Internet of Vehicles (IoV) has emerged as an advancement over the traditional Vehicular Ad-hoc Networks (VANETs) towards achieving a more efficient intelligent transportation system that is capable of providing various intelligent services and supporting different applications for the drivers and passengers on roads. In order for the IoV and VANETs environments to be able to offer such beneficial road services, huge amounts of data are generated and exchanged among the different communicated entities in these vehicular networks wirelessly via open channels, which could attract the adversaries and threaten the network with several possible types of security attacks. In this survey, we target the authentication part of the security system while highlighting the efficiency of blockchains in the IoV and VANETs environments. First, a detailed background on IoV and blockchain is provided, followed by a wide range of security requirements, challenges, and possible attacks in vehicular networks. Then, a more focused review is provided on the recent blockchain-based authentication schemes in IoV and VANETs with a detailed comparative study in terms of techniques used, network models, evaluation tools, and attacks counteracted. Lastly, some future challenges for IoV security are discussed that are necessary to be addressed in the upcoming research. With the huge increase in the number of vehicles on roads nowadays, more accidents and traffic congestion issues are encountered. This raises the need for serious arrangements to ensure roads’ safety and traffic efficiency. Different technologies have been introduced towards maintaining safer and time-efficient driving on roads, such as, Vehicular Ad-hoc Networks (VANETs) in which the vehicles exchange data about their speed, location, etc., and other road-related information to raise their awareness about surrounding road conditions and help them making better and effective decisions. However, with the rapid advancement in today’s technologies such as ubiquitous connectivity, wireless technologies, sensor devices, smart vehicles, and cloud computing platforms, the need for more powerful vehicular networks has increased. Hence, the IoV has appeared that can exploit and incorporate all these advanced technologies in order to provide more satisfying real-time services for vehicles’ drivers and passengers.IoV has emerged with great potential to support various services and offer several benefits to the transportation system such as cost effectiveness, time efficiency, road safety , trafficOn the other hand, blockchain technology has recently drawn the attention of both industry and academia due to its efficient characteristics represented in decentralization, immutability, consensus, fault tolerance, and enhanced security. Blockchain was first known as the enabling technology behind Bitcoin or cryptocurrency. Yet, it has recently attracted various emerging domains such as smart cities ,16,17, sNumerous surveys have recently been published discussing the authentication approaches and protocols in the vehicular networks of VANETs and IoV which are summarized in In order to highlight the contribution of this paper, a number of recent state-of-art surveys are summarized and compared in Highlighting the significance of the blockchain technology in IoV and VANETs by presenting a wide range of the blockchain-based authentication schemes that are proposed in the recent literature.Providing the first detailed survey focusing on the application of blockchain technology to a specific aspect of IoV security; that is, the authentication. Considering both IoV and VANETs technologies when surveying the different blockchain-based authentication schemes instead of restricting them to only one vehicular technology, which can provide a comprehensive source for researchers interested in the field of blockchain-based authentication.On the other hand, blockchain-based applications in IoV have been addressed by many surveys recently. For instance, the authors in  have surThe organization of the paper is as follows: IoV is an emerging field that mainly incorporates ITS and IoT technologies, while covering a wide range of other technologies and applications such as vehicular information services, advanced wireless communication technologies, cloud computing, edge computing, and automotive electronics to provide intelligent transportation services and enhance the quality of roads. It integrates the intelligent in-vehicle sensor devices with the intra-vehicle and inter-vehicle wireless communication technologies along with Internet technology to collect and exchange vehicle-related and traffic-related data that can be later used for making better road-related actions and decisions. IoV consists of three basic components: (1) the intra-vehicular network, (2) inter-vehicular network, and (3) vehicular mobile Internet . This inVANETs’ framework could not fully support the global and sustainable services targeted by ITS applications. This is caused by the pure ad-hoc communication architecture, in which an on-road vehicle can lose its granted services once it disconnects from an ad-hoc network. This is due to the inability of collaborating with other alternative reachable networks.Internet connectivity in VANETS could not be ensured, which affects the availability of commercial applications for vehicles’ drivers and passengers since those applications rely on reliable Internet connectivity.Despite the rapid technological advancement of personal mobile devices, they could not communicate with VANETs due to the incompatible network architecture.Intelligent decision making, and big data analytics applications were not possible in certain VANETs architectures. This could be related to the computing and storage constraints and the lack of cloud computing services.The application services could not guarantee high level of accuracy, since VANETs localize the computation and processing of traffic data information.The former technology to IoV is the VANETS which was basically introduced to improve the traffic efficiency and road safety by establishing connectivity and exchanging information between the moving vehicles with and without the aid of any pre-established roadside infrastructure via different communication modes namely, Wireless Access in Vehicular Environments (WAVE) based Wi-Fi, ad-hoc, and hybrid. Despite its efficiency in addressing road safety and traffic management issues with low operational cost, VANETs exhibit some commercialization problems which include but are not limited to the following :VANETs’ The above limitations of VANETs have drawn the attention of researchers and industrial developers to extend the capabilities of the existing vehicular networks to move further steps towards providing more efficient vehicular services and achieving the global objectives of ITS. Consequently, IoV has emerged as an advanced vehicular technology that attempts to overcome the shortcomings of conventional VANETs through supporting a high range of mobility, strong connectivity among vehicles and with roadside infrastructure, reliable Internet connection, and high interactivity with personal devices. IoV can also provide an immediate management of risk situations through maintaining low delay and delivering high reliability and robustness. Cloud and edge computing capabilities, processing, and analysis of collected data to transform them into useful information through big data analytics tools to provide services to consumers and businesses are as well positive points for the IoV.In addition, IoV has brought the ability to support diverse types of interaction models including Vehicle-to-Vehicle (V2V), Vehicle-to-Roadside-unit (V2R), Vehicle-to-Infrastructure (V2I), Vehicle-to-Pedestrian (V2P), Vehicle-to-Sensor (V2S), and many others as indicated in Different layered architectures for IoV frameworks have been proposed in the literature, which differ in the number and/or names of layers proposed. These architectures compete to optimize the number of layers and to enhance the distinguishability between the different layers ,61,62,63A blockchain, also called a Distributed Ledger Technology (DLT), can be defined as a group of connected blocks used to store transactions’ records or events and is maintained by all the participating users distributed over the network. Blockchain technology removes the reliance on a central authority, since it allows all users to generate and validate transactions directly in a peer-to-peer network which helps significantly in reducing the financial and time related costs associated with the intermediate party. Building the blocks of a blockchain relies on two key technologies, namely, cryptography ,65,66 anCryptography is adopted in blockchain to ensure the security and privacy of the data and the anonymity of the participants thereby using cryptographic hash functions and digital signatures . The adoConsensus mechanisms are used in the blockchain to establish trust in an untrustworthy environment and to verify the correctness and integrity of the transactions data being added to the public ledger. Blockchain consensus can be defined as “the agreement of a common value among a group of nodes in blockchain systems” . SeveralThree types of blockchains are commonly defined and agreed upon by most of the literature, namely, public blockchain, private blockchain, and consortium blockchain. The different types are distinguished from each other by their consensus making, read permission, immutability, and degree of centralization . Public However, other blockchain classifications have been suggested in the relevant literature. For instance, the authors in  have divIoV is a large-scale and heterogenous network that