1.1. Survey of a Few Existing Works

Various levels of privacy and security solutions have been proposed for VANET, and researchers have conducted extensive surveys of these solutions. Table 1 consists of a few surveys that have been done recently in VANET, which are discussed briefly in the following section.

In [4], the investigation of VANET authentication protocols and comparing them based on their security, performance, and efficiency has been discussed. The survey was lacking in comparing the conditional privacy-based schemes, unobservability, tracking of location, and scalability. They did not discuss the latest techniques such as Blockchain, which have the potential to revolutionize security and privacy in VANET. In [5], the authors have discussed the characteristics of VANET, the challenges of implementing them efficiently, the widely recognized security standards and architectures, and the grouping of attacks and their solutions. Nevertheless, the writers did not talk about simulators or provide a clear overview of the authentication schemes as described in [4]. In [6], the authors discussed the threats and prevention mechanisms for VANET identified in the existing literature and presented an onboard unit (OBU)–based solution. They claimed that the researchers have addressed the Sybil attack more than various kinds of intrusions. The authors additionally talked about the Internet of Vehicles (IoV) and stated that the majority of Internet of Things (IoT) gadgets will be in vehicles and that steps to enhance the multiple security difficulties need to be given attention. Nevertheless, the writer did not provide a comprehensive resolution to IoV security. The use of ITS in VANET has been discussed in [6], as well as the privacy and security matters involved. The writer also discussed the effectiveness of VANET and cloud-based computing in addressing security as well as privacy concerns and talked about the uses and unresolved problems with VANET. In [7], the writers talked about the VANET architecture, the grouping of security attacks as well as security resolutions for VANET. They also discussed trust in VANET, and the challenges involved in establishing trust and mitigation strategies. Additionally, they discussed various simulators that can be used to test and evaluate VANET security solutions. In [8], the authors discussed the key security, privacy, and message dissemination challenges in VANET and also reviewed the progress that has been made in addressing these challenges over the past 10 years. In [9], the authors discussed the different types of certificate revocation schemes, based on where the revocation information is stored. They also identified the key challenges and techniques at each stage of the certificate revocation process. In [10], the authors discussed the different types of VANET authentication schemes, their effectiveness in mitigating various attacks, and their advantages and disadvantages. They also identified the key research directions regarding the domain of VANET authorization. In [11], the authors discussed the security and privacy trade-offs in VANET by presenting a detailed comparison of different privacy and security solutions concerning privacy and security requirements, computational overhead, and security threats. Additionally, they identified the open challenges in VANET security and privacy. In [12], the authors have provided a thorough investigation of current security measures and classified different security breaches in VANET. They classify the security schemes based on various characteristics and discuss the performance analysis, controlled attacks, and security requirements of each classification briefly. The authors highlight some of the obstacles that must be addressed to deploy secure VANET. In [13], the authors present a state-of-the-art review of VPKI, including a discussion of the latest issues and advances in the field. They also evaluate existing VPKI proposals based on privacy and availability metrics and suggest future research. In [14], the writers offer a thorough analysis of VANET, covering topics such as their design, privacy protection, secure communication, communication standards, and authentication methods. Additionally, they analyze the authentication features and attack resistance of various authentication schemes, classify attacks, and summarize attack detection schemes. The location privacy mechanism and trust models in VANET have been discussed by Sheikh, Liang, and Wang [15]. They additionally addressed the issue of the potential applications that could be impacted by threats or attacks. In VANET, they also discovered a few unresolved issues. In their comprehensive analysis and comparison of the benefits and drawbacks of each approach, Sparsh and Kaula [16] explored several intrusion detection systems. The most advanced VANET techniques currently in use have been examined by Sheikh and Liang [17]. Later, they conversed about the in-depth examination of security and authentication scheme designs. They also spoke about some of the safety uses for VANET.

Existing surveys on VANET security have not provided a thorough and understandable review of the VANET authentication and key distribution methods. Furthermore, neither of these studies has covered how Blockchain and 5G technology might be used in VANET. This study will complement existing surveys by providing a lucid and easy-to-understand overview of authentication, key distribution, and other security topics in VANET.

