Nowadays, there are large amount of data generated by the IoT objects. For that, big data and Cloud and Fog Computing technologies have become increasingly important. Unfortunately, these environments still present many difficulties. And according to the statistics, we note that the major percentages of attacks come mainly from end users. Hence, we need an access control system. So, we focus our work on user access control. In fact, our contribution in this paper is mainly focused on securing user access to Fog Computing networks. For this, we propose, an efficient and complete protocol for access control service, which based on the trust assessment and the monitoring of the user's activities. Afterwards, we aim to improve our protocol by integrating the concept of certificates. We have shown through analysis studies and simulation results that the proposed scheme provides both security requirements in terms of confidentiality, integrity, availability, and anonymity; and network performance in terms of access response latency.