FR3093363A1 - Symmetric cryptography method and device for vehicle computer - Google Patents

Abstract

Symmetric cryptography method and device for vehicle computer The invention relates to a method and a device for decrypting encrypted data using a secret key. For this purpose, a random duration is generated. A first final value of a first internal counter of the first device is determined as a function of the random duration. One or more requests are transmitted to a second device. These requests allow the second device to determine a second final value of a second internal counter of the second device. The request or requests are transmitted in such a way that the second final value is identical to the first final value. The secret key is generated by the first device using the first final value and the same secret key is also generated by the second device using the second final value. The data to be transmitted by the second device to the first device is encrypted with the secret key that the second device has generated. Figure for the abstract: Figure 3

Description

Symmetrical cryptography method and device for vehicle computer

The invention relates to the exchange of data between different computer systems such as an electronic unit or a vehicle computer (in particular of the automotive type) and an off-board system. The invention also relates to methods and devices for the encryption and/or decryption of such data.

Technology background

Contemporary vehicles carry a number of computers, each performing one or more functions, such as, for example, the management of driving assistance, traction control, electronic brake distribution or even the control of actuators to ensure the operation optimal for a combustion engine.

These computers are also called UCE (“Unit of Electronic Control” or in English ECU “Electronic Control Unit”). These computers embed software which is executed to ensure the functions for which they are responsible. This software or its updates are downloaded from an off-board system (also called the cloud) via a wired or wireless connection, for example an OTA (over-the-air) type connection. French "by air").

To avoid any hacking of the data exchanged between the off-board system and the computer, it is known to encrypt or encrypt the data, for example by using symmetric encryption with a fixed secret key, asymmetric encryption or a mixed solution.

Concerning symmetric encryption with a fixed secret key, the same secret key is shared and used by the off-board system and by the computer to respectively encrypt and decrypt the data exchanged, the latter being stored in a secure area of the off-board system and in a secure area of the calculator. A major drawback of this technology is that if the secret key is intercepted by a third party, it is then necessary to redeploy a new, uncompromised secret key in all the computers using it.

Asymmetric encryption uses a combination of public key and private key (public key infrastructure type system) to encrypt and decrypt data. However, such technology requires significant computing capacity and slows down the data exchange process.

The mixed solution sets up a transmission of the computer's secret key encrypted with the public key of the off-board system. The off-board system can then encrypt the data to be transmitted to the computer with the computer's secret key. This solution is however complex to implement and requires the redeployment of a new public key in all the computers in the event of compromise of the public key of the off-board system.

An object of the present invention is to propose a symmetric encryption mechanism with reinforced security against the compromise of the secret key used in the encryption (also called ciphering) of the data.

According to a first aspect, the invention relates to a method for decrypting encrypted data from a secret key, the method being implemented in a first device, the method comprising the steps of:

- generation of information representative of a random duration;

- determination of a first final value of a first counter internal to the first device as a function of information representative of a random duration;

- transmission of at least one request intended for a second device for determination by the second device of a second final value of a second internal counter to the second device, the second final value being identical to the first final value ;

- generation of the secret key from the first final internal counter value;

- reception of data encrypted by the second device from the secret key generated by the second device from the second internal counter final value;

- decryption of the encrypted data received using the secret key generated by the first device;

- reinitialization of the information representative of a random duration, of the first final value and of the secret key;

- transmission of a request to reset the second final value and the secret key to the second device.

According to a variant, the decryption is implemented only if the duration between the transmission of at least one request and the start of the reception of the encrypted data is less than a determined duration.

According to yet another variant, the method further comprises a step of verifying that the encrypted data received has been encrypted with the secret key.

According to an additional variant, the incrementation time step of the first internal counter is identical to the incrementation time step of the second internal counter, the incrementation time step being greater than a maximum propagation time of a signal between the first device and the second device.

According to a second aspect, the invention relates to a device for decrypting encrypted data from a secret key, the device being called the first device and comprising:

- means for generating information representative of a random duration;

- means for determining a first final value of a first counter internal to the first device as a function of the information representative of a random duration;

- means for transmitting at least one request intended for a second device for determination by the second device of a second final value of a second counter internal to the second device, the second final value being identical to the first final value;

- Means for generating the secret key from the first final internal counter value;

- Means for receiving data encrypted by the second device from the secret key generated by the second device from the second final internal counter value;

- means for decrypting the encrypted data received using the secret key generated by the first device;

- means for reinitializing the information representative of a random duration, of the first final value and of the secret key;

- means for transmitting a request to reset the second final value and the secret key to the second device.

