DE102022124552A1 - Method for secure communication between a transmitter and a receiver in a motor vehicle and communication system - Google Patents

Abstract

The invention relates to a method for secure communication between a transmitter (12) and a receiver (14) in a motor vehicle. In an initialization phase, the sender (12) transmits an initialization message encrypted with a trustworthy key to the receiver (14), which contains a temporary key. The receiver (14) decrypts the encrypted initialization message with the trusted key to obtain the temporary key. A communication system (10) for a motor vehicle is also described.

Description

The invention relates to a method for secure communication between a transmitter and a receiver in a motor vehicle. The invention further relates to a communication system for a motor vehicle, which comprises at least one transmitter and one receiver.

In modern motor vehicles, the number of components that communicate with each other in order to exchange information or transmit control commands is constantly increasing. For security reasons, the corresponding communication between the components, i.e. the communication from a sender to a receiver, is secured, for which purpose encryption techniques are used.

Standards for this are known from the prior art, for example Secure Onboard Communication (SecOC) standardized in Autosar, which typically takes place between control devices within a motor vehicle. Here, for example, by means of an encryption standard, in particular the “Advanced Encryption Standard (AES)” encryption standard, an authentication identifier is generated using a hardware-based security module, in which the hardware-based security module secures the entire communication accordingly, which in combination with high-frequency communication, in particular less than five milliseconds a high load on the hardware-based security module.

In this respect, there are problems with securing communication, which has to be carried out with fast cycle times, especially cycle times of less than five milliseconds.

In order to reduce the load on the hardware-based security module, it is also known from the prior art to only secure the messages in blocks, which, however, means that the individual messages within a block are not secured.

The object of the invention is to realize a secure communication in a motor vehicle, which is also suitable for communication with fast cycle times.

The object is achieved according to the invention by a method for secure communication between a transmitter and a receiver in a motor vehicle. In an initialization phase, the sender transmits an initialization message encrypted with a trusted key to the recipient, which contains a temporary key. The recipient decrypts the encrypted initialization message with the trusted key to obtain the temporary key.

The basic idea of the invention is that a temporary key is used for the subsequent secure communication between the sender and the receiver instead of a permanent key, which ensures a high level of security because the key is only available temporarily. The temporary key is therefore a non-persistent key, whereas the trusted key is a persistent key. In addition, the load on a hardware-based security module during secure communication can be significantly reduced and limited to an initialization phase because the security provided for the communication is based on the temporary key, so that permanent integration of the hardware-based security module for cyclic communication is not necessary. In other words, the hardware-based security module can be avoided from being involved in high-frequency communication.

One aspect provides that the receiver generates an initialization vector before receiving the encrypted initialization message and passes it on to the sender. In particular, the initialization vector is unique for the corresponding transmitter-receiver pair. In addition, the initialization vector can be used by the sender to encrypt the encrypted initialization message, since the initialization vector serves as an initial value for encrypting the encrypted initialization message. In this respect, the receiver requests the encrypted initialization message from the sender by sending the receiver the initialization vector to the sender. The initialization message is encrypted based on the initialization vector, which ensures that the encrypted initialization message is unique for the sender-receiver pair because the initialization vector is unique for the receiver and therefore also for the corresponding sender-receiver pair. The initialization vector basically also serves to ensure that the communication is up to date.

The temporary key can be based on a random number, in particular on a hardware-generated random number. In particular, the (hardware-generated) random number is used as a temporary key. This applies in an analogous manner to the initialization vector, which can also be based on a random number, in particular on a hardware-generated random number. The hardware-generated random number is also called “True Random Number” (TRN). The randomly generated random number ensures that it is a temporary key, as the random number is generated anew for each session. A session can be triggered by switching on an ignition. In this respect, every time the ignition is switched on, a new random number is generated, based on which the temporary key and/or the initialization vector is generated. The temporary key is generated by the sender and the initialization vector is generated by the receiver. The sender and the receiver each generate a random number, which, however, is different from each other because they are both random numbers.

