature of the operating data strengthens the authentication codes;

[0032] the transmitter is a wheel unit and the receiver is a central unit, the transmitter and the receiver belonging to a system for monitoring the tire pressure of the motor vehicle;

[0033] the second symmetric encryption key is stored in a non-volatile memory of the transmitter and in a non-volatile memory of the receiver;

[0034] each arbitrary number is a random session number which is generated by the transmitter at the start of each driving cycle of the motor vehicle;

[0035] the transmission and reception of each message between the receiver and the transmitter are carried out by radio frequency.

[0036] The present invention also relates to a motor vehicle which comprises a tire pressure monitoring system comprising at least one wheel unit forming a transmitter and at least one central unit forming a receiver, characterized in that said central unit, and / or said wheel unit, are duly programmed to implement the method described above. Brief description of the drawings

[0037] Other characteristics and advantages of the invention will emerge on reading the description which follows, with reference to the appended figures, which illustrate: [Fig.l]: a schematic top view of a motor vehicle equipped with a tire pressure monitoring system comprising a wheel unit and a central unit; [Fig.2]: a flowchart representing the different operating modes adopted by the wheel unit of [Fig.l], during the method according to the invention; [Fig.3]: a flowchart representing the execution of the first step of the method according to the invention; [Fig.4]: a flowchart representing the execution of the second step of the method according to the invention.

[0038] For clarity, identical or similar elements are identified by identical or similar reference signs throughout the figures. [Detailed description of embodiments of the invention]

[0039] [Fig.l] shows a motor vehicle 10 which is equipped with a monitoring system of the “TPMS” type (English acronym for “Tyre Pressure Monitoring System”).

[0040] More particularly, the motor vehicle 10 is equipped with four wheel units which each form an electronic device, and which are each mounted on an associated wheel of the motor vehicle 10.

[0041] For the sake of clarity, only one wheel unit, referenced 12 and associated with a wheel 14, is described in the remainder of the description.

[0042] The wheel unit 12 comprises an electronic box which contains a set of sensors, such as a temperature sensor and a tire pressure sensor of the associated wheel 14.

[0043] Also, the wheel unit 12 comprises a computer 16, a battery 18 and a volatile memory 20a and a non-volatile memory 20b of the flash type for example.

[0044] The motor vehicle 10 comprises a central unit 22 which is equipped with a volatile memory 23a and a non-volatile memory 23b of the flash type, and which communicates with the wheel unit 12 via a radio frequency communication system.

[0045] For this purpose, the wheel unit 12 forms a transmitter which communicates with the central unit 22 forming a receiver.

[0046] The wheel unit 12 operates according to a plurality of distinct operating modes, each operating mode meeting a need depending on the instantaneous use of the wheel unit 12.

[0047] As can be seen in [Fig.2], the wheel unit 12 operates according to a stationary mode A, which is adopted when the motor vehicle 10 is stationary, a first intermediate mode B1 which is adopted at the exit from the stationary mode A, when the motor vehicle 10 begins a new driving cycle, a second intermediate mode B2 which is adopted following the first intermediate mode B1, a driving mode C and an interim mode D which are detailed later.

[0048] The term “driving cycle” means all of the phases which correspond to the driving of the motor vehicle 10, from the exit from stationary mode A, until the wheel unit 12 enters stationary mode A again.

[0049] It will be noted that the stationary mode A of the wheel unit 12 is adopted when the motor vehicle 10 has been stationary for a determined time, as detailed below.

[0050] The invention relates more particularly to a method for the secure transmission of messages in real time between the wheel unit 12, forming a transmitter, and the central unit 22, forming a receiver.

[0051] “Message” means a computer frame which comprises a block of information.

[0052] The method is executed at each driving cycle of the motor vehicle 10. In the present description, a current driving cycle is described in detail and reference is made to a previous driving cycle which precedes the current driving cycle.

[0053] The method comprises a first step, which is carried out during the first intermediate mode B1 and which consists of generating a first arbitrary current number Nbx which is a session number intended to guarantee the obsolescence of the validity of the transmitted messages.