According to a variant, the decryption is implemented only if the duration between the transmission of at least one request and the start of the reception of the encrypted data is less than a determined duration.

According to yet another variant, the device further comprises means for verifying that the encrypted data received has been encrypted with the secret key.

According to an additional variant, the incrementation time step of the first internal counter is identical to the incrementation time step of the second internal counter, the incrementation time step being greater than a maximum propagation time of a signal between the first device and the second device.

According to a third aspect, the invention relates to an on-board system in a vehicle comprising one or more devices as described above according to the second aspect of the invention.

According to a fourth aspect, the invention relates to a vehicle, for example of the automobile type, comprising one or more devices as described above according to the second aspect of the invention.

According to a fifth aspect, the invention relates to a computer program which comprises instructions adapted for the execution of the steps of the method according to the first aspect of the invention, this in particular when the computer program is executed by at least one processor.

Such a computer program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other desirable form.

According to a sixth aspect, the invention relates to a computer-readable recording medium on which is recorded a computer program comprising instructions for the execution of the steps of the method according to the first aspect of the invention.

On the one hand, the recording medium can be any entity or device capable of storing the program. For example, the medium may comprise a storage means, such as a ROM memory, a CD-ROM or a ROM memory of the microelectronic circuit type, or even a magnetic recording means or a hard disk.

On the other hand, this recording medium can also be a transmissible medium such as an electrical or optical signal, such a signal being able to be conveyed via an electrical or optical cable, by conventional or hertzian radio or by self-directed laser beam or by other ways. The computer program according to the invention can in particular be downloaded from an Internet-type network.

Alternatively, the recording medium may be an integrated circuit in which the computer program is incorporated, the integrated circuit being adapted to execute or to be used in the execution of the method in question.

Brief description of figures

Other characteristics and advantages of the invention will emerge from the description of the non-limiting embodiments of the invention below, with reference to the appended Figures 1 to 7, in which:

schematically illustrates an environment for the exchange of encrypted data between one or more first devices on the one hand and a second device on the other hand, according to a particular embodiment of the present invention;

schematically illustrates a first device of FIG. 1, according to a particular embodiment of the present invention;

schematically illustrates a second device of FIG. 1, according to a particular embodiment of the present invention;

illustrates a flowchart of the different steps of a data exchange process between one of the first devices and the second device of FIG. 1, according to a particular embodiment of the present invention;

illustrates a timing diagram associated with part of the operations of the process of FIG. 4, according to a particular embodiment of the present invention;

illustrates a flowchart of the different steps of a method for decrypting encrypted data implemented in one of the first devices of FIG. 1, according to a particular embodiment of the present invention;

illustrates a flowchart of the different steps of a data encryption method implemented in the second device of FIG. 1, according to a particular embodiment of the present invention.

A method for decrypting encrypted data, a method for encrypting data as well as the associated devices and system will now be described in the following with reference in conjunction with FIGS. 1 to 7. The same elements are identified with the same reference signs throughout the following description.

According to a particular and non-limiting embodiment of the invention, a method for decrypting encrypted data using a secret key is implemented in a first device, for example a computer of an on-board system in a vehicle. Data decryption is also called data decryption. For this purpose, the method comprises the generation of a random duration, for example expressed in seconds or milliseconds. A first final value of a first internal counter of the first device is determined based on the random duration. One or more requests are transmitted to a second device corresponding for example to an off-board system responsible for transmitting data to the first device, for example data for updating software installed on the first device. These requests allow the second device to determine a second final value of a second internal counter of the second device. The request(s) are transmitted in such a way that the second final value is identical to the first final value. The secret key is generated by the first device using the first final value and the same secret key is also generated by the second device using the second final value. The data to be transmitted by the second device to the first device is encrypted (or ciphered) by the second device with the secret key that the second device has generated. These encrypted data are then received by the first device and then decrypted (or decrypted) by the first device using the secret key generated by the first device. After the encrypted data is received and decrypted, the first device resets the random duration value, the first final value, and the secret key. The first device also transmits to the second device a request for the second device to reset the second final value and the secret key.

