The IoT is a technology that enables the inter-connection 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.