Cooperative Intelligent Transportation Systems are rapidly gaining momentum in the scenario of modern wireless communications. Within these environments, messages are exchanged continuously. The latter should be secure and ensure users’ privacy. Public Key Infrastructures (PKIs) represent the major solution to meet security needs. On the other hand, communications between the PKI and the vehicle stations or the Road Side Unit stations should also be secure. Main current security standards do not address and define a complete detailed secure end-to-end mechanism to send requests to the PKI and receive the associated responses. In this paper, we propose a detailed security protocol, based on ETSI security standards, that ensures confidentiality, integrity and authentication. The evaluation of the latter shows its ability in meeting security needs.