combines a large number of connected vehicles, roadside infrastructure, mobile personal devices, central and distributed storage, and computation servers in case of incorporating cloud and edge computing platforms. This along with the open-channel wireless communication model and the public Internet connectivity that dominate most of the communication makes the IoV network vulnerable to a variety of security attacks that could threaten the applications of IoV such as navigation, accident detection and notification, dynamic alternative routing, route optimization, and congestion management which consequently constitutes a threat and danger on drivers and passengers on the road. Furthermore, since IoV scenarios include high mobility and exchange of huge amount of data as well as requiring real-time services and decision making, more efficient, powerful, and reliable technologies must be adopted in IoV frameworks in place of the conventional techniques. On the other hand, blockchain technology has emerged recently as a decentralized storage mechanism in various industry applications due to its strong capabilities not only in distributed storage aspect, but also in terms of security, privacy, performance, automation, and reduced computational cost. Recently, blockchain has also been brought to the IoV paradigm to serve different purposes such as data protection and management, resource trading, resource sharing, ride sharing, traffic control and management, and forensic applications. Decentralization: Unlike the centralized-storage platforms where both data storage and management are handled by a trusted centralized node, blockchain technology exhibits a decentralized nature in which data records are kept and managed by all participating entities. This reduces the resource bottlenecks issue and maintenance cost associated with the centralized sever arrangements and avoids the single point of failure issue which all can be beneficial for IoV environments.Immutability: Since the creation and validation of new blocks of transactions should be agreed upon by all or most of the peers via the different consensus mechanisms before being added to the blockchain, the blockchain is almost impossible to be tampered with or modified.Security and privacy: The cryptographic nature of blockchain where both cryptographic hash functions and digital signatures are adopted can ensure the security of transactions data and the privacy of the participating users in IoV.Transparency: Since all participants keep a replica of the public ledger, they can access all the timestamped blockchain transactions. This enables the peers to manage, look up and verify transactions at any time in a transparent manner without an intermediary. This self-auditability and transparency not only promote the relief of the peers by managing their own transactions, but also mitigates the time and financial costs associated with the intermediate party.Automation: Blockchain technology supports the adoption of smart contracts which are software scripts that can be executed automatically by a triggering event or upon meeting some pre-defined set of rules. This automation property of blockchain can enhance the efficiency of many IoV applications and help provide various services autonomously without a need for a trusted entity.Traceability: Each transaction record is kept in the blockchain with a timestamp indicating its time of occurrence and joining the public ledger. This timestamped recording nature helps identifying the events in a chronological order which enhances the traceability and can support the non-repudiation requirement in IoV.The various features a blockchain can provide have motivated researchers and the industry to incorporate blockchain technology into the IoV. These properties include the following ,86:DecenIoV and VANETs have various features that are advantageous to vehicles’ drivers and passengers, pedestrians as well as the whole industrial business. However, like any new-emerging technology, IoV and VANETs come with several risks and security threats. The continuous mobility of vehicles, the existence of a third party acting as an authority to certify the nodes, and the wireless mode of communication among the different nodes make these vehicular networks vulnerable to wide range of security threats and attacks. Identifying the different security requirements and exploring the possible attacks that threaten these vehicular frameworks is the first step towards resisting them. Accordingly, in this section we first present the different security requirements and challenges for vehicular networks in The IoV networks are an amalgamation of diverse technologies with different standards and regulations ; which make IoV vulnerable to various types of security attacks. Depending upon the attacker objective(s), the attack launched might be passive or active, generated internally or from an external source. However, regardless of the attack’s source or activity type, these security threats are commonly classified into different categories based on the security aspect(s) of the network being compromised. For example, an attack could affect the authenticity of the users, the integrity of IoV data, or the availability of the provided services. Authentication: It means ensuring that the received data is generated by a legitimate sender, or in other words, making sure that the entity that sent the data must be the true actual entity it claims to be. It guarantees that the entities involved in the communication are authentic, not masqueraded by some attacker who forwards the messages on their behalf. Sybil attack, masquerading attack, impersonation attack, spoofing attack, replay attack, and wormhole attacks are examples of attacks that target the authenticity of IoV users.Availability: It is a basic requirement in IoV environment especially for the real-time critical applications where even a minor delay cannot be tolerated and made the information useless. Therefore, the IoV system should be available all the time to provide real-time information and services to all legitimate users and be able to tolerate partial system faults and failure issues through backups and replications. Moreover, a mature IoV system must have the ability to function under intense network load with the increasing number of participants. The common attacks that target the availability service are the denial-of-service and distributed denial-of-service attacks.Confidentiality: Some IoV applications include sensitive information that are accessed only by certain legitimate users. Therefore, confidentiality of this information must be insured through encryption to prevent it from being revealed and interpreted by any illegal entity even upon eavesdropping.Data integrity: It means there is no distortion—whether intentionally or accidentally—in the received data. In other words, the sent data and the received data are identical. Typical attacks on integrity can be data manipulation attack and malware attack.Non-repudiation: It guarantees that any involved user in the IoV environment cannot deny any of its past activities, i.e., sending or receiving any piece of information. This ensures that an attacker can be identified, and all its communicated messages can be retrieved if needed for subsequent actions.Access control: Each participating entity in the IoV network is assigned different rights and privileges to access the network resources. This security requirement guarantees that each node performs its functions based on the services it is entitled to.Privacy and anonymity: The users’ real identities may need to be made hidden using anonymous identities or pseudonyms to protect their privacy. Additionally, some location information such as the driving traces and tracks followed by the vehicles are sometimes preferred to be anonymous in order to prevent unauthorized location tracking.Data verification: Since malicious entities can modify the information sent by the sender, a regular data verification process is usually performed to identify the manipulated or tampered messages and thus reject or drop them (if found) to prevent misleading the receiving entities into taking improper decisions.Real-time guarantee and efficient routing: The majority of IoV and VANETs applications are real-time, such as accidents detection and warnings dissemination, which must be carried out within certain time constraints, otherwise the safety of drivers and passengers could be threatened, and the delayed information will become worthless. To be able to meet these time constraints, efficient secure routing protocols should be adopted to guarantee delivering the packets in their entirety and on time.Traceability and revocability: Despite the need for preserving the privacy of IoV users in general by hiding their real identities, the legal authorities should have the ability to retrieve the vehicles’ real identities in case of misbehaving to revoke them as well as in case of disputes.Scalability: With the increasing number of vehicles on the roads nowadays, more vehicles and entities are joining the network. Thus, a good vehicular network should be able to scale-up accordingly. However, this nodes extension may expose the network to higher security issues if not controlled and monitored properly. Therefore, monitored scalability is another security requirement of the IoV system.Guided by the various security threats an IoV system can suffer from, different security aspects have been defined in the literature. These security aspects can be classified into: (1) Security requirements that an efficient IoV system should maintain and (2) Security challenges that face any security subsystem of an IoV environment. High mobility: Data packets