2FLIP: A Two-Factor Lightweight Privacy Preserving Authentication Scheme for VANET Authentication in vehicular ad hoc network (VANET) requires not only secure and efficient authentication with privacy preservation but also applicable flexibility to handle complicated transportation circumstances. In this paper, we proposed a TWO-Factor LIghtweight Privacy preserving authentication scheme (2FLIP) to enhance the security of VANET communication. 2FLIP employs the decentralized certificate authority (CA) and the biological password based two factor authentication (2FA) to achieve the goals. Based on decentralized CA, 2FLIP only requires several extreme lightweight hashing process and a fast MAC operation for message signing and verification between vehicles. Compared with previous schemes, 2FLIP significantly reduces computation cost by 100~1000 times and decreases communication overhead by 55.24 % ~ 77.52%. Furthermore, any certificate revocation list (CRL) related overhead on vehicles is avoided. 2FLIP makes the scheme resilient to Denial of Service (DoS) attack in both computation and memory, either caused by deliberate invading behaviors or jammed traffic scenes. The proposed scheme provides strong privacy preservation that the adversaries can never succeed in tracing any vehicles even with all RSUs compromised. What’s more, it achieves strong non-repudiation that any biological anonym driver could be conditionally traced even he is not the only driver of the vehicle. Extensive simulations reveal that 2FLIP is feasible and has an outstanding performance of nearly 0 ms network delay and 0% packet loss ratio, which are especially appropriate for real-time emergency reporting applications.