Such a method avoids the exchange of the secret key between the first device and the second device, which limits the risks of interception and compromise of the secret key by an unauthorized third party, since the first device and the second device generate the same secret key on the basis of final values of identical internal counters but determined independently at the level of the first device on the one hand and of the second device on the other hand.

Furthermore, the reinitialization of parameters internal to the first device and to the second device, such as the secret key for example, makes it possible to generate a new secret key for each exchange of data between the first device and the second device. Thus, if a secret key used for a given exchange is stolen, the latter can no longer be used for subsequent data exchanges.

schematically illustrates an environment 1 for the exchange of encrypted data between one or more first devices 102, 103, 104 on the one hand and a second device 101 on the other hand, according to a particular and non-limiting embodiment of the present invention.

Each first device 102, 103, 104 corresponds for example to a computer, for example an UCE type computer. An ECU is made up of an electronic computer and one or more on-board software which perform one or more servo-controls. The first devices 102, 103, 104 belong for example to a system 10, advantageously embedded in a vehicle, for example of the automobile type. The computers 102, 103, 104 for example form a multiplexed architecture for the performance of various useful services for the correct operation of the vehicle and for assisting the driver and/or the passengers of the vehicle in controlling the vehicle.

The first devices 102, 103 and 104 communicate and exchange data between them via one or more computer buses, for example a communication bus 11 of the CAN data bus type (from the English “Controller Area Network”). or in French “Network of controllers”).

Each first device 102, 103, 104 is for example connected in communication with a second device 101. The second device 101 corresponds for example to an external device of an off-board system. Each first device 102, 103, 104 is connected to the second device via a wired or wireless connection. The second device 101 is for example a remote server of the “cloud” (or “cloud” in French). One or more first devices load, for example, the software updates stored for downloading into the second device 101 via, for example, the Internet with an OTA type connection (from English “over-the-air”, or in French “ by air”). The data representative of the downloaded updates are advantageously encrypted (or encrypted) by implementing the method described with regard to Figures 4, 6 and/or 7.

Of course, the number of first devices 102, 103, 104 of the system 10 is not limited to 3 and extends to any number, for example 10, 20, 50, 100 or more of first devices. An example of hardware embodiment of such a first device is described with reference to Figure 2. An example of hardware embodiment of a second device 101 is described with reference to Figure 3.

schematically illustrates a first device 2, according to a particular and non-limiting embodiment of the present invention. The first device 2 advantageously corresponds to the first devices 102, 103 and 104 of FIG. 1. The first device 2 advantageously corresponds to a computer of an on-board vehicle system, without however being limited thereto.

The first device 2 is for example configured for the implementation of at least part of the steps of the method described with regard to FIG. 4. Examples of such a first device 2 include, without being limited thereto, electronic equipment embedded such as a vehicle on-board computer, an electronic computer such as an ECU. The elements of the first device 2, individually or in combination, can be integrated in a single integrated circuit, in several integrated circuits, and/or in discrete components. The first device 2 can be made in the form of electronic circuits or software (or computer) modules or else a combination of electronic circuits and software modules. According to different particular embodiments, the first device 2 is coupled in communication with other similar devices or systems, for example via a communication bus or through dedicated input/output ports.

The first device 2 comprises one (or more) processor(s) 20 configured to execute instructions for carrying out the steps of the method and/or for executing the instructions of the software or software embedded in the first device 2. The processor 20 may include onboard memory, an input/output interface, and various circuits known to those skilled in the art. The first device 2 further comprises at least one memory 21, corresponding for example to a volatile and/or non-volatile memory.

The processor corresponds, for example, to a HSM Full hardware security module (Hardware Security Module) to protect access to the variables and functions used for data encryption/decryption.

The computer code of the on-board software or software comprising the instructions to be loaded and executed by the processor is for example stored on the memory 21.