must be preserved and kept unmodified during the entire uncertain routing process from the sender to the receiver and should also be delivered on time to satisfy the real-time security requirement. However, the high mobility of nodes in IoV and VANETs networks, and the continuously varying network topology have led to the transient nature of V2V and V2I interactions, which makes the real-time guarantee and non-repudiation security requirements much more difficult.Low errors tolerance: Any minor delay in receiving information or delivering packets in IoV may result in unacceptable situations or even road disasters, thus the time constraints are of high importance in such environments. However, the limited bandwidth and the unstable network quality in IoV caused by the huge number of vehicles being served, their mobility, and the unpredictable changes in wireless networks hinder the real-time security requirement. Therefore, some preventive security measures must be applied so that the drivers can be proactive in case of any emergency situations.Key management: Several authorities and stakeholders are considered important participants in IoV such as governmental institutions and vehicle manufacturers. Hence, it has always been a security challenge to delegate a certain authority among these stakeholders to serve as a fully Trusted Authority (TA) that is responsible for key distribution, management, and revocation, since choosing the wrong authority without considering its hidden intents and benefits could severely affect the security of the IoV system. Moreover, due to the scale of IoV and VANETs networks, the Certificates Revocation Lists (CRLs) that are responsible for the revocation of misbehaving and malicious vehicles become too long and thus not feasible which raises the overhead of the certificate revocation process. Thus, maintaining a balance between efficient key distribution/revocation process and low overhead is another challenge on IoV security.Tradeoff between security and privacy: In general, the more secure the system is, the less privacy it can provide for users. Many drivers and passengers may not be willing to sacrifice their privacy by sharing their private sensitive data such as their location and destination while caring for security at the same time. Thus, maintaining a good balance between high security and reasonable user privacy is another major security challenge in IoV.Compatibility and cooperation: Due to the divergent interests and targets of the various IoV participating parties such as vehicle manufacturers, consumers, and governmental organizations, it is a big challenge to align their interests properly.The following are different security challenges that might be faced by any IoV paradigm ,95,96,97Channel Interference Attack: This is also known as jamming attack, which is the act of interfering with the wireless communication links to hinder and disrupt the quality of communication between connected IoV devices.Denial of Service (DoS) Attack: In this attack the attacker floods the IoV network with fake unnecessary requests so that the service provider is no longer able to handle the legitimate requests issued by the genuine users.Distributed Denial of Service (DDoS) Attack: It is an advanced type of DoS attack, in which the fake requests are launched from several distributed nodes forming botnets.Man-in-the-Middle Attack: In this attack, an attacker places itself in the middle of two communicating legitimate entities to receive the data from the sender and then forward it to the receiver. When the message passes through the attacker, he/she could modify it before forwarding it to the receiver or just reveal its content.Message Tampering Attack: This attack aims to corrupt the messages and spread false information in the network by manipulating and fabricating the contents of the communicated messages. This can badly affect the road safety and the services provided by IoV thereby harming passengers and drivers.Eavesdropping Attack: It is also referred to as traffic analysis attack, in which the attacker monitors the network to track vehicle routes for some malicious purposes. The attacker eavesdrops on the communication channels to examine the messages flowing in the network.Malware Attack: In this attack, the attacker injects malicious software or viruses in the system files to contaminate the network, which can be leveraged to disrupt the network services and/or to manipulate users’ data.Message Holding Attack: Here, the attacker drops some sensitive messages that hold important data about road conditions that could greatly affect the decision of the drivers on road and thus the road safety is compromised. It also enables the drivers to keep these messages for future use (replaying) in the network.As stated previously, different types of security attacks can be launched against IoV or VANETs, whether passive or active, insider or outsider. However, the common factor of all of them is that they constitute a serious threat to the security of the users, the integrity of the flown data, and consequently the safety of the vehicles’ drivers and passengers. Various security attacks and threats in vehicular networks  have been defined in the literature ,95,98,99Sybil Attack: An adversary creates multiple fictitious vehicles on the road from a single node/vehicle by allocating it different identities to appear as it represents separate entities. In other words, an attacker creates (and then controls) more than one identity on a single physical node. This implies that other entities in the network will not be able to judge if the data originate from a single vehicle or multiple vehicles. This attack can be used to affect the route selection of a target vehicle by creating a fake traffic jam on the road which forces the targeted victim vehicle to choose an alternative path.GPS Deception/Spoofing Attack: It refers to the interception and manipulation of the GPS signals while being sent between two legitimate IoV nodes, to mislead the receiving entity by providing wrong location information about the sending vehicles. This can directly harm the drivers and the passengers by affecting their path decisions. Additionally, when a GPS receiver of a vehicle is attacked, it could provide the vehicle with false information about its location, speed, and other GPS information, which can greatly affect some IoV applications that depend on this GPS information such as navigation devices.Masquerading/Impersonation Attack: A node pretends to be another node by stealing its identity. Here, both the legitimate vehicle and the adversary vehicle can use the same identity simultaneously which can create a chaos in the network since other vehicles may receive contradictory information from the same identity. The malicious node can also enjoy the access privileges of the spoofed identity.Worm Hole Attack: Also known as tunneling attack, refers to the attack where at least two malicious vehicles create a private tunnel known as a worm hole which is used to forward the intercepted data between the two of them . In thisReplay Attack: In this attack, an adversary keeps iterating the old messages in the network to deceive the other nodes by dropping the messages with high priority from the queue. This can greatly affect the system’s efficiency and increase the cost of the bandwidth.The above-defined attacks are examples of common security threats that can affect the confidentiality and integrity of IoV data, as well as the availability of the entire IoV system. The focus of this survey as mentioned previously is on blockchain-based authentication mechanisms in vehicular networks. We surveyed a wide range of the existing schemes in the literature and categorized them based on the type of the blockchain used for authentication into three categories: (a) private, (b) public, and (c) consortium blockchain-based authentication. These categories are discussed below.A private blockchain is a limited access blockchain in which only a particular group of trusted entities (which is decided by a network administrator) is granted access permissions to the blockchain transactions for performing specific tasks.The nature of private blockchains enables establishing a high level of trust during authentication since only a small group of trusted nodes are allowed to access the vehicles’ authentication parameters stored in the blockchain. Additionally, being controlled by a single organization allows easier, more efficient management and supervision over the authentication data kept at the ledger. Consequently, private blockchains are implemented by many researchers to serve authentication purpose in IoV platforms.The authors in  incorporThe authors also addressed the authentication of registered vehicles by adopting a Public Key Infrastructure (PKI) technology based on cryptographic accumulator to enhance the authentication efficiency. The authentication process is performed in two phases; the first phase is to verify the vehicle’s identity to the roadside unit, and the subsequent phase is the mutual verification between the RSU and the cloud server. In the first phase, the vehicle sends its ID and public key to the RSU which verifies the vehicle identity and adds its own public key to the vehicle information and sends them to the corresponding cloud service provider authority (CA) after encryption with the public key of this CA. Upon receiving the message from the RSU, the CA starts the second phase by first decrypting the message with its own private key, and then searches the vehicle ID and the public key of the RSU in the blockchain to check whether the provided information is correct. If successfully found, a new session key is generated by the CA and