Abstract: An in-vehicle device includes: a reception unit configured to receive, via control area network communication (CAN communication), a message including data and an authentication code for authenticating the data; an authentication unit configured to authenticate the data by using the authentication code; and a control unit configured to perform control by using the data included in the message when the authentication unit has authenticated data included in a preset number of messages consecutively received by the reception unit. A method includes: receiving, via control area network communication (CAN communication), a message including data and an authentication code for authenticating the data; authenticating the data by using the authentication code; and when data included in two consecutively received messages is authenticated in the authenticating the data, performing control by using data included in a last received message among the two messages.

Abstract: A system includes a plurality of in-vehicle devices and having a function of determining abnormality of communication in the devices by authenticating a message including data, a first counter counted each time a power source is turned on, and a second counter incremented over time. The system includes an authentication unit for performing, when a message including control data to be exchanged between the devices is authenticated, authentication on a message sent and received between the devices and including at least the control data and the first counter by using at least the control data and the first counter, in which when it is confirmed whether synchronization is achieved between the devices, the authentication unit is for performing authentication on a message sent and received between the devices and not including the first counter but including the second counter by using the second counter without using the first counter.