According to a particular and non-limiting embodiment, the first device 2 comprises a block 22 of interface elements for communicating with external devices, for example the second device 101 of FIG. 1, a remote server or the "cloud" , odometer sensors, GPS sensor. Block 22 interface elements include one or more of the following interfaces:

- RF radio frequency interface, for example of the Bluetooth® or Wi-Fi® type, LTE (from English "Long-Term Evolution" or in French "Evolution à long terme"), LTE-Advanced (or in French LTE-advanced );

- USB interface (from the English "Universal Serial Bus" or "Universal Serial Bus" in French);

- HDMI interface (from the English “High Definition Multimedia Interface”, or “Interface Multimedia Haute Definition” in French).

Data are for example loaded to the first device 2 via the interface of block 22 using a Wi-Fi® network such as according to IEEE 802.11 or a mobile network such as a 4G network (or LTE Advanced according to 3GPP release 10 – version 10) or 5G.

According to another particular embodiment, the first device 2 comprises a communication interface 23 which makes it possible to establish communication with other devices (such as secondary computers of the on-board system) via a communication channel 230. The interface communication channel 23 corresponds for example to a transmitter configured to transmit and receive information and/or data via the communication channel 230. The communication channel 230 corresponds for example to a wired network of the CAN type.

schematically illustrates a second device 3, according to a particular and non-limiting embodiment of the present invention. The second device 2 advantageously corresponds to the second device 101 of FIG. 1.

Examples of such a second device 3 include, but are not limited to, various electronic devices such as a laptop computer, an ECU update tool. The elements of the second device 3, individually or in combination, can be integrated in a single integrated circuit, in several integrated circuits, and/or in discrete components. The second device 3 can be made in the form of electronic circuits or software (or computer) modules or else a combination of electronic circuits and software modules.

The second device 3 comprises one (or more) processor(s) 30 configured to execute instructions for carrying out the steps of the method. The processor 30 can include integrated memory, an input/output interface, and various circuits known to those skilled in the art. The second device 3 further comprises at least one memory 31, by a volatile at/or non-volatile memory and/or comprises a memory storage device which can comprise volatile and/or non-volatile memory, such as EEPROM, ROM, PROM, RAM, DRAM, SRAM, flash, magnetic or optical disk.

The processor corresponds, for example, to a HSM Full hardware security module (Hardware Security Module) to protect access to the variables and functions used for data encryption/decryption.

The computer code comprising the instructions to be loaded and executed by the processor is for example stored in the memory or the memory storage device 31.

According to a particular and non-limiting embodiment, the device 3 comprises a block 32 of interface elements for communicating with external devices, for example a first device 2. The interface elements of the block 32 comprise one or more of the following interfaces:

- RF radiofrequency interface, for example of the Bluetooth® or Wi-Fi® type;

- USB interface (from the English "Universal Serial Bus" or "Universal Serial Bus" in French);

- HDMI interface (from the English “High Definition Multimedia Interface”, or “Interface Multimedia Haute Definition” in French).

According to another particular embodiment, the device 3 comprises a communication interface 33 which makes it possible to establish communication with other devices via a communication channel 330. The communication interface 33 corresponds for example to a transmitter configured to transmit and receive information and/or data via the communication channel 330. The communication interface 33 comprises for example a modem and/or a network card and the communication channel can for example be implemented in a wired medium and/or wireless.

Data are for example loaded to the second device 3 using a Wi-Fi® network such as according to IEEE 802.11 or a mobile network such as a 4G network (or LTE Advanced according to 3GPP release 10 – version 10) or 5G.

According to an additional particular embodiment, the device 3 can provide output signals to one or more external devices, such as a display screen 340, one or more loudspeakers 350 and/or other peripherals 360 ( DVD player, a projection system) respectively via output interfaces 34, 35 and 36. According to a variant, one or the other of the external devices is integrated into the second device 3.

illustrates a flowchart of the different steps or operations of a data exchange process between one of the first devices, for example the first device 102, and the second device 101, according to a particular and non-limiting example embodiment of the present invention.

In a first operation 400, the second device 101 transmits a request to the first device 102 to initiate the data download procedure.

In a second operation 401, the first device 102 uses the value of its internal clock at the time of receipt of the request received in operation 400 to generate a random duration, called D_alea. This random duration is limited by a minimum time limit (for example equal to 0) and by a maximum time limit, the maximum limit being for example adjustable or configurable to limit the value of D_alea.

In a third operation 402, the first device 102 transmits a request to the second device 101 so that the latter starts an internal counter, called second internal counter count_Ks2.

