An Mobile Ad-hoc network (MANET) is formed when group of mobile wireless nodes collaborate between them to communicate through wireless links in the absence of the fixed infrastructure and any centralized control. Theses characteristics make it able to adapt and operate in difficult conditions, but also vulnerable to new security attacks not present in a traditional wired network. In this paper a new approach to secure MANETs has been proposed. Our solution is based on our efficient trust model and distributed algorithm to clustering network in order to distribute role of certification authority (CA) in each cluster. We use fully self-organized security and monitoring process to supervise behaviors of nodes with low trust level. Also, we propose clustering algorithm based on the trust and mobility metric to select CA and to establish public key infrastructure (PKI) in each cluster. Furthermore, we introduce new concept Dynamic Demilitarized Zone (DDMZ) to protect CAs and avoid the single point of failure in each cluster. The DDMZ is formed by set of the dispensable nodes which must be confident and located at one-hop from the CA. Our approach can be easily extended to other hierarchical routing protocols. The simulation results include an evaluation of the stability, availability and security.