Proposal of a Secure Modbus RTU Communication with Adi Shamir's Secret Sharing Method

Drinking fresh water, turning the lights on, travelling by tram, calling our family, or getting a medical treatment are usual activities, but the underlying SCADA (Supervisory Control and Data Acquisition) systems like CIS (Critical Infrastructure Systems), ICS (Industrial Control Systems) or DCS (Distributed Control Systems) were always the target of many types of attacks, endangered the above mentioned simple activities. During the last decades because of the fast spread of the internet based services and the continuous technical development these systems become more vulnerable than ever. Full reconstruction and innovative changes in older SCADA systems has high cost, and it is not always rewarding. Communication protocols as Modbus (1979) serve as a main basis for SCADA systems, so security of Modbus has a major impact of the security of SCADA systems. Our paper raises and answers questions about the security of the Modbus RTU protocol. We focus on the serial Modbus protocol, because in that method we found many unsolved problems, like lack of authentication of the participants, lack of secure channel and so on. The aim of this paper to propose a secure communication alternative for Modbus RTU @ RS485 wire. The main advantage of the proposed method is the coexistence with traditional slaves and bus systems and only software update is necessary.