1.2. Contributions From This Study

The VANET is an emerging technology, and a lot of changes are anticipated regularly in terms of security, privacy, and trust implementation by both the academia and the industry. In this regard, adding the advancement to the existing survey mandated the proposal of this survey work. Also, the researchers need to stay updated; this survey stands unique, as it discusses the taxonomy of the secure ITS requirement and a deep insight into vehicle to vehicle (V2V), vehicle to infrastructure (V2I), and vehicle to everything (V2X). Moreover, recent work related to hybrid authentication schemes, Blockchain, 5G, and 6G technologies has been incorporated.

The remaining sections of the article are structured like this: Section 2 provides an extensive summary of the VANET architecture, communication protocols, features, attack types, and security requirements of VANET. Section 3 provides a comprehensive overview of secure communication and validation techniques in VANET, focusing on node verification and message authentication. Section 4 discusses the taxonomy of secure ITSs. Section 5 discusses the categorization of VANET authentication methods, analyzing the safety, privacy, and scalability requirements of each category. Section 6 discusses some of the open challenges in VANET. To summarize, we have presented the conclusions and suggestions for future research in Section 7. Figure 1 shows the flow of the manuscript.

2.1. Architecture of VANET

Figure 2 illustrates a recommended VANET system comprising vehicles, OBUs, roadside units (RSUs), trusted authorities (TAs), and other components [18, 19].

2.2. Communication Standard Used in VANET

The communication range of DSRC [26, 27] varies from 100 to 1000 m with data rates ranging from 6 to 27 Mbps. Safety-related messages are typically transmitted every 100 to 300 s [28, 29]. Vehicles can communicate with each other or with RSUs, which periodically broadcast beacon messages. Figure 3 represents the DSRC-based communication for V2V and V2I. DSRC offers exceptional resilience in the face of obstacles and dynamic environments, ensuring uninterrupted connectivity at speeds reaching 500 km/h. Its non-line-of-sight (NLOS) capability enables communication even when direct paths are blocked, making it ideal for applications that demand reliable data exchange in complex scenarios. Additionally, DSRC’s ability to function in extreme weather conditions further expands its versatility and applicability.

The 5.85 GHz to 5.925 GHz band has a 75-MHz bandwidth spectrum [31] that has been set aside by the Federal Communications Commission (FCC) for “DSRC.” This dedicated spectrum allocation ensures reliable and interference-free communication for V2V and V2I applications. WAVE [32] is a dedicated wireless communication standard designed to facilitate communication between vehicles and infrastructure (V2I and V2V), building the basis for ITS. WAVE’s short-range communication capabilities, high data rates, and dedicated safety channel make it ideal for a variety of applications that boost traffic flow, increase road safety, and give drivers useful information. The key features of WAVE are as follows: (a) 5.9 GHz is the frequency range used for short-range communication. WAVE provides reliable and timely data exchange between vehicles and infrastructure within a range of up to 1000 m. (b) High Data Rates: With data transmission rates reaching up to 27 Mbps, WAVE effectively transfers traffic data, safety messages, and other kinds of critical information. (c) Dedicated Safety Channel: WAVE prioritizes public safety by allocating a dedicated channel for emergency messages related to accidents, road closures, and other critical events. (d) Ease of Deployment and Cost-Effectiveness: WAVE leverages existing DSRC technology, making it relatively simple to deploy and cost-effective compared to other communication standards.

IEEE802.11p—The Foundation of Short-Range Communications (DSRC) with Specialized Focus on Vehicle Networks [33]. IEEE802.11p functions as the foundational technology of DSRC, which utilizes channels with a 10-MHz bandwidth in the 5.9-GHz range (5.850–5.925 GHz) to facilitate communication in VANET. IEEE802.11p specifies the media access layer (MAC) and physical access layer (PHY) requirements for VANET operations [34].

2.3. The Characteristics of VANET

2.4. The Classification of the Attackers, Threats to Security, and Additional Needs for Protection

To ensure secure communication, VANET authentication schemes need to meet certain requirements, including node authentication, message authentication, privacy preservation, nonrepudiation, low computational overhead in communication, traceability, and unlinkability.

3.1. Secure Communication

During V2V communication, messages are transmitted in large quantities from one vehicle to another. These messages include alerts concerning accidents and gridlock on the road, group communications among vehicles sharing characteristics, and beacon communications that contain information about the vehicle’s current speed, acceleration, and other states [42]. Every message must be authenticated in order to guarantee security. Figure 4 illustrates how secure messages are authenticated in vehicles [43].

In V2I communication, the same high volume of beacon and warning messages are sent in V2I communication [44], in addition to messages from the vehicle to the RSU regarding service requests, like requests for information about hotels in the area. Figure 5 displays the requested services flowchart [30].