Another aspect provides that the sender has a hardware-based security module that is set up to provide the trusted key. Specifically, a random number is encrypted based on the trusted key. Furthermore, the recipient can have a hardware-based security module that is set up to provide the trusted key. In particular, based on the trusted key, the encrypted initialization message is decrypted by the recipient in order to obtain the random number, which later serves as a temporary key and has been transmitted from the sender to the recipient, among other things, by means of the encrypted initialization message. The respective hardware-based security module is therefore only used in the initialization phase for communication in order to provide the trustworthy key that is used for the encryption of the random number or the temporary key, in particular for the encryption of the encrypted initialization message.

In addition, both the transmitter and the receiver can each have a hardware-based random generator with which the hardware-based random number is provided. The hardware-based random generator can be designed separately from the respective hardware-based security module or as part of the respective hardware-based security module.

In principle, the initialization vector, which is generated by the receiver based on the random number provided at the receiver, can be sent to the sender unencrypted.

The sender receives the initialization vector and, based on the initialization vector and the trusted key, encrypts the random number provided by the sender, which later serves as a temporary key to receive the encrypted initialization message. The encrypted initialization message is sent from the sender to the recipient, who can decrypt it because the recipient also has access to the trusted key (and the previously generated initialization vector).

The trustworthy key is therefore a persistently stored symmetrical key, which was transmitted securely to the control unit from a trustworthy location using a suitable PKI (Public Key Infrastructure). In other words, the trusted key is an authentic, persistent key that is provided to the motor vehicle using a public-key infrastructure (PKI).

The temporary key that has been exchanged between the sender and receiver is in particular a trustworthy key, in particular a trustworthy symmetrical key. The key becomes trustworthy through the nature of the exchange. In other words, trustworthiness is inherited from the persistent key.

Furthermore, the sender can generate an authentication code (“Message Authentication Code” - MAC), which is transmitted to the recipient, who checks the authentication code. In particular, the authentication identifier is sent together with the encrypted initialization message. In addition, the authentication identifier is generated together with the encryption of the encrypted initialization message, for example based on the trusted key and/or the initialization vector. The authentication identifier is a hash value. Using the authentication identifier, the recipient can ensure that the encrypted message has been correctly decrypted on the recipient side, which ensures secure and reliable communication because an attack can be ruled out. The initialization vector previously transmitted from the receiver to the sender can represent a starting point for creating the authentication label. In this respect, the authentication label has been created based on the initialization vector.

The recipient who receives and decrypts the encrypted initialization message can connect directly and at any time. A confirmation of receipt or similar is therefore not required.

Another aspect provides for secure communication between the sender and the recipient when the temporary key has been exchanged, i.e. the initialization phase is complete. Individual messages of the secured communication are secured, in particular by means of security based on a secure hash function from the ARX algorithm family, in which the temporary key is used. The individual messages can be secured with a "hash-based message authentication code" (HMAC), i.e. a "message authentication code" (MAC), which is also referred to as a message authentication code, which uses the temporary key in conjunction with a hash function. In other words, the individual messages are each secured with a hash value that is obtained from the calculation of the message authentication code. The message authentication code can be calculated based on the temporary key, which is why this is used accordingly to secure the individual messages.

Secure communication can be started by the sender as soon as the temporary key has been generated using a random number. The at least one recipient, in particular all respective recipients, can check the correctness of the message authentication codes after the initialization phase has been completed. The temporary key, in particular the random number, has therefore already been transmitted from the sender to the recipient, in particular to all recipients.

ARX-based security is basically the application of ARX hash algorithms to calculate the message authentication code, which can be done outside of the hardware-based security module. “ARX” stands for “Addition, Rotation, and XOR”, which refers to the operations used in ARX-based protection. The ARX-based protection can be “Chaskey”.