sent to the RSU along with the corresponding vehicle ID after encryption with the RSU public key. The RSU decrypts the packet with its private key, stores the session key for securing further communication with the CA, and sends the issued digital certificate to the vehicle encrypted with its public key, upon which the authentication process is culminated.The proposed solution is evaluated using Veins open-source framework through a small-scale network of 13 nodes in terms of the time overhead and communication cost needed for the encryption process related to the blockchain technology and for the whole authentication process. Their scheme proves to provide high authentication efficiency in preventing malicious attacks with low time and communication costs. However, large packet loss is encountered during the registration of the vehicles and the key distribution process.The authors in  discusseThe network model consists of four types of nodes: vehicle nodes (VNs) and RSUs which are both considered as untrusted nodes in their threat model, TAs which are assumed to be semi-trusted nodes, and a data center (DC) which is assumed as a fully trusted network entity that stores all vehicle-related information. A smart contract is used to automate part of the authentication process and the delegated proof-of-stack  is adoptThe authentication phase has five steps. In the first step, the VN enters the RSU communication range and issues an authentication request to the RSU. Then, the RSU forwards a part of the request message to the TA in the second step asking for the VN-related authentication parameters. In the third step, the TA executes the smart contract in the blockchain. It checks first whether P exists in the blockchain, then retrieves the VN-related authentication parameters from the DC according to P, and then sends these parameters to the RSU. Once the RSU successfully authenticates the TA and the VN, it sends the updated parameters to the VN and the TA in the fourth step. In the final step, after the TA successfully authenticates the RSU, it sends the updated parameters to the DC via the secure channel. Once the DC has updated the parameters, the TA sends acknowledgment message to the RSU. After the VN successfully authenticates the TA, it updates the authentication parameters in its memory. After the VN and the TA negotiate a session key, it remains valid as long as the VN lies within the communication range of the RSU. Once the VN leaves the communication range of the RSU, the session key will be revoked.A detailed security analysis is performed on the proposed scheme using ProVerif tool  to checkThe authors in  proposedThe proposed scheme has three phases: the registration phase where the vehicles interact with the registration server, the authentication phase and the authorization phase which are handled by the different service providers. The registration phase starts when the vehicle sends a registration request message containing its original id  to the registration center and then a key exchange process takes place using the Elliptic Curve Diffie Hellman (ECDH) key exchange protocol  to exchaThe authors in  suggesteThe scheme is composed of three phases, namely, system initialization, registration of the vehicles, and mutual identity authentication and revocation. In the first phase, the CA initializes the system parameters including the elliptic curve parameters and hashing functions. Moreover, the public key pairs are generated and transferred to the blockchain network entities, i.e., CA, RA, and RSUs. When a vehicle first registers with the CA, it submits its original vehicle id obtained from the motor vehicle’s division. The CA verifies the vehicle’s original id and assigns for it a Pseudo Id (PID) along with the Elliptic Curve Cryptography (ECC) public–private key pair. The PID is then signed digitally by the CA using the Elliptic Curve Digital Signature Algorithm (ECDSA) , formingWhen the vehicle becomes active on the road, it initiates an authentication request containing its PID, hash pointer, and transaction id to the nearest RSU. The RSU then uses the received PID as an index to query the blockchain for the vehicle’s respective transaction, if verified, the RSU sends a challenge message to the vehicle encrypted with its public key and waits for the reply. If the vehicle successfully decrypts the challenge message and sends the correct response, it is authenticated successfully by the RSU. Extensive simulation using Vein’s framework and OMNeT++ network simulator proves the efficiency of this scheme in terms of authentication delay, transaction throughput, and packet-delivery ratio.The authors in  proposedThe proposed message authentication scheme includes six phases: initiation, registration, message signing, message verification, block generation, and block confirmation. In the initiation phase, the RTA generates its public–private key pair, the genesis block of the blockchain and the list of LTAs. During the registration phase, a vehicle’s owner sends the vehicle’s information and its biometric information to the RTA which in turn generates a pair of asymmetric keys and transfers it along with the system key to the vehicle’s local memory. The registration of LTAs also takes place in this phase by the RTA in the same approach as the vehicles’ registration. When a vehicle needs to broadcast a message into the network, it computes the message’s hash value h(m) and concatenates it with the hash values of the previous blocks in the blockchain and encrypts them together with its private key. This arrangement is then attached as a header to the actual message content along with a timestamp and encrypted with the system key before being broadcasted to other vehicles. This process of message signing guarantees the legitimacy of the sending vehicle and thus the authenticity of the broadcasted information since the unauthorized vehicles cannot have the system key used for message signing as it is distributed only to the registered vehicles during the registration phase.As the V2V messages are broadcasted among the vehicles, the corresponding LTA keeps collecting those messages until it forms a block after a certain number of messages. The block is then verified through the PBFT consensus by the vehicles and their LTA. Once the block is verified, the LTA transfers it to the other LTAs for block confirmation process. If confirmed by all LTAs, they send the confirmed block to the RTA which then concatenates the block officially into the blockchain. The simulation results reveal that the proposed scheme can prevent impersonation attacks and single point of failure issue while providing a highly efficient blockchain-based authentication in terms of the total consensus delay and throughput.The above-mentioned private blockchain-based authentication schemes can be found in A public blockchain is an open access blockchain in which everyone can access, send, receive, and verify the different transactions of the blocks. Since it is a fully opened blockchain, even the vehicles can participate in the authentication process by looking up the ledger for the targeted authentication parameters. This provides a better utilization of the available computing resources in the IoV environment compared to restricting the authentication process to a few trusted nodes, thus a more decentralized, time efficient authentication process is achieved. These characteristics of a public blockchain have attracted many researchers to adopt it for IoV authentication.The authors in  contribuMICAz mote—a hardware wireless sensor module—is installed on two vehicles to serve as a wireless communication interface in the V2V arrangement. Measurements are recorded indoor and outdoor in real-time and reported with the aid of MATLAB R2020a. The Pearson Correlation Coefficient  is compuThe authors in  proposedAfter registration and successful authentication, a vehicle can communicate with other entities in IoV environment and exchange data in the data transfer phase. The deployed dPoW consensus mechanism allows the protocol to scale based on the incoming traffic generated by the vehicles and their use of PUFs makes the vehicles immune to physical and impersonation attacks. A detailed analysis is conducted through software implementation and NS3 simulation tool to evaluate their scheme in terms of the four-way tradeoffs of distributed systems which are scalability, decentralization, security, and latency. Two types of delay were measured: (1) the authentication delay at the RSU, which is time needed to authenticate a vehicle by the RSU and (2) the time to finality, which is the time needed to form a block, mine it, and reach a consensus on the mined block. The results show that their authentication scheme efficiently satisfies all the above-mentioned four properties without sacrificing any of them.The authors in  designedBeside the setup and registration phases, the protocol has other six phases, namely, message signing, authentication, group key management, blockchain formation, AI-based secure big data analytics using blockchain and dynamic nodes addition. During the authentication phase two types of authentications exist, i.e., V2V and batch authentication. The V2V authentication enables each vehicle to authenticate its neighbors in its cluster in the smart city. The batch authentication is then used by the RSU to authenticate all its clusters’ vehicles simultaneously which saves time and reduces the computation overhead of the whole scheme. After that, a group key is agreed upon and granted to each cluster to be used for securing communication among intra- and inter-cluster vehicles and their managing RSU. The blockchain formation phase is handled by fog and cloud servers