In a fourth operation 403a, the first device 102 starts its internal counter, called first internal counter count_Ks1. The first internal counter is started after a time delay calibrated to be equal to the maximum propagation time of the request transmitted to operation 402, this maximum propagation time depending on the communication channel used. The second internal counter will be stopped at the end of the random duration D_alea, the random duration being equal to the time elapsed between the start of the first internal counter and the stopping of the first internal counter. This operation is used to determine the first final value of the first internal counter taken at the end of the random duration D_alea. This first final value corresponds to the number of incrementing time steps counted between the start and the stop of the first internal counter. The incrementation step Tcount is for example adjustable in duration and must be greater than the maximum propagation time of the request transmitted to operation 402. It is considered that the temporal drift of the internal clock of the first device 102 is negligible.

According to a variant, several internal counters are used, the internal counters having different time increment steps. The first final value is determined by juxtaposing the final values of each of the internal counters.

According to another variant, the duration of the temporal incrementation step is generated randomly on each transmission of data from the second device 102 to the first device 101. The random duration of the incrementation step is for example generated as in the operation 401. For the first device and the second device to use the same time incrementation step, the first device transmits synchronization requests to the second device at the start and end of the time incrementation step generated, as described in operation 404 .

In a fifth operation 403b, the second device 101 starts its internal counter, called second internal counter count_Ks2, upon receipt of the request transmitted to operation 402.

In a sixth operation 404, the first device 102 transmits a request to the second device 101 so that the latter stops its internal counter, called second internal counter count_Ks2. The transmission of the request by the first device 102 corresponds to the end of the operation 403a during which the first device 102 has stopped its internal counter, called the first internal counter count_Ks1, at the end of the random duration. Upon receipt of this request, the second device 101 stops its internal counter which takes the second final value. This second final value corresponds to the number of incrementing time steps counted between the start and the stop of the second internal counter. The increment step Tcount of this second internal counter is also adjustable in duration. The duration of the incrementation step of the second internal counter is chosen to be equal to the duration of the incrementation step of the first internal counter. It is considered that the time drift of the internal clock of the second device 101 is negligible.

The second final value taken by the second internal counter count_Ks2 is therefore equal to the first final value taken by the first internal counter count_Ks1.

In a seventh operation 405a, a hash function of the first device 102 uses the first final value to generate the secret key K_sec.

In an eighth operation 405b, a hash function of the second device 101, which is identical to the hash function of the first device 102, uses the second final value (which is identical to the first final value) to generate the same secret key K_sec .

The secret key K_sec is therefore generated both by the first device 102 and by the second device 101, which avoids having to transmit this secret key K_sec from one device to another.

In a ninth operation 406, the second device 101 encrypts (or encrypts) the data to be transmitted to the first device 102 with the secret key K_sec generated by the second device 101 during operation 405b. Encryption is achieved by implementing a block or afloat encryption algorithm, for example an AES (Advanced Encryption Standard) type algorithm, Blowfish, Serpent or Twofish.

In a tenth operation 407, the second device 101 transmits to the first device 102 the data encrypted (or encrypted) during the ninth operation 406. According to an optional variant embodiment, the first device 102 checks that the start of the downloading of the encrypted data begins before a determined delay, this delay beginning at the end of the sixth operation 404. According to this variant, the maximum time between the end of the sixth operation 404 and the start of the tenth operation 407 must be less than the determined delay, called Cste_Dmax_start, this delay being for example chosen by a user or fixed in the system. According to this variant, if the transmission of the encrypted data (and consequently the reception of these data by the first device 102) begins beyond this variable Cste_Dmax_start, then the first device 102 refuses the reception of the encrypted data.

In an eleventh operation 408, the first device 102 decrypts (or decrypts) the encrypted data received during the previous operation, the decryption being performed using the secret key K_sec generated by the first device 102 during operation 405a.

In a twelfth operation 409, the first device 102 verifies that the first decrypted or decrypted datum is equal to a determined datum, called Cste_verif. The verification operation that the start of the downloading of the encrypted data begins before a determined time Cste_Dmax_start is for example carried out during this twelfth operation which is then implemented in parallel with the eleventh operation 408. If one and/or the If the other of the conditions associated with Cste_verif and Cste_Dmax_start is not verified, then the first device 102 interrupts the reception of the encrypted data and the process goes directly to operation 412a. If the 2 conditions associated with Cste_verif and Cste_Dmax_start are verified, then the reception and the decryption of the encrypted data continue and the process then continues with the following operation 410.