3.2. Authentication Mechanism

The following steps comprise the authentication mechanism in general [45, 46].

4.1. Authentication Based on Entities

When it comes to ITSs, the first thing to ensure is the entities involved. Entities play an important role in the ITS, and there are basically three entities involved (i) V2V, (ii) V2I, and (iii) V2X.

V2V Authentication: For ITS to facilitate safe communication between automobiles, V2V authentication is a vital security feature. To stop impersonation, replay, and message modification attacks, it makes sure that cars can confirm each other’s identities and the veracity of the messages they exchange. Information about traffic and warning messages is transferred in vehicular communication network types. To reduce accidents and traffic congestion, vehicle communication systems are useful. The study of V2V communication has increased recently due to the growing significance of road safety. IEEE 802.11p establishes a global standard for wireless connectivity in vehicle settings (WAVE). V2V and V2I are the two main networking protocols found in vehicular environments when using wireless access; these protocols are grouped under the umbrella of ITS. Additionally, several applications such as traffic management and road conditions will be made possible by vehicular communication networks [53].

A combination of three networks, namely, intravehicle, intervehicle, and vehicular ad hoc, is known as the V2V network. V2V refers to a spread wireless communication and exchange of data system that is large in scope and connects nearby automobiles in accordance with standard data interaction protocols and communication protocols, predicated on the notion that three networks can merge to form one. It is an integrated network for smart management of traffic, intelligent data service, and smart vehicle control, and it represents a typical use of the IoT technology for innovative transportation systems (ITSs). It will be crucial to acquire core technologies and standards to succeed in this developing market. But just as crucial as developing the V2V technologies themselves should be the integration of those technologies with other infrastructures. This will lead to the V2V being a crucial component among the most extensive IoT [53].

Mahmood et al. [54] suggested a secure V2V message transmission system for VANET, enhancing privacy preservation and meeting security and performance requirements.

Bansal et al. [55] provide an ID-based computationally efficient privacy-preserving authentication (ID-CEPPA) V2V communication system for V2V (VANET) networks and elliptic curve cryptography (ECC). In V2V communications, the scheme effectively ensures message originality and nonrefutation, source authentication, and vehicle’s absence of ID.

Lim and Tuladhar [56] discussed that driving safety and traffic efficiency are increased when vehicles are authenticated locally without the use of infrastructures or trusted authorities thanks to LIDAR, a lidar information-based authentication mechanism for V2V communication.

Hasrouny et al. [57] have looked into a secure design that ensures integrity in V2V communication. It suggests group-oriented V2V communication and authentication for the distribution of safety messages with a lightweight solution that is decentralized through group leaders (GLs), effective, affordable, and suitable in real-world scenarios.

Snehal and Hajare [58] have provided a review on group-oriented V2V communication authentication and authentication in VANET.

Aliev and Choi [59] put forth a scalable, secure, and group key management approach for message encryption. The suggested approach for managing group keys is scalable and can effectively manage different situations, such as joining a node or departing the group, using an algorithm called rekeying.

4.2. Authentication Requirements

To guarantee safe and dependable communication between cars and RSUs, authentication in VANET is essential. In order to stop malicious actors from posing as authorized users and introducing erroneous or misleading information into the network, it allows the ID of communicating entities to be verified.

In VANET, there are two levels of authentication: message level and node level. RSUs and vehicles undergo node-level authentication, while messages are shielded from manipulation through message-level authentication.

4.2.4. Steps Involved in Node-Level Authentication

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  Initial Authentication Setup: Before joining the network, each car is outfitted with cryptographic keys and certificates (if PKI is being used).

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  Requesting and Verifying Credentials: Authentication credentials, such as digital certificates, are requested by a vehicle seeking to establish communication with another.

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  Certificate Verification: The receiving vehicle uses cryptographic techniques to verify the sender’s certificate’s authenticity or credentials, confirming they come from a reliable source and are not revoked or expired.

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  Secure Communication Establishment: After a successful verification, the cars can create a secure communication channel and encrypt data to guarantee integrity and confidentiality.

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  Periodic Authentication Updates: The credentials for validation may need to be updated regularly as cars move and communicate to ensure secure communication.

Figure 7 shows the steps involved in node-level authentication.