in two steps; first, each fog server receives the list of transactions and their compact signatures from the associated RSUs and verifies them, if the signatures are valid, it transmits the partial blocks to a cloud server in the blockchain center. Then, the cloud servers convert the received partial blocks into complete blocks which are then mined and voted for through the PBFT consensus mechanism to decide on their eligibility to be added to the blockchain. The signature verification and block verification applied in this scheme ensures that data poisoning attacks which inject malicious transactions in the blockchain are avoided in the proposed BBAS-IoV protocol. This results in fully trusted blockchain transactions that can be a great asset for deriving highly accurate ML models and thus correct predictions and decision making can be achieved.Detailed formal security analysis using the Automated Validation of Internet Security Protocols and Applications (AVISPA) tool as well as informal security analysis is provided to evaluate the security features of the proposed BBAS-IoV scheme. The results obtained show that various security attacks namely, replay attacks, man-in-the-middle attack, vehicle and RSU impersonation attacks, privileged-insider attack, and ephemeral secret leakage attack are prevented. Extensive performance evaluation through simulation with the aid of Multi-precision Integer and Rational Arithmetic Cryptographic Library (MIRACL) reflects high efficiency of the scheme in terms of computation, communication, and storage overheads.The authors in  designedThe proposed scheme assumes the existence of a trusted registration authority that is completely responsible for initializing the system parameters including elliptic curve, one-way hash function and its own private–public key pair as well as enrolling all the entities in the network, i.e., cloud servers, edge servers, RSUs, and vehicles. The authentication phase then takes place in two steps: V2CH authentication in which a vehicle and its associated CH mutually authenticate, then CH2RSU authentication where the CH and its associated RSU authenticate each other. Upon a successful authentication process, each vehicle can securely report accident-related transactions to its associated CH if it detects an accident on the road. The cluster head then securely transfers the received transactions to its associated RSU which in turn sends them secretly to the corresponding edge server. The edge server prepares a partial block containing the accident-related transactions, the Merkle tree root, and a digital signature. This incomplete block is forwarded to its associated cloud server in the blockchain center forming a complete block from the received partial block. At this point, all the cloud servers in the blockchain center participate in the block verification process through the PBFT consensus and if verified, the complete block containing the vehicle accident-related information is added to the blockchain center and made available for use by other vehicles for optimal route selection and better road-related decisions.Using the Automated Validation of Internet Security Protocols and Applications (AVISPA) simulation tool, the proposed BCAS-VADN proves to be secure against multiple attacks including replay, man-in-the-middle attacks, impersonation and privileged-insider attacks, physical vehicle capture attack, and ephemeral secret leakage attack.The authors in  introducExtensive simulation using Vanet-MobiSim and NS2 demonstrates that the proposed BCPPA can resist several attacks such as replay attack, impersonation attack, DDoS attack, man-in-the-middle attack, stolen verifier attack, side-channel attack, message modification attack, birthday collision attack, and hijacking attack. The authors claim good efficiency in terms of communication cost, time cost, average packet delay, and packet loss ratio.The authors in  proposedThe DB-KMM provides an automatic public key management including update and revocation via the smart contract. For the update process, when a vehicle sends a key update request containing its ID, old public key, and validity period to the VSP, the VSP triggers the smart contract to generate a new public/private key and a new validity period for the requesting vehicle. The VSP then forms a new transaction containing the vehicle ID, new public key, and validity period and sends it to the BN. The RSUs forming the blockchain network perform the consensus on the received transaction and if the mining succeeds, the updated transaction is added to the blockchain, and the new public key is transferred to the requesting vehicle. Lastly, when a malicious behavior is noticed on some vehicle, the VSP initiates a revocation transaction to the BN and triggers the smart contract to remove the vehicle’s identity and public key from the blockchain.The performance of the proposed DB-KMM is tested in terms of the end-to-end packet latency and the packet loss ratio using OMNeT++ and Veins simulators and the results prove that it greatly improves the cost of public key management compared to the traditional PKI management. Further, security analysis shows that the scheme can resist DoS attack, public key tampering attack, internal attacks, as well as collusion attacks.The authors in  extendedThe root of CMT is considered as the transaction root whereas the root of MPT is taken as the certificate root. The transaction root and the certificate root are then written immutably in the blockchain. The significance of this extended blockchain being developed is represented by two aspects. First, it provides a simplified authentication technique whether a certain certificate is in the MPT or not. Given the certificate root and a record containing the nodes along the path, the authenticator can compute a hash using the given record. If the hash value is equal to the certificate root in the blockchain, it is proven that the certificate exists in the MPT. Second, it provides transparency within the activities of the authorities by using transactions roots; since, if the transaction root is given, it can be verified when a certain certificate is issued or revoked. Conditional privacy preserving is provided by allowing each vehicle to utilize several certificates, while the mapping between the certificates and the real identities is encrypted by the LEA’s secret key and stored in the blockchain and can only be revealed by the LEA in case of malicious behavior. Security investigation proves that BPPA is resistant to forgery attack, man-in-the-middle attack, replay attack, identity revealing attack, and authority abuse attack. Further, experimental results demonstrate the efficiency of the scheme in terms of communication and computation costs, low latency, and high throughput.The above-mentioned public blockchain-based authentication schemes can be found in A consortium blockchain, also known as hybrid blockchain, is a combination of both private and public blockchain in which the read access can be open or restricted, and only a small group of nodes belonging to different organizations is responsible for making the consensus.Since consortium blockchains incorporate the best of public and private blockchains, that is, a mixture of decentralization with good level of trust at the same time, it has been the most popular blockchain type being implemented by researchers for IoV authentication. In such way, the authentication can be performed with a high degree of trust since only a group of trusted entities perform the consensus of the authentication data blocks, while guaranteeing an efficient performance in terms of resource utilization and authentication time delay due to the semi-decentralized nature of consortium blockchains.The authors in  optimizeIn the BCA-TG protocol, each RCN has an Update Information (UI) vector containing the Local Information (LI) of all RCNs in the consensus network as elements. Initially, the UI of each RCN will have only its own LI while all the other LI elements corresponding to the other RCNs are set to null. For example, if 5 RCNs are used for consensus making, the UI of The authors in  handled The RSUs within a region serve as access points to the vehicles in the IoV paradigm and also as fog computing units for providing real-time services to authorized OBUs. The proposed BLA protocol includes five phases: initialization, registration, authentication, consensus, and service-delivery phases. The initialization phase takes place only once via the initial setup of system parameters by the audit department. Then, the registration of SMs and OBUs is conducted by the audit department during the registration phase in which each registered entity is allocated a pair of private and public keys using Elliptic Curve Cryptography (ECC) and Diffie–Hellman (DH) cryptography mechanisms. During the authentication phase, each SM authenticates the OBUs within its region, and the authentication results are then passed to the subsequent phase where they are written to the ledger by the consensus makers, i.e., WPs, through PBFT consensus protocol. The way they addressed the cross-datacenter authentication is by allowing a flexible option to vehicles to choose whether to be reauthenticated or not upon entering a new VFD during the service-delivery phase.The noninteractivity property that BLA has, in which a vehicle sends only one message to its SM for authentication or service requesting without the need for exchanging any acknowledgement messages between them, makes it lightweight in both communication and computation cost. In addition, the utilization of pseudonym preserves the privacy of vehicles and ensures anonymity. Security analysis proves that the proposed BLA protocol