Public key infrastructure (PKI) has been recognized as the most efficient and effective tool providing key management in classical networks. However providing such infrastructure for ad hoc network is a challenging task, due to the dynamic topology and infrastructure-less nature of these networks. In this paper we propose a new scheme for the implementation of PKI infrastructure in ad hoc network based on clustering technique in order to simplify the key management. Our solution combines two techniques to distribute the PKI functionalities between the network nodes. The first one is used to affect the full certificate authority (CA) functionalities to specified nodes in the network named cluster-heads, and the second to manage certificate distribution over network nodes. We also propose to use a mechanism of multi-signature intended to give more trust to certificate signed by more than one cluster-head, used after for cluster-head election in order to elect the most trusted node as cluster-head.