Since the temporary key can be used in ARX-based security, it is in turn ensured that the load on the hardware-based security module is reduced, since the hardware-based security module is no longer (or no longer) involved in securing the individual messages, i.e. during the cyclic phase (“ComCyclic”). This makes it possible to ensure secure communication with fast cycle times between the transmitter and the receiver, in particular cycle times less than five milliseconds.

Furthermore, the sender and the receiver can each have a processor, with the respective processor being set up to calculate a message authentication code. In this respect, the protection is carried out by means of the processor, so that the hardware-based security module is relieved accordingly.

The message authentication code (MAC) is calculated using an algorithm that requires two input parameters, namely data to be protected and a secret key, whereby the algorithm calculates a checksum from the two input parameters, namely the message authentication code.

The secret key in this case is the temporary key.

The cyclic communication is secured by calculating the MAC (Message Authentication Code) using the temporary key with hash algorithms of the ARX family (Add Rotate Xor) outside the hardware-based security module. Cyclic communication can therefore be efficiently secured in terms of computing load without using a hardware-based security module. This requires the use of a key outside the hardware-based security module and therefore outside the trusted memory area.

Furthermore, a message counter that counts the number of secured messages within a block and/or a block counter that counts the number of blocks can be provided. In particular, message counter information is included in the secured messages. The message counter ensures that communication is protected against replay attacks. Furthermore, the information from the message counter or the block counter can be used to calculate the message authentication code.

Furthermore, the object is achieved according to the invention by a communication system for a motor vehicle which comprises at least one transmitter or a receiver, the communication system being set up to carry out the method of the type described above. The aforementioned advantages therefore arise in a similar manner for the communication system.

Basically, the trusted key provided by the respective hardware-based security module is is only used indirectly to exchange the temporary key between the sender and the receiver, in particular by means of the encrypted initialization message in the initialization phase. However, during secure communication, i.e. the cyclic communication phase, access to the hardware-based security module is not necessary.

In other words, for the final secured communication, i.e. after the initialization phase, a secure hash function from the ARX algorithm family is used, in which the messages are secured based on the temporary key. As already explained, the temporary key was passed from the sender to the recipient in the initialization phase by means of the encrypted initialization message, which was encrypted based on the trusted key.

According to the invention, it is therefore possible to reduce the computing load and latency requirement on the hardware-based security module (HSM) for secure communication with fast cycle times, in particular cycle times of less than five milliseconds. The calculation of the hash value or the message authentication code, i.e. the MAC (Message Authentication Code), takes place outside the hardware-based security module (HSM) using ARX algorithms, which are optimized for calculating the message authentication codes on processors. The trusted key, which is stored in the hardware-based security module (HSM), is used indirectly to transmit the temporary secret key, so that it is a temporary secret shared key. The random number can be used as a temporary key.

The encrypted initialization message that is transmitted to the recipient can therefore contain the temporary key as well as information from the message counter and the block counter, in particular the respective counter readings.

The recipient can therefore decrypt the encrypted initialization message with the trusted key in order to obtain the temporary key and the counter information, i.e. the respective counter readings.

As already described, the trusted key is a shared key. Consequently, this can also be referred to as a trustworthy, shared key.

Secure communication is started when the temporary key has been exchanged and information from the message and block counters has been synchronized, i.e. their counter readings. The synchronization can be carried out by checking with the next message received whether the message counter, in particular its count, is less than that in the initialization message. If this is the case, a new block has begun and the block counter must be incremented by one. This created a synchronization window for the block counter the same size as the message counter.

• - 1 eine schematische Darstellung eines erfindungsgemäßen Kommunikationssystems für ein Kraftfahrzeug,
• - 2 eine Übersicht eines spezifischen Ausführungsbeispiels des erfindungsgemäßen Kommunikationssystems im Vergleich mit aus dem Stand der Technik bekannten Kommunikationssystemen, und
• - 3 eine Übersicht, die das erfindungsgemäße Verfahren zur sicheren Kommunikation verdeutlicht, welches mit dem Kommunikationssystem gemäß 1 oder 2 ausgeführt werden kann.

