Vehicular Ad hoc NETworks (VANETs) are a burgeoning research focus, aimed at creating communication among vehicles to improve the road safety and enhance driving conditions. For such networks, security is one of the most challenging issues due to their nature of wireless transmission and high topology changing frequency. In this paper, we propose a secure and distributed certification system architecture for safety message authentication in VANET, which resists against false public-key certification. To increase the availability of the authentication service, our proposal is designed through a decentralized system, supervised by a root authority. The latter authority delegates to a set of regional certification authorities the privilege of issuing public-key certificates to the vehicles. Each regional certification authority cooperates with its subordinates RSUs to sign public-key certificates using threshold signature. The main purpose of our solution is to ensure the messages authentication while respecting the imposed constraints by the real-time aspect and the nodes mobility. We demonstrate through the practical analysis and simulation results the efficiency of our solution with comparison to other concurrent protocols.