ensures most of the security aspects in IoV network, such as confidentiality, integrity, traceability, and non-repudiation. Moreover, extensive simulations are performed to measure the performance in terms of response time. The results obtained show that low time overhead is achieved which reflects the suitability of BLA algorithm for real-time VFS.The authors in  improvedThe authors in  proposedDuring the system initialization phase, the TA initiates the system parameters including the ECC, the ABE, and the blockchain parameters. A smart contract is then deployed by the TA into the blockchain to automate the authentication process. Each vehicle then needs to be registered with the TA to obtain its secret authentication parameters during the registration phase. First, the owner of a vehicle must choose a physical identity, a password along with his/her biometrics which will be sent to the TA. The TA then calculates a corresponding blinding identity (AID) and a Vehicle Public Key (VPK) from the received parameters and uploads them as a unique tuple {AID, VPK} to the Vehicle Public Key Table (VPKT) in the smart contract. The AID is also sent to the vehicle’s OBU which is assumed to be tamper-proof. When a vehicle wishes to send data to nearby vehicles or RSUs, the vehicle’s owner provides the login information  which are verified by the OBU. If correct, the OBU encrypts the vehicle’s AID using the ABE and sends it along with the message and a timestamp to the receiver , otherwise, the OBU rejects the message. The receiving entity then validates the freshness of the message through the timestamp. If valid, it initiates a transaction to the blockchain managers requesting for the associated vehicle public key. The blockchain managers then lookup the VPKT using the AID to obtain the corresponding VPK which is then transmitted to the transaction issuer to be verified. Upon successful validation, the authentication is completed, and the message is accepted.BPAS also supports conditional traceability and vehicle’s revocation in case of detecting any malicious behavior by simply allowing the TA to delete the corresponding tuple in the VPKT. The proposed scheme is evaluated in terms of time cost and security features and is found to be resistant to replay attacks, impersonation attacks, DDoS, and password guessing attacks.The authors in  proposedThe proposed scheme contains five phases: initialization, registration, mutual authentication and key exchange, consensus, and certificate update. A central secured entity, named TA, is responsible for initializing the system and the public parameters used for the cryptographic functions, as well as registering the OBUs and RSUs during the first two phases of EASBF. Then, mutual authentication and key exchange takes place between OBUs and BMs, and a session key is shared among them for subsequent interactions. Upon successful authentication and key exchange, the BM shares the results of authentication with all AMs, which store them into a new block and add it to the large public register under the PBFT consensus. During the consensus phase, one of the AMs acts as a “Speaker” which is responsible for initiating the consensus process while the rest serve as the congress members who participate in the voting scenario initiated by the Speaker. Finally, the certificate update phase which supports two scenarios: the ability to move from one fog area to another transparently without the need of re-authentication and certificate update upon a vehicle’s request. The Random Oracle Model (ROM)  and AVISThe authors in  proposedThis arrangement in which the blockchain keeps only pointers to the pseudo-ids while the pseudo-ids themselves are kept in the unlimitedly huge TCS improves the scalability of the system. In addition, an illegal vehicle can be determined by checking whether its pseudo-id has a revocation tab instead of looking up the whole certificate revocation list which greatly reduces the computational overhead. Further, detailed security analysis proves that the proposed scheme can resist replay attacks and prevent single point of failure problem.The authors in  addresseThe scheme is composed of two phases: offline registration phase and authentication phase in which each RSU authenticates a group of vehicles simultaneously as a batch which helps significantly in reducing the computation cost. Elliptic curve cryptography, one-way hash function, and bilinear pairing are utilized for generating secret key pairs and session keys during the offline registration phase and for securing communication during the authentication process. In the authentication phase, the shared session key of a vehicle is constructed independently which helps mitigate the interference in the regular V2R data exchange. In addition, an efficient group key management mechanism that employs the Chinese Remainder Theorem (CRT)  is suggeThe authors in  designedThe UTXO data structure is formed by three key items, namely, basic, in, and out items. The basic item includes the transaction id, the operation name, the timestamp, and the signature of the requester to prove its ownership. The in item represents the information of the sending entity while the out item includes information related to the receiving entity. The authors define several operations that are as follows. The Issue operation is used by trusted authorities to generate new tokens for the entities, which can take place only upon two circumstances: the initial enrollment of a vehicle and periodically generated for well-behaved entities. Similarly, the Query operation can be used to retrieve the UTXO of a transaction on the blockchain through transaction id in order to check the trustworthiness of a sending entity. The Revoke operation is the key operation introduced by the authors for revocation management instead of the conventional certificate revocation list that implies extra storage and computation overheads. However, the main operation used for authentication purposes is the Transfer operation and the procedure is as follows: when an entity The above-mentioned consortium blockchain-based authentication schemes can be found in The choice of the consensus mechanism used by the blockchain depends on many factors such as the amount of power consumption the system can afford, the percentage of faulty nodes the network can tolerate, and others. However, the size of the IoV network is also an important factor to be considered. For example, although the PBFT consensus is lightweight in power consumption and has a good fault-tolerance of approximately 33%, it does not support high network scalability; thus, it is chosen only by the small-scale IoV platforms such as in , or by fSome of the papers only considered a V2V model to investigate their authentication schemes such as , while mMost of the papers support anonymous authentication schemes, while some of them provide pure authentication without supporting such privacy preservation option as in ,126,128.In terms of performance analysis, the use of blockchain for authentication in IoV and VANETS networks introduces the need to include additional evaluation measures that reflect the efficiency of the blockchain itself such as the throughput which refers here to the number of transactions that can be verified and added to the blockchain per second ,127,139,Regarding the security analysis, the different authentication schemes have shown uneven levels of resistance against the various security attacks introduced in the previous sections.In To conclude, a comprehensive view of the recent blockchain-assisted authentication schemes in IoV and VANETs was provided in this survey while highlighting the main differences between them in terms of security performance and operational performance. This helps in identifying the research gaps and creates a map that guides the future researchers in this area of IoV blockchain-based authentication. Although the adoption of blockchain for IoV authentication has brought many benefits such as the increased time-efficiency due to the decentralization of the authentication scheme among the distributed blockchain servers, and the more secure authentication reflected by the high resistance against different security attacks, a lot of work still must be done. This area needs more research and development to design new IoV-specific consensus mechanisms that can provide a more efficient balance between scalability and power consumption by consuming less power, supporting highly scalable IoV networks while maintaining the same good level of security represented by the high fault tolerance.In this section, a number of research challenges in IoV security that need to be addressed as well as some potential research directions to be explored are suggested.Despite the many advantages that blockchain technology has brought to the field of IoV authentication represented by a decentralized, autonomous, fault-tolerant, and more secured authentication protocols which are all properties that are required by any efficient authentication process, many challenges also arise with blockchains when being employed for authentication in environments with high performance expectations and critical security requirements such as the IoV. Most of these challenges are associated with the consensus mechanisms used by the blockchain, as different consensus protocols show uneven levels of support for the various IoV requirements such as scalability, fault tolerance, power consumption, as well as real-time response.