Further advantages and features of the invention will become apparent from the following description and the drawings to which reference is made. In the drawings:

• - 1 a schematic representation of a communication system according to the invention for a motor vehicle,
• - 2 an overview of a specific embodiment of the communication system according to the invention in comparison with communication systems known from the prior art, and
• - 3 an overview that illustrates the method according to the invention for secure communication, which can be used with the communication system according to 1 or 2 can be executed.

In 1 a communication system 10 for a motor vehicle is shown, which includes a transmitter 12 and a receiver 14, between which a communication channel 16 is formed.

The communication channel 16 is designed as a bidirectional communication channel, since messages can be sent from the transmitter 12 to the receiver 14, but the receiver 14 can also transmit messages to the transmitter 12 via the communication channel 16.

In the embodiment shown, the transmitter 12 has a hardware-based security module 18, which is set up to provide a trustworthy key that is used to establish communication between the transmitter 12 and the receiver 14, as will be explained below.

The receiver 14 also has a hardware-based security module 20, which is also configured to provide the trusted key to decrypt a message encrypted by the sender 12. In this respect, both the sender 12 and the receiver ger 14 each has a hardware-based security module 18, 20.

In addition, the transmitter 12 has a processor 22, which is also in communication connection with the hardware-based security module 18. This is a bidirectional connection in which work orders are transferred by the processor 22 to the hardware-based security module 18 and answered by it. In particular, the work orders involve cyber security calculations such as encryption and decryption or MAC calculations. This ensures that only the hardware-based security module 18 can access the trusted key and that there are no read accesses on the part of the processor 22 in relation to the trusted key.

The receiver 14 also has a processor 24, which is in communication connection with the hardware-based security module 20 of the receiver 14, in particular without direct access to the trustworthy key.

Furthermore, both the transmitter 12 and the receiver 14 each include a random number generator 26, 28, via which a random number can be generated.

The respective random number is generated for a session by the corresponding random number generator 26, 28. The session can be triggered by the ignition of the motor vehicle, so that the respective random number generator 26, 28 generates a new random number when the engine of the motor vehicle is restarted. The random number generators 26, 28 can in particular be integrated in the respective hardware-based security module 18, 20.

Using the random number generator 26 of the transmitter 12, a temporary key is generated, as described below in relation to 2 and 3 is explained in more detail, which is used for secure communication between the transmitter 12 and the receiver 14 after an initialization phase.

In addition, the transmitter 12 also has a message counter 30 and a block counter 32, which are provided, for example, by the processor 22, as described below with reference to 2 and 3 is explained. The receiver 14 also has a message counter and a block counter, although this is not shown here.

In 2 the communication system 10 is shown using a specific exemplary embodiment in which the transmitter 12 is designed as part of an integrated platform (IP). The receiver 14, on the other hand, is designed as an actuator.

Likewise are in 2a and 2 B two communication systems known from the prior art are shown, on the basis of which the in 2c illustrated invention becomes clear. In 2a a communication system known from the prior art is shown, in which every message is secured using the hardware-based security module (“HSM”), whereas in 2 B a communication system known from the prior art is shown, in which the individual messages are not secured, but several messages are secured in blocks.

As already mentioned in relation to 1 As explained, both the transmitter 12 and the receiver 14 each have at least one processor 22, 24 and a hardware-based security module (“HSM”) 18, 20.

In addition, the communication channel 16 is formed between the transmitter 12 and the receiver 14 so that the transmitter 12 can transmit messages to the receiver 14. As in 2 is also shown, however, it is also possible that the receiver 14 transmits at least information to the transmitter 12 via the communication channel 16.

The relevant news is in 2 referred to as the “controller function”.

In addition, in 2 the respective loads of the hardware-based security modules 18, 20 and the processors 22, 24 are shown, in particular for the systems known from the prior art.

It is clear from this that, according to the invention, the loads have been shifted from the hardware-based security module 18, 20 towards the respective processor 22, 24, since at least the calculation for the secured communication, in particular the calculation of a message authentication code ("Message authentication code" - MAC), is carried out by the respective processor 22, 24 and no longer by the hardware-based security modules 18, 20.

