Abstract-The IoT is a technology that enables the interconnection of smart physical and virtual objects and provides advanced services. Objects or things are generally constrained devices which are limited by their energy, computing and storage capacity. A Wireless Sensor Networks (WSN) is a network composed of devices managed by a CPAN (Personal Area Network Coordinator). The network is used in order to gather and process data of a given environment. It is characterized by their low bit rate and low power consumption, and it uses small size packet in their transmissions. In order to protect the WSN, a mutual authentication between devices is required during the association of a new device. The exchanged data should be authenticated and encrypted. In this work we propose a robust, lightweight and energy-efficient security protocol for the WSN systems. The real tests we made and a performance evaluation of our security protocol are provided.