The Controller-Area Network (CAN) bus protocol is a bus protocol invented in 1986, originally intended for automotive use. By now, the bus can be found in devices ranging from cars and trucks, over lightning setups to industrial looms. Due to its nature, it is a system very much focused on safety, i.e., reliability. Unfortunately, there is no build-in way to enforce security, such as encryption or authentication. In this paper, the authors investigate the problems associated with implementing a backward compatible message authentication protocol on the CAN bus.