According to the invention, the hardware-based security modules 18, 20 are only supervised with a corresponding task at the beginning, namely during an initialization phase, as described below with reference to 3 is explained, whereby the load for the respective hardware-based security module 18, 20 is significantly compared to the method known from the prior art is reduced. Furthermore, the trustworthy key serves to ensure authenticity between the communication partners without it having to be known outside the hardware-based security module.

Nevertheless, it is ensured how a comparison of the 2a and 2c shows that each of the messages is individually secured, although the load for the hardware-based security modules 18, 20 according to the invention is even lower than the load for block-by-block protection, as a comparison of 2 B and 2c clarified.

In 3 shows how secure communication between the transmitter 12 and the receiver 14 is set up and carried out.

In a starting phase “ComStart”, a random number is generated by the random number generator 26 of the transmitter 12.

The random number generator 26 may be provided in the hardware-based security module ("HSM") 18 of the transmitter 12, which is indicated by the expression "HSM_TrueRandomNumberGen()". The random number TRNX may comprise 128 bits.

In an initialization phase “ComJoin”, the receiver 14 first generates an initialization vector (“IV”), which can comprise 128 bits. The initialization vector can be generated based on the random number provided by the random number generator 28 of the receiver 14, which is indicated by the expression “IV[128] = HSM_TrueRandomNumberGen()”. 3 is specified. The random number generator 28 of the receiver 14 can also be provided in the hardware-based security module (“HSM”) 20 of the receiver 14, as is clear from the expression above.

The receiver 14 transmits the initialization vector via the communication channel 16 to the transmitter 12 (“ArxComJoin”), which receives the initialization vector accordingly.

During the initialization phase, the sender 12 generates an encrypted initialization message (“CiphTxt”) that contains the temporary key, which is based on the random number (“TRNX”) that the random number generator 26 of the sender 12 generated. In particular, the temporary key corresponds to the random number that the random number generator 26 of the transmitter 12 generated in the initial “ComStart” phase.

The encrypted initialization message (“CiphTxt”) is generated or encrypted using the trustworthy key in that the processor 22 of the sender 12 has received an encryption job from the hardware-based security module (“HSM”) 18.

Basically, at least the temporary key, i.e. the random number generated by the random number generator 26 of the sender 12, is encrypted based on the initialization vector received by the receiver 14 and the trusted key. Furthermore, the message counter and the block counter are provided confidentially in the encrypted initialization message ("CiphTxt") in order to enable synchronized entry into an ongoing communication.

In addition, the sender 12 generates an authentication tag (“AuthTag”), which is transmitted to the receiver 14 together with the encrypted initialization message (“CiphTxt”).

The authentication tag (“AuthTag”) is generated based on the trusted key.

In this respect, the encrypted initialization message (“CiphTxt”) and the authentication tag (“AuthTag”) are transmitted together to the recipient 14 (“ArxComSecret”).

The receiver 14 receives the encrypted initialization message together with the authentication identifier, whereby the receiver 14 checks the authentication identifier, i.e. compares it using the trustworthy key.

In addition, the receiver 14 decrypts the encrypted initialization message with the trusted key (and based on the previously generated initialization vector), which is provided via the hardware-based security module 20 of the receiver 14.

The receiver 14, which has the trustworthy key, can thus decrypt the temporary key as well as the message counter and block counter from the encrypted initialization message, so that the receiver 14 has the random number generated by the sender 12 and the current counter contents.

Using the authentication tag (“AuthTag”), the recipient 14 can also check whether the decryption of the encrypted initialization message was successful.

In this respect, the recipient 14 is able to decrypt the temporary key and use it later. Through the values of news counter and block counter, the receiver 14 is also able to synchronize them with the transmitter 12.

The initialization phase “ComJoin” is completed if the receiver 14 was able to successfully decrypt the temporary key as well as the information of the message counter and block counter from the encrypted initialization message, so that the receiver 14 connects and establishes secure communication, in particular cyclic communication, with the sender 12 can be done (“ComCyclic”).

