IN-CAR NETWORK SYSTEM, ELECTRONIC CONTROL UNIT AND UPDATE PROCESSING METHOD

An update processing method for updating data used to generate a MAC in an in-vehicle network system including a plurality of electronic control units that communicate a data frame having a MAC added thereto with one another via a bus in accordance with the CAN protocol is provided. The method includes the detection step of detecting a state of a vehicle having the in-vehicle network system mounted therein and the update step of updating a MAC key used to generate a MAC under the condition that the state of the vehicle detected in the detection step is a predetermined state.