4.2.7. Challenges of Message-Level Authentication

There are various obstacles to overcome when implementing efficient message-level authentication in VANET:

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  Real-Time Requirements: Because VANET require real-time communication, the computational overhead of complex authentication mechanisms renders them impracticable.

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  Resource Constraints: Authentication schemes can be limited in complexity by the limited computational and battery power of vehicles and RSUs.

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  Dynamic Network: Consistent message authentication is challenging to maintain in dynamic VANET where vehicles are constantly entering and leaving the network.

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  Privacy Concerns: To stop illegal tracking or personal profiling, it is essential to strike a balance between robust authentication and user privacy.

In VANET, message-level authentication is a vital security feature that guarantees the authenticity and integrity of messages sent back and forth. By confirming that messages have not been altered or tampered while in transit, it guards against malevolent parties introducing inaccurate or deceptive data into the network. Figure 8 shows that the message-level authentication.

For message-level authentication in VANET, the following steps must be followed:

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  Step 1: Message Generation: The sender composes a message with the required details, like vehicle status, safety alerts, and traffic updates.

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  Step 2: Message Signing: Digital signatures or message authentication codes (MACs) are used by the sender to sign the communication.

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  Step 3: Message Transmission: When another vehicle or RSU comes within communication range, the sender broadcasts the signed message to them.

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  Step 4: Message Reception: The broadcast message is sent to receiving entities along with the digital signature or MAC.

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  Step 5: Signature Verification: Using the public key of the sender or a shared secret key, the recipient confirms the signature or MAC.

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  Step 6: Message Integrity Validation: The integrity and authenticity of the message are verified if the signature or MAC verification is successful.

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  Step 7: Message Rejection: A failed signature or MAC verification indicates tampering or unauthorized modification, and the message is rejected.

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  Step 8: Message Processing: The recipient can process the message and make use of the information it contains after the message’s integrity has been confirmed.

4.3. Privacy-Preserving Schemes in VANET

In VANET, protecting privacy is essential to protecting user data and anonymity, as well as facilitating safe and effective communication between vehicles and RSUs. Effective privacy-preserving mechanisms are becoming more and more important as VANET become more common.

4.4. Overhead

In VANET, “overhead” refers to the extra processing power and network resources used by security mechanisms to guarantee the security and integrity of communications, such as message signing, authentication, and encryption. In addition to adding extra overhead that may affect network performance and resource usage, these mechanisms are necessary to safeguard VANET against hostile attacks and preserve data confidentiality.

VANET security mechanisms have an inherent overhead component. Reduced overhead is necessary to guarantee network performance, resource utilization, and energy efficiency, even though protecting VANET from malicious attacks and maintaining data confidentiality are paramount. Secure and dependable communication between vehicles and RSUs can be ensured by VANET by carefully choosing lightweight security mechanisms, putting adaptive protocols into place, and using effective data encoding and scheduling techniques.

5.1. Cryptography-Based Authentication Schemes

In the realm of authentication schemes rooted in cryptography, there exist three primary types: asymmetric, involving two keys—a first key for encryption and a second for decryption; symmetric, relying on a single key for both encryption and decryption purposes; and ID-based, resembling asymmetric schemes but distinct in the generation of the public key, which is derived from the user’s ID information.

A framework known as PKI binds public keys to user identities. The binding is completed by certificate issuance and registration by the CA. The person who registers is registering authority (RA): This body guarantees the authenticity of the asking party. The secret key, sometimes known as the private key, is utilized for signing, and verification is done using the public key. Figure 10 illustrates how a PKI-based system functions.

Elliptic Curve Digital Signature Algorithm (ECDSA), a variant of the digital signature algorithm, incorporates ECC. Its primary function involves encrypting messages, thereby restricting data access solely to authorized users, significantly bolstering security measures. The digital realm offers a range of diverse digital signature algorithms to explore.

Examples of asymmetric cryptography schemes are PKI and ECDSA. While privacy preservation is a crucial requirement that should not be overlooked, authentication is an essential prerequisite for secure communication in VANET. The ability to reveal personal information only to specific individuals or organizations is known as privacy.

The user’s private information, such as the owner’s ID, address, and vehicle location, should not be disclosed to any other party or vehicle during the authentication process other than the appropriate authority, such as a CA or TA. Thus, privacy preservation must be guaranteed by the authentication scheme. Therefore, privacy-preserving requirements like location tracking, unlinkability, and unobservability need to be protected.