In a thirteenth operation 410, the decrypted or decrypted data is downloaded into the memory of the first device 102.

In a fourteenth operation 411, the first device 102 transmits to the second device 101 a request to reset the variables associated with the encrypted data exchange process, namely the second final value Cpt_alea of the second internal counter count_Ks2 and the secret key K_sec.

In a fifteenth operation 412a, the first device 102 reinitializes the variables associated with the encrypted data exchange process, namely the random duration D_alea, the first final value Cpt_alea of the first internal counter count_Ks1 and the secret key K_sec.

In a sixteenth operation 412b, the second device 101 reinitializes the variables associated with the encrypted data exchange process upon receipt of the request transmitted during operation 411, namely the second final value Cpt_alea of the second internal counter count_Ks2 and the key secret K_sec.

Operations 400 to 412b are advantageously repeated when a new transmission of encrypted data from the second device 101 to the first device 102 must be performed, the encryption/decryption process being based on a new secret key generated both by the first device 102 and the second device 101.

illustrates a timing diagram associated with part of the operations of the process of FIG. 4, in particular operations 402 to 404, according to a particular and non-limiting example embodiment of the present invention.

FIG. 5 illustrates two timing diagrams 52 and 54 associated with operations 402 to 404 for the first device 102 and two timing diagrams 51 and 53 associated with operations 402 to 404 for the second device 101.

A first request 541 is transmitted by the first device 102 to the second device 101 at a time t. This request is received by the second device at a time t+δt in a time interval corresponding to the maximum propagation time of the signal carrying the request Tpropag_max 531. The second device 101 starts counting 512 with its internal counter count_Ks2. The first device 102 starts the counting 522 with a delay relative to the instant t corresponding to the maximum propagation time of the signal transporting the request Tpropag_max 531. The starting of the counter 522 also corresponds to the start of the random duration D_alea 521 At the end 523 of the random duration D_alea 521, the first device 102 determines the number of incrementation time steps Tcount counted by the first counter count_Ks1, this number corresponding to the first final value of the first internal counter count _Ks1.

A second request 542 is then transmitted by the first device 102 to the second device 101 when the first internal counter coun_Ks1 stops counting the number of time steps of increments elapsed during the random duration D_alea. Upon receipt of this second request 542, the second device stops its internal counter count_Ks2 and determines the number of incrementation time steps elapsed 513 until receipt of this second request. This number of elapsed incrementation time steps 513 corresponds to the second final value of the second internal counter count _Ks2.

As shown in FIG. 5, the duration of a time step for incrementing the first internal counter count_Ks1 is equal to the duration of a time step for incrementing the second internal counter count_Ks2, this time being greater than the propagation time maximum of the signal Tpropag_max 531.

The incrementation time step of the first internal counter, which is identical to the incrementation time step of the second internal counter, corresponds for example to a preprogrammed constant. According to a variant, the incrementation time step of the first internal counter, which is identical to the incrementation time step of the second internal counter, is a randomly generated constant. For this, the first device generates additional information representative of a random duration which will make it possible to obtain a random time step for the first counter internal to the first device. Then the first device transmits at least one request intended for a second device for determination by the second device of a duration which will make it possible to obtain a time step for the second counter internal to the second device identical to the time step of the first counter internal to the first device.

illustrates a flowchart of the different steps of a method for decrypting (or decrypting) encrypted data, according to a particular and non-limiting embodiment of the present invention. The method is advantageously implemented by a first device, for example a computer of the on-board system of FIG. 1.

In a first step 61, information representative of a random duration is generated, for example as described in operation 401.

In a second step 62, a first final value of a first counter internal to the first device is determined according to the information representative of a random duration.

In a third step 63, at least one request is transmitted to a second device to allow the second device to determine a second final value of a second internal counter of this second device. The second final value is identical to the first final value.

In a fourth step 64, a secret key is generated from the first final internal counter value, for example by implementing a hash function.

In a fifth step 65, data encrypted by the second device from the secret key generated by the second device from the second final internal counter value is received by the first device.

