The Internet of Things (IoT) has overturned the information technology world. This new phenomenon is becoming inescapable and already covers almost all fields, from watchmaking to automated factories. IoT simplifies our everyday life and creates value for people and businesses. Things, also called entities, are very heterogeneous, use different communication technologies and, generally, are limited capacity devices. Therefore securing such systems raises many challenges. Communicating entities should authenticate each other and protect the integrity and the confidentiality of the data they exchange while using lightweight, fast and energy-efficient algorithms. In this paper, we propose a robust security protocol, designed especially for constrained IoT devices. We carried out a real implementation and the obtained results prove the efficiency of our protocol.