[0054] At each new rolling cycle, a new current arbitrary number Nbx is generated to replace a previous arbitrary number Nbx-1, during the transition from the stationary mode A to the first intermediate mode B1 adopted by the wheel unit 12.

[0055] Additionally, each arbitrary number is randomly generated using a random number generator.

[0056] Also, the first step of the method consists of establishing, by the wheel unit 12, a first message transmitter authentication code MAC1-E. A message authentication code, often designated by its English acronym MAC for “message authentication code”, is a code accompanying data in order to ensure the authenticity of the latter, by making it possible to verify that they have not been the subject of identity theft, after a transmission for example.

[0057] With reference to [Fig.3], the first transmitter authentication code MAC1-E is produced by an authentication algorithm Al with a first authentication parameter Pal as input to the algorithm.

[0058] The first authentication parameter Pal comprises the second previous arbitrary number Nbx-1 which was generated by the wheel unit 12, and stored by the associated volatile memory 20a, during the previous rolling cycle.

[0059] In addition, the first authentication parameter Pal comprises a first encryption key TL

[0060] The first encryption key Tl is a unique key produced by a random number generator, also referred to by its English acronym TRNG for “True Number Generator”.

[0061] The goal is to have a population of first encryption keys Tl sufficiently numerous to limit the probability that an attacker finds the first encryption keys by brute force.

[0062] Also, the first authentication parameter Pal comprises functional data recorded by the sensors of the wheel unit 12, namely the pressure DI of the tires, the temperature D2 and the acceleration D3 of the wheel unit 12.

[0063] The first encryption key T1 is recorded in the non-volatile memory 20b of wheel unit 12 during the manufacture of wheel unit 12.

[0064] Following the production of the first transmitter authentication code MAC1-E, the first step of the method consists of transmitting, by the transmitter wheel unit 12, a plurality of first messages M1, as can be seen in [Fig.3].

[0065] The first messages M1 are transmitted during the first intermediate mode B1 adopted by the wheel unit 12, at the exit from the stationary mode A.

[0066] During the first intermediate mode B1 and the first step of the method, the first messages M1 are transmitted successively according to a defined periodicity, for example sixteen seconds.

[0067] The first messages M1 are identical and each respect a determined architecture, or frame.

[0068] More precisely, the first messages M1 each comprise functional data, namely the pressure DI of the tires, the temperature D2, the acceleration D3, the identifier D4 of the wheel unit 12, the mode adopted D5 and a synchronization data item D6.

[0069] In addition, the first messages M1 each comprise the first encryption key T1, the first current arbitrary number Nbx, and the first transmitter authentication code MAC1-E previously established by the wheel unit 12.

[0070] The first encryption key T1 and the first arbitrary running number Nbx are encrypted using a second symmetric encryption key T2 according to the standard known by the English acronym AES, meaning “Advanced Encryption Standard”.

[0071] It will be noted that the second encryption key T2 is identical for all the wheel units 12 and the central units 22 of the same customer.

[0072] Preferably, the second encryption key T2 is saved in a secure space, such as a secure database.

[0073] Also, the second encryption key T2 is recorded in the non-volatile memory 23b of the central unit 22 and in the non-volatile memory 20b of the wheel unit 12.

[0074] Still during the first step of the method, corresponding to the first intermediate mode B1, the first messages M1 are received by the central unit 22.

[0075] Upon receipt of each first message M1, the central unit 22 establishes a first receiver authentication code MAC1-R produced by the authentication algorithm A1 with the first authentication parameter Pal as input to the authentication algorithm A1.

[0076] The first authentication parameter Pal comprises the second previous arbitrary number Nbx-1 received by the central unit 22 and recorded in the volatile memory 23a of the central unit 22 during the previous rolling cycle, and the first encryption key Tl received in each of the first messages Ml previously received.

[0077] For each first message M1 received and after having established the first associated receiver authentication code MAC1-R, the central unit 22 compares the first receiver authentication code MAC1-R established with the first transmitter authentication code MAC1-E received from the wheel unit 12.

