In recent years, vehicles such as automobiles have been equipped with many vehicle control devices, i.e., vehicle control devices that electronically control various pieces of in-vehicle equipment, such as an engine and a brake, and vehicle control devices that control pieces of equipment, such as meters, that display various vehicle states, including a vehicle control device that controls a navigation system. Additionally, in the vehicle, a vehicle network is formed by electrically connecting those vehicle control devices together by means of communication lines, and various pieces of data are transmitted and received between the vehicle control devices through this vehicle network. On the other hand, in the thus formed vehicle network, each vehicle control device connected to the vehicle network serves to control various pieces of in-vehicle equipment, such as an engine and a brake, that are mounted in the vehicle, and hence is required to have significantly-high security.
Therefore, recently, the development of an authentication system to verify the correctness of communication targets or of communication data between the vehicle control devices connected to the vehicle network has been advanced. In this kind of system, communication with a communication target is started on condition that authentication has been established.
Additionally, for example, a data distribution system described in Patent Document 1 has been known as a technique that increases the security of such a specific network. This data distribution system communicates dummy data that is a dummy representation of normal data between a transmitting terminal and a receiving terminal, besides the normal data. The receiving terminal determines whether received data is dummy data or normal data. As a result, it is difficult to identify normal data even if data transmitted to the network is illegally obtained.