The Internet of Things enables the interconnection of smart physical and virtual objects, managed by highly developed technologies. WSN, is an essential part of this paradigm. The WSN uses smart, autonomous and usually limited capacity devices in order to sense and monitor industrial environments. However, if no authentication mechanism is deployed, this system can be accessible, used and controlled by non-authorized users. In this paper, we propose a robust WSN mutual authentication protocol. A real implementation of the protocol was realized on OCARI, one of the most interesting Wireless Sensor Network technologies. All nodes wanting to access the network should be authenticated at the MAC sub-layer of OCARI. This protocol is especially designed to be implemented on devices with low storage and computing capacities.