Abstract: An onboard communication network of a vehicle is monitored to detect a request message that includes a first cipher based message authentication code (CMAC) and a request. A challenge message is determined based on the request message. The challenge message includes a counter and a random number output from a random number generator. A second CMAC is generated based on the challenge message and the request. The request message is authenticated based on determining that the second CMAC matches the first CMAC. The vehicle is operated based on the authenticated request message.