While cloud computing and Internet of Things (IoT) technologies have gradually matured, mobile intelligent transportation systems have begun to be widely used. In particular, the application of vehicular ad hoc networks (VANETs) is very convenient for real-time collection and analysis of traffic data. VANETs provide a great convenience for drivers and passengers, making it easier to choose routes. Currently, most research on VANETs obtains data through cloud servers. However, there are few studies on cloud servers obtaining vehicle information through the roadside unit (RSU). In the process of reading traffic information, there will be some private and sensitive information, which may be intercepted or tampered with in untrusted public channels. Therefore, it is necessary to propose a protocol to protect vehicle data during the information reading phase. In this paper, we propose a new provably secure authentication protocol to negotiate a session key before transmitting traffic information. This protocol can complete mutual authentication and generate a session key. Finally, security analysis and performance analysis show that our protocol is secure and efficient.