The pervasive communications between vehicles and dynamic mobility may significantly increase data exchange and therefore bring a huge amount of traffic data in InVANETs. Due to some environmental factors, like the vulnerability of wireless connection, limitation of in-car computing ability, and speed of vehicles, it is extremely challenging to design identity authentication protocols satisfying the requirements of both high security and efficiency simultaneously. To this end, the aggregate signature technology has been employed in InVANETs. However, the technology still suffers from high computational overhead due to the management of certificates, as well as the key escrow problem (i.e., the dependence on a fully trusted third party). In this article, we propose the SE-CLASA protocol for InVANETs in order to tackle the aforementioned problems. In addition, a novel factor-contained aggregation mechanism is proposed to resist an information injection attack investigated in our analysis. Moreover, we prove the security of the proposed SE-CLASA and conclude that it meets most known security requirements in a general InVANET scenario. Simulation results show the superiority of the proposed SE-CLASA, in terms of security and efficiency, compared to the most recent authentications in InVANETs.