In the initialization phase, the hardware-based security modules (“HSM”) 18, 20 were therefore only used once, as well 2c emerges.

In secured communication, the individual messages are secured using ARX-based protection based on hash algorithms from the ARX family, in which the temporary key is used.

The ARX-based algorithm can be, for example, Chaskey.

Specifically, ARX-based security is used to generate message authentication codes (“MAC”) based on the previously exchanged temporary key, where the message authentication codes are used to secure individual messages. The message authentication codes are, in particular, hash values.

The individual messages are thus secured by means of the message authentication code (“Message Authentication Code” - MAC), which is calculated by the corresponding processor 22, 24 of the sender 12 or the receiver 14 based on the exchanged temporary key, as well as from 2c emerges.

The message authentication code is calculated using an algorithm that requires two input parameters, namely the data to be protected, i.e. the content of the message to be transmitted including information (counter values) from the message counter and block counter as freshness, as well as a secret key, which corresponds to the temporary key. The algorithm calculates a checksum (“hash value”) from the two parameters, namely the message authentication code (“MAC” or “HMAC”), which is attached to the respective message in order to secure it.

Furthermore, in 3 shown that the individual secured messages contain counter information (“MsgCnt” or message counter). The message counter 30 is used here, which counts the messages, for example from 0 to 255, to ensure that several messages with the same content still differ from one another.

The message counter 30 then overflows, so that a new block is started, which in turn is counted by the block counter 32 to determine the freshness of the communication.

The block counter 32 is initialized with 0 for each new session, so that it starts anew for each temporary key. Furthermore, the block counter 32 is incremented by 1, which means that with an increment every 256 ms and a block length of 32 bits, for example, the block counter 32 only overflows after 35 years for communication with a cycle time of 1 ms.

Obviously, secure communication between the transmitter 12 and the receiver 14 is guaranteed.

Claims (10)

Method for secure communication between a transmitter (12) and a receiver (14) in a motor vehicle, wherein the transmitter (12) transmits an initialization message encrypted with a trustworthy key to the receiver (14) in an initialization phase, which contains a temporary key, wherein the recipient (14) decrypts the encrypted initialization message with the trusted key to obtain the temporary key. Procedure according to Claim 1 , characterized in that the receiver (14) generates an initialization vector before receiving the encrypted initialization message and passes it on to the transmitter (12). Procedure according to Claim 1 or 2 , characterized in that the temporary key is based on a random number, in particular on a hardware-generated random number. Method according to one of the preceding claims, characterized in that the transmitter (12) has a hardware-based security module (18) which is set up to provide the trusted key, in particular based on the A random number is encrypted using a trusted key. Method according to one of the preceding claims, characterized in that the transmitter (12) generates an authentication identifier which is transmitted to the receiver (14), which checks the authentication identifier, in particular wherein the authentication identifier is sent together with the encrypted initialization message. Method according to one of the preceding claims, characterized in that a secured communication between the transmitter (12) and the receiver (14) is started when the temporary key has been exchanged, with individual messages of the secured communication, in particular by means of a security based on a secure hash function from the ARX algorithm family, in which the temporary key is used. Method according to one of the preceding claims, characterized in that the transmitter (12) and the receiver (14) each have a processor (22, 24), the respective processor (22, 24) being set up to calculate a message authentication code. Method according to one of the preceding claims, characterized in that a message counter (30) which counts the number of secured messages within a block, in particular wherein information from the message counter (30) is contained in the secured messages and/or in the calculation of the message authentication code enter. Method according to one of the preceding claims, characterized in that a block counter (32) is provided which counts the number of blocks, in particular information from the block counter (32) being included in the calculation of the message authentication code. Communication system (10) for a motor vehicle, comprising at least one transmitter (12) and one receiver (14), wherein the communication system (10) is configured to carry out the method according to one of the preceding claims.

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