When someone is anonymous, their true ID is not disclosed to others. Large anonymous certificates, approximately 43,800, and corresponding private keys are preloaded into PKI authentication schemes. Anonymous authentication was proposed by Raya and Hubaux [35], where privacy was maintained by using anonymous public keys instead of a single public key. This prevents the receiver from knowing who owns the keys. The list is growing because malicious nodes have a high revocation rate.

Challenges identified in VANET, as highlighted by the authors in [76, 77], encompass substantial storage demands, latency concerns in verifying revocation lists, and the timely distribution of CRLs.

In their work (Calandriello et al. [78]), the suggestion entails the creation of pseudonyms in real time, employing a combination of self-certification and a fundamental pseudonym strategy alongside group signature. This approach aims to tackle the storage constraints and latency issues outlined in [35].

If the ID is anonymous and the node is not malicious, this is known as conditional privacy. A method for achieving conditional privacy was suggested by Rajput et al. [79]. The approach involved assigning two tiers of pseudonyms, namely, foundational and short-term pseudonyms, to individual vehicles.

By using a digital signature, VANET security can be improved and the message’s authenticity, integrity, and nonrepudiation can be guaranteed. IEEE 1609.2 recommends ECDSA schemes for message verification [80]. Compared to authentication schemes that use the Rivest–Shamir–Adleman (RSA) algorithm, ECDSA produces less computational overhead. Investigators [81–83] have talked about ECDSA.

Symmetric key cryptography, also known as private key cryptography, relies on a single secret key for both encryption and decryption purposes. Compared to asymmetric cryptography schemes, it is faster to execute and easier to design because the sender and the recipient share the same secret key.

The idea of a symmetric random key set technique has been employed by Xi et al. [84] to guarantee privacy and reduce OBU overhead. Nevertheless, since both the sender and the recipient use the same secret key for authentication, symmetric cryptography schemes cannot ensure nonrepudiation.

Symmetric key cryptography encompasses authentication schemes like hash function, timed efficient stream loss-tolerant authentication (TESLA), and MAC.

The message and secret key are entered into the MAC algorithm, which creates the tag and adds it before sending. The same secret key is utilized at the receiver’s end to compute the label. The authenticity of a message is only established when both tags are equivalent. Message integrity is guaranteed by the MAC algorithm and genuineness.

Lin et al. [85] introduced the timed, efficient, and secure vehicular communications (TSVC) scheme in their work. For packet authentication, each packet has a short MAC tag appended to it. According to simulation results, under high-traffic conditions, TSVC outperforms other PKI-based schemes with regard to packet loss ratio.

A CPP scheme based on a MAC was proposed by the author in [86]. In this, message generation and authentication are accomplished through verifiable secret sharing, which allows a car to obtain its group key. It requires less computation and communication overhead and satisfies the fundamental security and privacy requirements.

The verification of message integrity involves employing a hash function. This function appends a message’s hash value or digests to the message itself before transmission, ensuring its integrity. If an attacker manipulates the message during transit, any alterations will result in a distinct hash value, prompting the recipient to reject the modified message.

In [87], authors have put forth an authentication scheme that preserves conditional privacy by utilizing the Chinese remainder theorem (CRT). To cut down on computational overhead, they have done away with storing the master key inside the tamper-proof device (TPD). Because the scheme does not employ binary pairing or operations like map to point during the authentication process, it verifies signatures more quickly even when the number of signatures increases. The system can withstand frequent attacks and provide improved performance with reduced computational and communication overhead.

Authors in [88] used the Trust-Extended Authentication Mechanism (TEAM), a decentralized lightweight authentication scheme, for V2V communication. To determine the safe secret key set, they employed a hash chain. It complies with security requirements, including anonymity.

The TESLA makes use of hash chains in addition to precise MAC. With broadcast or multicast data streams, TESLA enables the recipients to verify the legitimacy and integrity of the source for every packet [89].

Bao et al. [90] introduced a lightweight authentication system that combines TESLA and Bloom Filters in their work. This system aims to safeguard against active attacks while ensuring privacy preservation.

ID-based cryptography (IBC) generates the public key using ID information, like email addresses. Since a certificate is not needed for message authentication, there are fewer messages overhead. Additionally, because of the management and lack of maintenance for CRL, it enhances VANET communication.