[0078] After comparison, each first message M1 received is rejected if the first transmitter authentication code MAC1-E differs from the first receiver authentication code MAC1-R.

[0079] Conversely, each first message M1 received is accepted if the first transmitter authentication code MAC1-E is identical to the first receiver authentication code MAC1-R.

[0080] The method comprises a second step which is carried out during the second intermediate mode B2 and which is carried out following the first step.

[0081] The second step of the method consists in establishing, by the transmitting wheel unit 12, a second message transmitter authentication code MAC2-E produced by the authentication algorithm Al with a second authentication parameter Pa2 as input to the algorithm Al.

[0082] The second authentication parameter Pa2 comprises the first current arbitrary number Nbx stored by the wheel unit 12 during the first step of the method corresponding to the first intermediate mode Bl.

[0083] In addition, the second authentication parameter Pa2 includes the first encryption key TL

[0084] Following the production of the second transmitter authentication code MAC2-E, the second step of the method consists of transmitting, by the transmitter wheel unit 12, a plurality of second messages M2, as can be seen in [Fig.4].

[0085] During the second step which corresponds to the second intermediate mode B2, the second messages M2 are transmitted successively according to a defined periodicity, for example sixteen seconds.

[0086] The wheel unit 12 switches from the second intermediate mode B2 to the driving mode C when the second messages M2 transmitted reach or exceed a predetermined number, for example the number of twenty second messages M2 transmitted.

[0087] The second M2 messages are identical and each respect a determined architecture, or frame.

[0088] More precisely, the second messages M2 each comprise functional data, namely the pressure DI of the tires, the temperature D2 and the acceleration D3.

[0089] In addition, the second messages M2 each comprise the second transmitter authentication code MAC2-E previously established by the wheel unit 12.

[0090] Still during the second stage of the process, and the second intermediate mode B2, the second messages M2 are received by the central unit 22.

[0091] Upon receipt of each second message M2, the central unit 22 establishes a second receiver authentication code MAC2-R produced by the authentication algorithm Al with the second authentication parameter Pa2 as input to the authentication algorithm Al.

[0092] The second authentication parameter Pa2 comprises the first current arbitrary number Nbx and the first encryption key Tl which are extracted from the first messages Ml and stored in the volatile memory 23a of the central unit 22 during the first step.

[0093] For each second message M2 received and after having established the associated second receiver authentication code MAC2-R, the central unit 22 compares the second receiver authentication code MAC2-R established with the second transmitter authentication code MAC2-E received from the wheel unit 12.

[0094] After comparison, each second received M2 message is rejected if the second transmitter authentication code MAC2-E differs from the second receiver authentication code MAC2-R.

[0095] Conversely, each second M2 message received is accepted if the second transmitter authentication code MAC2-E is identical to the second receiver authentication code MAC2-R.

[0096] In certain situations, the identifier of the wheel unit 12 is unknown to the central unit 22, in particular when the wheel unit 12 has just been mounted on the associated wheel 14, for example to replace a faulty wheel unit 12.

[0097] In such a situation, the first step of the method constitutes a learning phase, by the successive transmission of first messages M1 from the wheel unit 12 to the central unit 22.

[0098] The number of first messages M1 necessary for the central unit 22 to learn the identifier of the wheel unit 12 concerned during the learning phase is for example twenty messages.

[0099] Conversely, the identification of the wheel unit 12 is validated by the central unit 22 upon receipt of the first message M1 by the central unit 22 if the first transmitter authentication code MAC1-E transmitted is identical to the first receiver authentication code MAC1-R.

[0100] Indeed, the concordance of the first transmitter authentication code MAC1-E and the first receiver authentication code MAC1-R means that the transmitter wheel unit 12 was already present on the wheel 14 in the previous rolling cycle.

[0101] This characteristic makes it possible to limit the learning time necessary for the identification of a new wheel unit 12 by the central unit 22.

[0102] Similarly, in the event of a reset of the wheel unit 12 occurring for example due to of a power cut, the wheel unit 12 loses the data recorded in its volatile memory 20a, in particular the first current arbitrary number Nbx and the second previous arbitrary number Nbx-1.