(1)Scalability: The scalability of a consensus mechanism depends on its way of reaching the consensus, whether it is Proof-based or Vote-based approach. Since Proof-based consensus protocols such as PoW and PoS do not require all the network entities to submit their individual decisions on the information to be verified, their scalability is not affected with more nodes being added to the network as at the end of the day they allocate the role of announcing the conclusions of all the participating nodes about the information to a single node. Thus, they can be suitable for large-scale networks such as the IoV environment. However, the huge amount of power consumed by these Proof-based consensus approaches undermines their efficiency and their suitability for all IoV environments especially the resource constrained ones. On the other hand, the Vote-based blockchain consensus making approaches such as the PBFT consensus mechanism exhibit negligible power consumption, yet their scalability is restricted to small-scale networks with limited number of nodes since all the nodes in the network are engaged in transaction verification and should submit their individual votes in order to reach consensus. For such a trade-off that is faced by the existing consensus mechanisms when employing blockchain for IoV, the challenge is represented in how to tune and improve these mechanisms in the future to be able to support efficient authentication in the highly scalable resource constrained IoV environments.(2)Fault tolerance: the different blockchain consensus mechanisms have uneven capabilities in terms of how many faulty nodes each can tolerate while still being able to ensure the integrity of the participants and data in the network. In such wide and publicly opened environments as IoV where a variety of attacks can be encountered in which attackers impersonate the authentic nodes, high fault-tolerant consensus protocols should be adopted to ensure that the integrity of the communicated data can still be guaranteed even if the authenticity of a considerable percentage of participants is compromised. Some existing consensus protocols such as PoW can offer the high fault tolerance required for IoV environments; however, as discussed before it exhibits a large power consumption which can be an issue when used for authentication in some resource constrained IoV platforms. Thus, the need appears for developing new consensus making approaches that can guarantee high fault tolerance for the vulnerable IoV arrangements while maintaining an acceptable level of power consumption. Alternatively, mitigating the power consumption effect of the already existing blockchain consensus protocols by finding other solutions such as the charge-as-go solution represented by providing mobile charging units to charge the vehicles’ batteries as they move to be able to tolerate the high power consumed during the authentication is a challenging proposal to be explored in the future.(3)Communication mode: Due to the two types of communication used by the different consensus mechanisms which are synchronous and asynchronous communication modes, different time responses are expected. In the asynchronous mode, the sending entity does not wait for acknowledgement from the receiving node on a previous request, instead, it directly proceeds with the following communication steps. The consensus protocols that use this mode of communication such as Proof of Work (PoW) and Proof of Time (PoT) could be thought of as perfectly suitable to be used for authentication in the different IoV applications which are mostly time-critical applications in which even a small delay of milli seconds is not forgivable. However, when considering some availability issues scenarios where the receiving entity can be temporarily out of network, waiting for an acknowledgement from the receiving entity before proceeding on with further interaction would save a lot of time and bandwidth that were used for such useless communication. Therefore, developing novel consensus making protocols that combine synchronous and asynchronous modes into one hybrid mode that incorporates the advantages of both communication modes, for example, by setting some thresholds for the number of acknowledgments to be received before proceeding the rest of communication asynchronously, or by using timeouts. However, such proposed solution is in turn full of challenges, since this hybrid mode of operation needs to be repeated periodically and regulated with other thresholds, that is, the synchronous mode should be injected once after every consecutive asynchronous interaction to check periodically that the receiving entity can be reached. This arrangement imposes an extra overhead associated with designing the optimal thresholds, monitoring, and managing the different time windows and timeouts. Thus, the feasibility of developing such complicated hybrid consensus protocols to be used for IoV authentication while maintaining a relatively low operational cost is another future challenge to be investigated.Thus, to design an efficient authentication protocol for IoV based on blockchain that maintains a good performance balance while highly considering all the above-mentioned factors is a great challenge that should be addressed by future researchers. In the way to achieve this, serious efforts should be dedicated to developing all-in-one IoV-specific consensus mechanisms that can meet all the requirements of IoV applications including high scalability, high fault tolerance, real-time response with low energy consumption. Beside these factors, another important challenge in this area to consider is how to achieve the optimal assignment of the various blockchain-related tasks such as blocks creation, validation, and consensus making to the different IoV nodes based on their computing capabilities and the energy consumption requirements of the adopted consensus protocols in order to achieve an efficient authentication process. In  for instIn this survey paper, we targeted the adoption of blockchain in IoV for the authentication security requirement. However, we believe that blockchain can be of great benefits for other IoV security requirements such as data integrity, secure routing, and availability which can all be explored and discussed as part of the future research directions.(1)Data Integrity: Blockchain can be adopted by IoV environments to ensure that the data sent and received between two communicating IoV entities are identical. In other words, to make sure that no unauthorized modifications on the data take place during transmission. For instance, when a sender aims to send some data to another IoV node, it can send a copy of the data to the publicly accessible blockchain as well. At the other end, when the receiving entity receives the data, it can compare it with the copy stored in the immutable blockchain, if matched that means no manipulation was performed and data integrity is guaranteed. Otherwise, the received data cannot be trusted. The use of blockchains for ensuring data integrity in IoV may become even more important to explore when considering using them for big data analytics and data mining which require the data to have high level of accuracy to result in developing accurate and efficient decision making and AI-based applications.(2)Secure Routing: Blockchain technology can be used for guaranteeing a secure routing in IoV environments via different arrangements. An example could be maintaining a list of all possible legal next-hop nodes in the public ledger which can be accessed by all IoV entities. This list should be firstly developed by gradually adding the ids of the nodes that were successfully registered and authorized by the network. This means that any malicious unauthorized node will not be valid in this routing list. In this way, when an entity receives a packet that was routed through at least one illegal forwarder that does not exist in the routing list kept in the blockchain, this means the routing processes have been compromised and some illegal entity had access to the transmitted packet.(3)Availability: A feasible suggestion that could be investigated is the integration of blockchain with some physical identification modules in an attempt to prevent some availability attacks and threats. That is, obligating each single service request to include the physical identity of the request originator, e.g., MAC address of the NIC of the user’s mobile device or the vehicle’s hardware id assigned by the vehicle manufacturer. Then, upon receiving the request by an infrastructure node, i.e., a RSU, it should extract the physical identity of the requester and store it as a record in the blockchain while keeping track of a counter that counts the number of successive requests and a timer to monitor the time intervals between these successive requests. In such arrangement, availability attacks specifically the denial of service (DoS) attack that originates from a single physical entity with different logical identities, i.e., IP addresses, can be efficiently detected and thus blocked by any IoV entity through monitoring the public ledger records and noticing any extraordinary behavior of rapid successive requests originated from the same physical id.The above-mentioned examples are a few humble suggestions for using blockchain in the context of the different IoV security requirements. However, there is a strong belief that blockchain capabilities can cover more aspects and offer a variety of solutions regarding these IoV security concerns, yet to be explored.Since IoV paradigm is based on big data and high-performance computing, cloud computing infrastructures constitute the most important building blocks for providing data storage. Even in blockchain-assisted IoV platforms, the cloud servers (CS) cluster constitutes the blockchain network.Thus, developing the cloud technologies is a critical aspect to guarantee the security and the privacy