Authors in [91] have presented a decentralized privacy preservation scheme that makes use of hash-based message authentication codes (HMAC) and asymmetric identities. The outcome of the simulation indicates that the proposed scheme is resilient, lightweight, and able to withstand typical attacks. Nevertheless, conditional privacy, location tracking, and unobservability are not guaranteed by the scheme.

In [92], the authors detail a privacy-focused authentication framework comprising a single hash function, secret key, pseudo-ID, and segmented into four distinct stages. Validation of its security and privacy adherence was conducted using the ProVerif tool. Leveraging only hash and exclusive-OR operations, the scheme stands out for its resilience, lightweight nature, and minimal computational and communication burdens. Furthermore, the authors discuss potential enhancements for this framework concerning its applicability within 5G and edge computing scenarios in VANET.

Azees and Deboarh [93] employed tracking mechanisms with conditional privacy to withdraw the automobile in the event of any misbehavior and anonymous authentication to prevent malicious vehicles from entering the VANET. Bilinear pairing was the method they employed. Five distinct steps are involved in achieving anonymous authentication: (1) key generation and registration, (2) generation of anonymous certificate, (3) generation of signature, (4) verification, and (5) conditional tracking. The certificate’s computational cost, RSU serving capacity, and signature verification procedure have all been examined in terms of performance analysis. With location tracking, it offers the lowest cost for verifying certificates and signatures. The system can fend off frequent attacks and guarantee privacy protection.

5.2. Signature-Based Authentication Schemes

Authentication in cryptographic schemes such as asymmetric, symmetric, and ID-based ones often relies on single-user signatures. However, this approach introduces complexities like key management, frequent alterations to public and private key pairs, as well as computational and communication overhead. To address these challenges, researchers have proposed group signature-based authentication systems. With a single modification, group signature authentication demonstrates a resemblance to the functionality of a public and private key pair. An essential feature of group signature is its ability to offer anonymous authentication to safeguard privacy, as discussed in [94]. In this system, any member within the group can sign the safety message using its private key. The recipient validates the sender by verifying the signature with the group public key, thereby revealing only the ID of the group manager.

On the basis of group communication in VANET, the authors in [95] have employed a dual authentication scheme. The plan relies on each user’s fingerprint and the vehicle’s secret key.

To reduce the computation in this case, CRT-based key management is employed. Additionally, there is a minimal amount of information needed to update the group key in the event that a group is joined. The plan has less overhead in terms of computation and communication and can withstand common attacks, but it cannot withstand conditional privacy or location tracking.

The anonymous authentication scheme (AAAS) in VANET has been implemented by the authors in [96] using regional trust authority. To ensure anonymity and conditional privacy, they have employed group signatures.

To achieve conditional privacy, Islam et al. [97] used group key generation protocols and password-based systems. Because the protocol does not use an elliptical curve or bilinear pairing, it has a lower computational overhead.

5.3. Verification-Based Authentication Schemes

A vehicle using VANET communication can send safety messages, which need to be confirmed by the RSU or other nearby vehicles in less than 300 milliseconds. In high-traffic areas, the quantity of messages increases and begins to build up for validation. The messages will be invalidated or discarded if they are not verified within the allotted time. It might also result in an accident or other dire circumstances. Messages can be verified cooperatively or in batches to address the issue of timely message verification with less delay, communication overhead, and computational overhead. The types of authentication algorithms that can be designed are as follows: (1) authentication schemes based on batch verification and (2) authentication schemes based on cooperative verification.

A batch-aided verification scheme has been proposed by Wu et al. [98] to validate the message more quickly and with less delay. They have enlisted the help of RSU and a few terminals to work together on the message verification task in this scheme. Additionally, the scheme scales 10 times faster than schemes in which the only message verifier is RSU.

To guarantee security and conditional privacy, authors in [99] have suggested an ID-based batch verification scheme. When compared to schemes that employ the binary pairing technique, the scheme performs admirably.

In VANET, every vehicle undertakes the verification of safety messages upon reception—a departure from the traditional system where this redundant verification process often leads to delays, particularly during high message volume scenarios. Employing cooperative message authentication stands out as a viable solution to mitigate this issue.

An effective cooperative messaging authentication (ECMA) scheme has been proposed by Lin and Li [100] to lessen the need for duplicate authentication for each vehicle within the range of the same message. Selfish vehicles utilize free-riding attacks. The strategy involves implementing an evidence token aimed at assessing the vehicle’s authentication process contribution independently, eliminating the reliance on TA assistance to thwart free-riding attacks. Upon a vehicle passing the RSU, it acquires an evidence token reflective of its past contributions.

