We propose in this paper a security protocol based on periodic change of pseudonyms. The idea is to avoid illegal traceability of vehicles during their communications and preserve their privacy and confidential information. Two different approaches are proposed. In the first approach, each vehicle asks the central authority a new communication pseudonym after a time t. While in the second approach, each vehicle generates itself after a time t, a new communication pseudonym. Our objective is to permit at least two vehicles to change their pseudonym in the same time interval. We evaluate in this work, the bandwidth used by considering the vehicles speed in each approach. The proposed protocol is based on equidistant distribution of the road side unit and uses the average of speed permitted on the road to evaluate lifetime t of the communication's pseudonyms and certificates. The exchange of information is based on asymmetric and symmetric cryptography scheme and it uses hash function. Our protocol provides authentication, non-repudiation and privacy.

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