The use of Internet of Things (IoT) networks is increasing daily both in terms of the number of active IoT devices and the number of sectors and applications using IoT technologies. This widespread deployment increases the importance of protecting these networks and secure their data from theft or manipulation. The IoT routing protocol (RPL) is the primary protocol used in resource constrained IoT networks. However, this protocol contains a number of security vulnerabilities that practically restrict its use in many sensitive applications. This research proposes a lightweight secure operating mode for the RPL protocol called Lightweight Security Mode RPL (LSM-RPL) that use Hash-Based Message Authentication Code (HMAC) for source authentication and data integrity. The proposed mode employs two types of pre-configured secret keys: The first is secret but shared by all network members to protect broadcast messages and prevent external intrusion. The second type of key is private key that shared only between the node and the root. It used to protect unicast messages and prevent internal intrusion. Simulation results demonstrated the proposed security layer strong ability to overcome the negative impact of several common RPL attacks. These results were compared with several state-of-the-art studies in this field. For example, under the Decrease Rank Attack scenario with two malicious nodes, LSM-RPL demonstrated significant improvements over RPL, achieving up to 10.1% higher Packet Delivery Rate (PDR), and reducing energy consumption and total packets received by 45.9% and 26.4%, respectively.
