  The automotive market is profitable for cyberattacks with the constant shift
toward interconnected vehicles. Electronic Control Units (ECUs) installed on
cars often operate in a critical and hostile environment. Hence, both carmakers
and governments have supported initiatives to mitigate risks and threats
belonging to the automotive domain. The Local Interconnect Network (LIN) is one
of the most used communication protocols in the automotive field. Today's LIN
buses have just a few light security mechanisms to assure integrity through
Message Authentication Codes (MAC). However, several limitations with strong
constraints make applying those techniques to LIN networks challenging, leaving
several vehicles still unprotected. This paper presents LIN Multiplexed MAC
(LINMM), a new approach for exploiting signal modulation to multiplex MAC data
with standard LIN communication. LINMM allows for transmitting MAC payloads,
maintaining fullback compatibility with all versions of the standard LIN
protocol.
