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Encrypted signal transmission with AUTOSAR in CAN-FD

Posted: 17 Aug 2015     Print Version  Bookmark and Share

Keywords:vehicle networks  data transmission  bus system  AUTOSAR-3.x  ECU 

In today's vehicle networks, data transmission is for the most part implemented without any special security measures. That is, in accessing a vehicle's bus system, it is possible to read the data transmitted in raw format or to even play it into the bus system in modified form. Encrypted data transmission would not only ensure that this information can only be evaluated by authorised recipients, it would also make it much more difficult to intercept or alter the messages.

Media reporting about vehicle manipulation [1, 2] raises the question of whether data in the vehicle network can actually be influenced by manipulation. Can a manipulated device or internally implanted device with a remote control function influence vehicle behaviour? And what countermeasures can be taken to prevent such manipulation?

Today's vehicles are highly complex systems, which consist of networked sensors and actuators and continually transmit important data over bus systems. In the vast majority of cases, the information being transmitted is in raw data format. A plausibility check, if such a check is even possible, has limited effectiveness. The receiver is unable to verify whether the data was actually supplied by the desired sender or whether it was fed in by an outside electronic control unit, i.e. whether it is authentic data. The data is freely accessible as well, so an analysis of the bus information can be used to determine signal contents. The transmission is neither confidential nor authenticated.

This was the problem that engineers at Vector were confronted with. Their task was to come up with an implementation for secure communication over a CAN network which could be used flexibly and could also be integrated with AUTOSAR-3.x basic software. Key protection goals, along with authentication, included preventing replay attacks. It would be desirable to implement communication that could not be accessed externally.

For the encryption method, the specialists chose the AES algorithm [3]. From today's perspective this method is considered cryptographically secure. It involves symmetrical block encryption with a block length of 128 bits. It generates 16B or a multiple of 16, which the sender transmits to the receiver. It is advantageous that some microcontrollers already have very fast hardware implementations of this algorithm.

Since a CAN message can transmit a maximum of 8 data bytes per frame, a decision was made to utilise the ISO transport protocol (TP) that was already included in the communication stack for the transfer. This required simplifying the CAN configuration and protocol for unidirectional communication with a fixed 1:1 relationship between sender and receiver.

Symmetrical encryption requires that both the sender and receiver have the same key. The software modules that are used permit dynamic allocation of the keys at runtime, so that the user or OEM can freely choose them.

A higher-level method such as an (asymmetrical) key exchange method might be implemented, or a static allocation might be made, such as in end-of-line programming. When a vehicle-specific key is used, whenever an ECU is replaced, the automotive service shop must train the new ECU by an authorisation method, because the key must be kept confidential under all circumstances.

Preventing replay attacks
In this configuration, encrypted transmission of messages is now possible, where the information is, however, still purely static, i.e. a unique key text can be assigned to the plain text signals. This means that replay attacks, i.e. recording excerpts of a desired communication and replaying it into the system at a later time, can still be made. That is because the receiver cannot check whether the message actually originates from the sender at this time point.

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