[0103] Consequently, the wheel unit 12 is no longer able to establish either the first transmitter authentication code MAC1-E or the second authentication code MAC2-E.

[0104] To overcome this problem, a new learning phase, such as the learning phase described above, is launched, if the current mode adopted by the wheel unit 12 is the first intermediate mode B1 which corresponds to the first step of the method, or a previous mode.

[0105] If the current mode adopted by the wheel unit 12 is a mode subsequent to the first intermediate mode B1, the central unit 22 knows the identifier of the transmitting wheel unit 12 which is transmitted via the messages sent by the wheel unit 12, and the central unit 22 can detect an anomaly since the transmitter authentication code MAC2-E received by the central unit 22 is invalid.

[0106] In such a situation in which the central unit 22 knows the identifier of the wheel unit 12 and the central unit 22 invalidates the associated MAC2-E transmitter authentication code, a new learning phase dedicated solely to the wheel unit 12 concerned is launched.

[0107] With reference to [Fig.2], the transition from one operating mode of the wheel unit 12 to another is organized in the following manner.

[0108] The first intermediate mode B1 is adopted by the wheel unit 12 at the exit from the stationary mode A, when the motor vehicle 10 begins a new driving cycle.

[0109] The wheel unit 12 is equipped with a message counter (not shown) which counts the first messages M1 and second messages M2 transmitted during the intermediate modes B1, B2.

[0110] The wheel unit 12 switches from the first intermediate mode B1 to the second intermediate mode B2 when the message counter reaches or exceeds a predetermined number, for example the number of twenty messages.

[0111] The wheel unit 12 switches from the second intermediate mode B2 to the driving mode C when the motor vehicle 10 is moving and the counter reaches or exceeds a predetermined number, for example the number of forty messages.

[0112] In driving mode C, the exchange of messages between the wheel unit 12 and the central unit 22 continues in accordance with the second step of the method described previously, with the difference that the periodicity of sending the second messages M2 increases from sixteen seconds to sixty-four seconds.

[0113] In the event of the motor vehicle 10 stopping in intermediate mode B1, B2, the wheel unit 12 switches to interim mode D.

[0114] Similarly, if the motor vehicle 10 stops in driving mode C, the wheel unit 12 switches to interim mode D.

[0115] In the interim mode D, the exchange of messages between the wheel unit 12 and the central unit 22 continues in accordance with the second step of the method described previously.

[0116] The wheel unit 12 leaves the interim mode D to selectively adopt the first intermediate mode B1 if the counter has not reached a predetermined number, for example the number of forty messages, or the driving mode C if the counter has reached the number of forty messages and the motor vehicle 10 is moving, or the stationary mode A if the motor vehicle has been stationary for a predetermined time, for example nineteen minutes.

Claims

Claims

1. Method for the secure transmission of messages in real time between a transmitter (12) and a receiver (22) which are on board a motor vehicle (10), characterized in that it comprises at least a first step which is executed during a current driving cycle of the motor vehicle (10) and which consists of: - generating a first current arbitrary number (Nbx), - establishing, by the transmitter (12), a first transmitter authentication code (MAC1-E) produced by an authentication algorithm (A1) with a first authentication parameter (Pal) as input to said algorithm, the first authentication parameter (Pal) comprising at least: i. a second previous arbitrary number (Nbx-1) stored by the transmitter (12) during a previous driving cycle of the motor vehicle (10), and ii. a first encryption key (Tl), - transmit, by the transmitter (12), at least a first message (Ml) which comprises: i. said first encryption key (Tl) and the first current arbitrary number (Nbx) which are encrypted by a second encryption key (T2), and ii. said first transmitter authentication code (MAC1-E), - receiving, by the receiver (22), said first message (Ml), - establishing, by the receiver (22), a first receiver authentication code (MAC1-R) produced by an authentication algorithm (A1) with the first authentication parameter (Pal) as input to said algorithm, the first authentication parameter (Pal) comprising at least: i. the second previous arbitrary number (Nbx-1) received and stored by the receiver (22) during said previous rolling cycle, and ii. the first encryption key (Tl) received in said first message (Ml) previously received, - comparing, by the receiver (22), the first transmitter authentication code (MAC1-E) received with the first receiver authentication code (MAC1-R), the first message (Ml) being rejected if the first transmitter authentication code (MAC1-E) differs from the first receiver authentication code (MAC1-R), and the first message (Ml) being accepted if the first sending authentication code (MAC1-E) is identical to the first receiving authentication code (MAC1-R).

