  The automotive market is increasingly profitable for cyberattacks with the
constant shift toward fully interconnected vehicles. Electronic Control Units
(ECUs) installed on cars often operate in a critical and hostile environment.
Hence, both carmakers and governments have decided to support a series of
initiatives to mitigate risks and threats belonging to the automotive domain.
The Controller Area Network (CAN) is the primary communication protocol in the
automotive field, and the integrity of the communication over this network is
assured through Message Authentication Codes (MAC). However, limitations in
throughput and frame size limit the application of this technique to specific
versions of the CAN protocol, leaving several vehicles still unprotected. This
paper presents CAN Multiplexed MAC (CAN-MM), a new approach exploiting
frequency modulation to multiplex MAC data with standard CAN communication.
CAN-MM allows transmitting MAC payloads maintaining full-back compatibility
with all versions of the standard CAN protocol. Moreover, multiplexing allows
sending DATA and MAC simultaneously.