A few key concerns must be resolved to have an ECMA. One of the serious issues with the implementation of an ECMA is handling potential failures. Failures are the most critical problems, and causations arise from a variety of sources, including system crashes, network errors, or even malicious attacks. To diminish these failures, you must have a solid design in place which will be able to detect and recover from such failed conditions soon.

Another key issue is the design of participation incentives, which is essential for motivating users to take part in the cooperative message authentication in an interface. It can be achieved by designing some mechanisms to rewarding, including encouraging rewards, reputation system, and other benefits, the users for contributing messages in an interface system.

Privacy is also a major concern in ECMA since it requires gathering and processing personal data, thus, in this context, privacy regulations that should be considered that includes European General Data Protection Regulation (GDPR), and California Consumer Privacy Act (CCPA) for instance to obtain the user’s consents, apply data protection, and ensure the transparency of data processing.

Furthermore, other eminent issues need to be discussed in the context of ECMA, i.e., security, scalability, usability, and interoperability in order to develop a cooperative messaging authentication system that is secure, scalable, easy to use, and interoperable.

The authors of [101] have used a trustworthy cooperative authentication system. To prevent synchronization issues between cooperative and non-cooperative vehicles, they have employed success reports in this. According to the simulation results, even at 200 vehicles per km, there will be no message loss.

Tables 5 and 6 provide a comparison of schemes (A, B, and C) covered in Section 5 considering the criteria for (1) security, (2) privacy, and (3) scalability.

5.4. Advanced Authentication Schemes in VANET

VANET are built to handle the special difficulties of securing communication in constantly changing and limited resources vehicular environments. This is why advanced authentication schemes are implemented in VANET. With the least amount of overhead and flexibility to accommodate the dynamic nature of VANET, these schemes seek to offer robust authentication, privacy protection, and efficiency. The types of authentication algorithms is designed as (i) hybrid scheme and (ii) Blockchain-based authentication.

An authentication scheme based on the hybrid proxy (HPBS) was suggested by Liu, Wang, and Gu [102]. Three issues in the VANET environment were resolved in HPBS by introducing the idea of agent vehicles and combining ID-based and PKI-based hybrid authentication: RSUs can be made more efficient in terms of message authentication; their computational load can be decreased; and user privacy can be safeguarded.

Tangade, Manvi, and Lorenz [103] suggested a hybrid cryptography (TMHC)–based trust management system to further secure VANET. Since secure communications and the establishment of trust between vehicles depend on authentication, the suggested TMHC incorporates hybrid cryptography–based authentication for an effective and reliable trust management system. HMAC and asymmetric ID-based digital signature (ID-based) are two components of hybrid cryptography.

Wang et al. introduced an innovative group signature-based algorithm in their work [104], aiming for mutual authentication within V2V communication within a hybrid D2D message authentication (HDMA) scheme designed for 5G-enabled VANET. Furthermore, to reduce the computational load of the modular exponentiation operation, they incorporated a precomputed lookup table.

Addressing the security prerequisites of diverse V2I communications, Ali and colleagues introduced a hybrid signcryption protocol based on elliptic curve cryptosystems (ECCHSC) in their work [105]. This protocol offers a single-step implementation and fulfills essential security requisites, encompassing message confidentiality, integrity, nonrepudiation, and ID anonymity.

Blockchain technology has been used by Ma et al. [106] to propose a decentralized key management protocol for VANET that automatically registers, updates, and revokes the user’s public key. They have suggested using bivariate polynomials for both key agreement and mutual authentication. The plan can counteract denial-of-service attacks, collusion attacks, internal attacks, and public key tampering attacks. In addition, the protocol operates efficiently with reduced overhead in terms of computation, storage, and communication. The plan eliminates the reliance on TA, as with a PKI system, while also guaranteeing the anonymity needed to protect privacy.

A PPA scheme for VANET has been proposed by Lu et al. [107]. Revocation list-free distributed authentication schemes are offered by the Merkel Patricia tree (MPT). Multiple certificates could be used by vehicles to obtain conditional certification. Every entity’s performance has been examined using the Hyperledger Fabric (HLF) platform. Based on the simulation results, which show that every vehicle can authenticate in less than 1 millisecond, the scheme meets the real-time time constraint. Also, compared to the previously used schemes, storage and processing times have been significantly decreased.

