Abstract: An electronic control unit, which receives a message and a freshness value given to the message via a connectionless communication, includes a storage unit storing a freshness value list, which indicates a predetermined number of freshness values in a descending order from a reference value, which is a largest freshness value among the freshness values received in a past. The electronic control unit compares a received value, which is the received freshness value, with the freshness value list to provide a verification result, and updates the freshness value list so as to hold the received value in response to the verification result indicating that (i) the received value is not larger than the reference value and not smaller than a permissible value which is a smallest freshness value in the freshness value list, and (ii) the received value is not in the freshness value list.

Abstract: An electronic control unit, which receives a message and a freshness value given to the message via a connectionless communication, includes a storage unit storing a freshness value list, which indicates a predetermined number of freshness values in a descending order from a reference value, which is a largest freshness value among the freshness values received in a past. The electronic control unit compares a received value, which is the received freshness value, with the freshness value list to provide a verification result, and updates the freshness value list so as to hold the received value in response to the verification result indicating that (i) the received value is not larger than the reference value and not smaller than a permissible value which is a smallest freshness value in the freshness value list, and (ii) the received value is not in the freshness value list.

Abstract: A communication system includes a transmission-side electronic control unit and a reception-side electronic control unit. In the transmission-side electronic control unit, one key is selected from a plurality of keys in a memory based on synchronization information which is information synchronized in communication between the transmission-side electronic control unit and the reception-side electronic control unit. Further, a message is generated and a message authentication code (MAC) is generated using the selected key. The message to which the message authentication code is added is transmitted to the reception-side electronic control unit.

Abstract: In a communication system, a management node includes: a high-order count value holding unit holding a high-order count value; and a high-order count value distribution unit updating the high-order count value under a high-order update condition and distributing the updated high-order count value to normal nodes. In the communication system, a normal node includes: a count value holding unit holding a count value; a low-order update unit updating a low-order count value held in the count value holding unit under a low-order update condition; and a high-order update unit updating a high-order count value held in the count value holding unit to the high-order count value distributed from the management node together with a reset of the low-order count value.

Abstract: In a communication system, a management node includes: a high-order count value holding unit holding a high-order count value; and a high-order count value distribution unit updating the high-order count value under a high-order update condition and distributing the updated high-order count value to normal nodes. In the communication system, a normal node includes: a count value holding unit holding a count value; a low-order update unit updating a low-order count value held in the count value holding unit under a low-order update condition; and a high-order update unit updating a high-order count value held in the count value holding unit to the high-order count value distributed from the management node together with a reset of the low-order count value.