In a sixth step 66, the encrypted data received in the previous step is decrypted (or decrypted) by the first device using the secret key generated by the first device.

In a seventh step 67, the information representing a random duration, the first final value and the secret key generated or determined by the first device are reset by the first device.

In an eighth step 68, a request to reset the second final value and the secret key is transmitted by the first device to the second device.

illustrates a flowchart of the different steps of a method for enciphering (or encrypting) data intended for the first device, according to a particular and non-limiting embodiment of the present invention. The method is advantageously implemented by a second device, for example a device of the off-board system of FIG. 1.

In a first step 71, at least one request transmitted by the first device is received by the second device.

In a second step 72, a second final value of a second internal counter of this second device is determined from the request or requests received in step 71.

In a third step 73, a secret key is generated from the second final value of the second internal counter, for example by implementing a hash function.

In a fourth step 74, the data intended for the first device is encrypted (or encrypted) using the secret key generated by the second device in the previous step.

In a fifth step 66, the data encrypted in the previous step is transmitted to the first device.

In a sixth step 67, a request to reset the second final value and the secret key transmitted by transmitted by the first device is received by the second device.

Of course, the invention is not limited to the embodiments described above but extends to a data encryption method and to the device configured for the implementation of such a method. The invention also relates to a communication method between the first device and the second device implementing the encryption and decryption of data.

The invention also relates to a vehicle, for example an automobile or more generally a land motor vehicle, comprising the device 2 of FIG. 2 or the on-board system 10 of FIG. 1.

Claims (10)

A method of decrypting encrypted data from a secret key, said method being implemented in a first device (102), said method comprising the steps of:
- generation (61) of information representative of a random duration;
- determination (62) of a first final value of a first counter internal to said first device (102) as a function of said information representative of a random duration;
- transmission (63) of at least one request (541, 542) intended for a second device (101) for determination by said second device (101) of a second final value of a second internal counter to said second device (101), said second final value being identical to said first final value;
- generation (64) of said secret key from said first final internal counter value;
- receiving (65) data encrypted by said second device (101) from said secret key generated by said second device (101) from said second internal counter final value;
- decryption (66) of said received encrypted data using said secret key generated by said first device (102);
- reinitialization (67) of said piece of information representative of a random duration, of said first final value and of said secret key; and
- transmission (68) of a request to reset said second final value and said secret key to said second device (102). Method according to claim 1, for which said decryption is implemented only if the duration between said transmission of at least one request and the start of said reception of encrypted data is less than a determined duration. Method according to one of claims 1 and 2, further comprising a step of verifying that said encrypted data received has been encrypted with said secret key. Method according to one of Claims 1 to 3, for which the time step of incrementation of said first internal counter is identical to the time step of incrementation of said second internal counter, the said time step of incrementation being greater than a maximum propagation duration of a signal between said first device and said second device. Device for decrypting data encrypted from a secret key, said device being called the first device and comprising:
- means (20) for generating information representative of a random duration;
- means (20) for determining a first final value of a first counter internal to said first device as a function of said information representative of a random duration;
- means (22) for transmitting at least one request intended for a second device for determination by said second device of a second final value of a second internal counter to said second device, said second final value being identical to said first final value;
- means (20) for generating said secret key from said first final internal counter value;
- means (22) for receiving data encrypted by said second device from said secret key generated by said second device from said second final internal counter value;
- means (20) for decrypting said encrypted data received using said secret key generated by said first device;
- Means (20) for resetting said piece of information representing a random duration, said first final value and said secret key; and
- Means (22) for transmitting a request to reset said second final value and said secret key to said second device. Device according to Claim 5, for which the said decryption is implemented only if the duration between the said transmission of at least one request and the start of the said reception of the encrypted data is less than a determined duration. Device according to one of Claims 5 and 6, further comprising means (20) for verifying that said encrypted data received has been encrypted with said secret key. Device according to one of Claims 5 to 7, for which the time step of incrementation of said first internal counter is identical to the time step of incrementation of said second internal counter, the said time step of incrementation being greater than a maximum propagation duration of a signal between said first device and said second device. Motor vehicle comprising the device according to one of Claims 5 to 8. Computer program product comprising instructions adapted for the execution of the steps of the method according to one of Claims 1 to 4, when the computer program is executed by at least one processor.