Lin et al. introduced an efficient certificate management scheme [108] employing the ECDSA rooted in the Ethereum public Blockchain to ensure secure communication among vehicles. This innovative system minimizes the need for vehicles to store private keys, subsequently reducing verification costs and time. The scheme underwent verification on Rinkeby, the Ethereum test network, and simulation was conducted using NS-2, with VanetMobiSim chosen for its efficacy. Further exploration is warranted to ascertain how this approach aligns with the prerequisites for conditional privacy and security in VANET deployment.

In [109], authors present a secure payment scheme based on Blockchain technology for VANET, taking into account two situations: (1) management of park toll systems and (2) electronic toll collection.

The payment plan, which consists of two transactions: (i) V-R and (ii) V-Rs, is reliable and efficient. In this scenario, the smart contract handles all transactions automatically, and only RSU participates in the consensus. Additionally, it can lessen security and privacy requirements.

A consortium Blockchain-based unlinkability authentication scheme was employed by Liu et al. [110]. The distributed nature of the service manager (SM) creates a distributed database that facilitates data sharing. Every vehicle starts the authentication process and creates a unique pseudonym. To confirm the legitimacy of the car, SM consults local data. Stronger anonymity and unlinkability can be ensured by this scheme, but cooperation among SMs makes it vulnerable to collusion attacks and linkability.

Bi et al. [111] suggested a productive Blockchain-based, anonymous, certificate-less authentication system for VANET. In order to lower the computational overhead in VANET, they first suggest a novel signature scheme devoid of bilinear pairing. Second, they use Blockchain technology to enable the sharing of trustworthy information. Third, they offer privacy-preserving by using anonymous keys and allow vehicle ID tracing in case of malicious activity. Table 7 gives a comparative analysis of the schemes which were discussed so far in the article.

The integration of Edge computing, Fog computing, and Cloud computing forebodes a lot of improvement in VANET. Of these techniques, some salient features are being highlighted here that make VANET better.

5.5. 5G and 6G Networks in VANET

Research on the integration of 6G and 5G networks in VANET is still ongoing. Research has been done to address the design and implementation issues that the emergence of 5G networks presents for VANET security [117]. The expectations for 6G networks are that they will overcome the shortcomings of 5G and offer massive connections, excellent dependability, and extremely low latency. As a result, they will be a major contributor to the advancement of the ITS and the concept of the Smart City [118]. The IoV is anticipated to have a high degree of security and privacy thanks to the incorporation of cutting-edge technologies such as edge computing, Blockchain, artificial intelligence (AI), network automation, slicing, terahertz links, visible light communications, backscatter communications, physical layer authentication, and cell-free massive multiple-input multiple-output (MIMO) [118]. Still, there are unresolved issues with VANET, including the absence of international standards and the requirement to address inadequate system reliability, low intelligence levels, and weak network security [119].

Research and interest in the integration of 5G and 6G networks in VANET are high. 5G network development has brought up design and implementation concerns for VANET security. Studies have been carried out to examine the security characteristics of 5G networks and how they can be utilized in automobile networks to facilitate a smooth and effective integration [117]. Moreover, it has been emphasized how important automotive applications will be to the architecture of future 6G networks. In autonomous vehicles, 5G communications and related technologies are integrated to enable cooperative perception [120]. SDN control and virtual network functions (VNFs) are two well-known technologies that are essential to 5G-based flexible security. As a result, 5G uses these technologies to enable the integration of VANET and 5G security [117]. Still, there are unresolved issues with VANET, including the absence of international standards and the requirement to address inadequate system reliability, low intelligence levels, and weak network security [121].

To sum up, the amalgamation of 5G and 6G networks within VANET represents a multifaceted and dynamic field of study. While 5G networks have brought new security features that VANET can take advantage of, 6G networks are anticipated to emerge in order to address 5G’s shortcomings and offer the ultra-low latency and high reliability needed for ITSs of the future. In order to successfully integrate 5G and 6G networks in VANET, it will be imperative to address the outstanding challenges in VANET.

5.6. VANET Security and Privacy Applications

VANET allows data exchange and communication between infrastructure and vehicles, which is critical to improving transportation systems. There are several known uses for VANET, including infotainment, safety, and traffic efficiency. The application of VANET into three categories, viz., safety application, nonsafety application, and other applications, is depicted in Table 8.