## 2 Related Works and Limitations  
In VANETs, the source authentication and message integrity of traffic-related information form a very important security requirement in the system. Satisfaction of these security requirements ensure the trust and proper functionality of all versatile technologies that comes with a VANETs system by simply securing moving vehicles, RSUs, Application Servers, and roadside sensors. To this effect many research works have been done to provide the needed security for such an advent technology of smart city [24].  
The key management problem posed by the certificate based PKI cryptosystem paved the way to the pioneering work of a certificate-less public key signature (CL-PKS) scheme by [PERSON] and [PERSON] [31]. This idea caught much research interest in the aspect of improving the security and performance. In [32], [PERSON] and [PERSON] presented a general procedure to construct a CL-PKS scheme from any ID-based signature scheme. The first CL-PKS scheme was bilinear pairing based proposed by [PERSON] et al. in [33]. Whereas in [34], [PERSON] et al. presented a new security model for CL-PKS schemes which considers inside attack scenario. The first bilinear pairing free CL-PKS scheme was first proposed by [PERSON] et al. in [35], which was found to be vulnerable to other attacks in [36]. In [37] a scheme ideal for IoT deployment was proposed; however, it was found to bear some flaws concerning inside attack performance by KGC in [38]. In order to provide the needed security property of anonymous authentication in [39; 40] the idea of pseudonym-based authentication was employed. Despite providing privacy preservation, the limitation of overburdened TA in storing these pseudonyms for each vehicle was encountered as has shown out as the shortfall for their approach. In [41], having foreseen the problem of overburdened TA and sought to provide a solution they designed a scheme by using anonymous certificates but this was done at the expense of interactions between the infrastructures. In [42] et al., privacy protection for VANETs communications was achieved based on the technique of ID-based ring signature, but they failed to provide conditional privacy, since there was no any tracking mechanism in their algorithm [43]. Many more researchers demonstrated the need to formulate robust schemes in terms of security and privacy protection. To this cause, [PERSON] et al. [44] proposed an anonymous authentication scheme, however it is deemed computationally intensive in [45]. In [46], [PERSON] et al. proposed a scheme that utilizes the methods of a cuckoo filter and binary search to facilitate batch verification for vehicular communication of V2V and V2I. He at al. [17] designed an ECC based certificate-less based signature scheme for VANETs system with batch verification feature. However, [PERSON] et al. [31] states that their scheme still vulnerable to side-channel attack since some of sensitive information like TA's master private key is stored in a tamper-proof devices (TPD). A scheme in [47] uses pseudonyms instead of real identities in trying to secure VANETs communications. The scheme in [47] achieves efficiency and provides batch verification but falls short in terms of providing all security requirements like unlinkability.