2. Method according to claim 2, characterized in that it comprises at least a second step which is carried out following the first step during said current driving cycle and which consists of: - establishing, by the transmitter (12), a second transmitter authentication code (MAC2-E) produced by the authentication algorithm (A1) with a second authentication parameter (Pa2) as input to said algorithm, the second authentication parameter (Pa2) comprising at least: i. the first current arbitrary number (Nbx) stored by the transmitter (12) during the first step, and ii.the first encryption key T1, - transmitting, by the transmitter (12), at least one second message (M2) which comprises said second transmitter authentication code (MAC2-E), - receiving, by the receiver (22), said second message (M2), - establishing, by the receiver (22), a second receiver authentication code (MAC2-R) produced by the authentication algorithm (A1) with the second authentication parameter (Pa2) as input to said algorithm, the second authentication parameter (Pa2) comprising at least: i. the first current arbitrary number (Nbx) received and stored by the receiver (22) during said first step, and ii.the first encryption key (Tl) received and stored by the receiver (22) during said first step, - comparing, by the receiver (22), the second transmitter authentication code (MAC2-E) received with the second receiver authentication code (MAC2-R), the second message (M2) being rejected if the second transmitter authentication code (MAC2-E) differs from the second receiver authentication code (MAC2-R), and the second message (M2) being accepted if the second transmitter authentication code (MAC2-E) is identical to the second receiver authentication code (MAC2-R).

3. Method according to any one of the preceding claims, characterized in that the first step constitutes a learning phase during which the transmitter (12), which is associated with an identifier (D4), transmits a plurality of successive first messages (M1) to the receiver (22) to allow the identification of the transmitter (12) by the receiver (22) if the transmitter (12) is unknown to the receiver (22), the identifier (D4) of the transmitter (12) being able to be validated upon receipt of the first message (Ml) by the receiver (22) if the first MAC1-E transmitter authentication code transmitted is identical to the first MAC1-R receiver authentication code received.

4. Method according to claim 3, characterized in that the learning phase is capable of being executed in the event of memory loss of the transmitter (12).

5. Method according to any one of the preceding claims, characterized in that each authentication parameter (Pal, Pa2) comprises functional data (Dl, D2, D3) of the motor vehicle (10) which participate in the establishment of said authentication codes (MAC1-E, MAC1-R, MAC2-E, MAC2-R).

6. Method according to any one of the preceding claims, characterized in that the transmitter (12) is a wheel unit (12) and the receiver (22) is a central unit (22), the transmitter (12) and the receiver (22) belonging to a system for monitoring the pressure of the tires of the motor vehicle (10).

7. Method according to any one of the preceding claims, characterized in that the second symmetric encryption key (T2) is recorded in a non-volatile memory (20b) of the transmitter (12) and in a non-volatile memory (23b) of the receiver (22).

8. Method according to any one of the preceding claims, characterized in that each arbitrary number (Nbx, Nbx-1) is a random session number which is generated by the transmitter (12) at the start of each driving cycle of the motor vehicle (10).

9. Method according to any one of the preceding claims, characterized in that the transmission and reception of each message between the receiver (22) and the transmitter (12) are carried out by radio frequency.

10. Motor vehicle (10) which comprises a tire pressure monitoring system comprising at least one wheel unit (12) forming a transmitter and at least one central unit (22) forming a receiver, characterized in that said central unit (22), and / or said wheel unit (12), are duly programmed to implement the method according to any one of the preceding claims.

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