of the blockchain-based authentication system in vehicular networks. Cloud-related security and privacy issues might be encountered during the process of transmitting vehicles’ data to/from the cloud or while being stored in the different cloud servers. This imposes different security challenges in both processes which should be addressed in the future to guarantee a high level of efficiency for the blockchain-based cloud-assisted IoV authentication systems.Data transmission challenges: Transmitting data back and forth between the cloud servers, the vehicles, and the other IoV entities via an open wireless network may impose high risks on the security of the blockchain-based authentication system and the privacy of the users. Some schemes attempt to mitigate the security risk by adopting wired communication wherever possible, i.e., between the cloud servers and the intermediate nodes. However, this arrangement is not practical nor feasible for all IoV-based scenarios. Moreover, this can only mitigate the risk at the stationary end of the system while the mobile part, i.e., the V2I communication is still exposed to different security threats due to its wireless nature. Thus, the existing wireless communication protocols should be well investigated and developed to insure private and secure exchange of users’ authentication data between the different parts of the IoV-based system.Data storage challenges: Having users’ sensitive data such as the authentication data including users’ real identities, public keys and certificates stored in a common-access platform such as the cloud is another threat to consider. The privacy of users is threatened to be violated if no proper encoding schemes exist within the cloud. Thus, the need for developing efficient encryption algorithms that can be adopted in both data storage and transmission to protect the privacy and security of the whole IoV cloud-based paradigm while ensuring full compatibility with the rapidly evolving and heterogenous cloud technologies is another important challenge which we hope to be tackled in the future.In this survey, security aspects of the emerging vehicular technology, IoV, and the preceding VANETS which have made an evolution in the intelligent transportation systems were discussed. The power of the emerged blockchain technology in general and specifically in IoV was also highlighted. Moreover, different security requirements, challenges, and potential security attacks and threats in vehicular networks were presented. After that, more focus was dedicated to discussing a wide range of recent blockchain-based authentication techniques in IoV and VANETs environments and a comprehensive comparison between them was then provided. At last, some possible security challenges and research directions in IoV and VANETs that need to be addressed in the future were highlighted. In this paper, we focused only on the conceptual comparison between the surveyed blockchain-based IoV authentication schemes, i.e., in terms of the different techniques, network architectures, and evaluation tools used, as well as features and limitations. However, we believe that including more quantitative measures in comparisons is a direction that can be considered and improved in future surveys.


There is a need for more knowledge on how people with substance use problems (SUPs) understand and experience user involvement when receiving care. In this systematic review, we identify and reanalyse the existing qualitative research that explores how people with lived experiences of substance use understand user involvement, and their experiences of key practices for achieving user involvement. We systematically searched seven electronic databases. We applied Noblit and Hare’s meta-ethnography, revised by Malterud, to identify, translate, and summarise the studies. The electronic search resulted in 2065 articles. We conducted a full-text evaluation of 63 articles, of which 12 articles met the inclusion criteria. The primary studies’ synthesis reveals three different understandings of user involvement: user involvement as joint meaning production, points of view represented, and user representation in welfare services. Key practices for achieving user involvement involved seeing and respecting the service user as a unique person, the quality of the interactional process, and the scope of action for people with SUPs, as well as professionals, including issues of stigma, power, and fatalism. The metasynthesis recognises the ambiguity of the concept of user involvement concept and the importance of including the service user’s perspective when defining user involvement. The analysis of key practices emphasises the importance of relational processes and contextual aspects when developing user involvement concepts. User involvement in substance use care is a fundamental goal of welfare services and has been emphasised in a number of strategies, plans, and declarations ,2,3. HowSecond, the conceptualisation of user involvement is complex, variable, and multidimensional ,2,3. TamThird, people with experiences of substance use have a different understanding of key concepts related to user involvement than researchers and health professionals. Bee et al.’s  systematThere is a need for more knowledge of how people with SUPs understand, define, and experience user involvement in substance use care. In order to fill this knowledge gap, we developed a study to explore and synthesise qualitative research on the perspectives of people with SUPs on user involvement. Meta-analyses of the included qualitative studies provide a systematic description and synthesis of the current knowledge base, which are essential for achieving the goal of user involvement in research and professional practice. In this systematic review, we identify and reanalyse the existing qualitative research that explores how people with lived experiences of substance use understand user involvement, and their experiences of key practices for achieving user involvement. The aim is to gain a thorough understanding of the perspectives of people with SUPs on user involvement.We chose meta-ethnography as our method of analysis, which is a stepwise strategy for synthesising findings across qualitative studies , revisedWe exported the search results (2065 hits from the systematic search and 6 hits from the hand search) into the reference citation manager EndNoteX6 and removed duplicates (N-667). We then exported the references into the systematic review app Rayyan. Three independent reviewers  examined the titles and abstracts, and excluded irrelevant reports and papers using the inclusion and exclusion criteria (N-1340).(1)The study was an empirical study, written in English, and published in a peer-reviewed journal;(2)The study included qualitative data that explored the understandings and experiences of user involvement in substance use care from the perspective of people with lived experiences of substance use. In cases of mixed-method studies, only the qualitative results were included;(3)The study sample consisted of people who were 18 years old or older, had lived experiences of substance use, and were receiving help from the welfare system to reduce any difficulties resulting from drug use;(4)The study was published between January 2008 and June 2021.The included articles met the following criteria:Furthermore, any articles that did not focus specifically on user involvement were also excluded. All reviewers conducted a full-text screening of 63 publications, assessed their relevance, and excluded irrelevant papers (N-51). L.B.S., H.A., and B.M.F. evaluated the quality of the 12 eligible publications independently, and in accordance with the Critical Appraisal Skills Programme (CASP) checklist. Any disagreements in the screening and quality check process were resolved through discussions until a consensus was reached. An overview of the data collection can be seen in the flow diagram in This meta-ethnography is based on findings from 12 primary studies conducted in Australia, Canada, Finland, Ireland, Norway, and the UK between 2008 and 2019 . The studies included a total of 346 persons with experiences of substance use problems, with samples ranging from six to 83 informants. Although not all studies give details of the demographic variables of the participants, our impression is that they are broadly representative of age groups (18–69) and of male and female participants . Seven studies were based solely on open-ended or semi-structured individual interviews, one study was based on focus groups, and four studies were based on combinations of these and other data sources, such as field notes. Six studies conducted thematic analyses, two were based on grounded theory, three used different variations of a participatory analysis, and one conducted a phenomenographic analysis. Six of the studies used the concept of service user involvement or user involvement, while the other six studies featured concepts, such as collaborative practice, consumer participation, empowerment/decision-making, peer engagement, stakeholder involvement, and the user’s choice/self-determination. Eight studies examined service user’s experiences being involved in their own drug treatment or substance use care. Three studies explored experiences from people engaged in user organisations providing services to others. One study examined drug users’ perspectives on peer engagement in health and harm reduction settings.The results of these primary studies can be considered as the first-order analysis. Our analysis is a second-order analysis in which we synthesised the results of the first-order analysis. In line with Noblit and Hare , all autStarting with the index article, we used Microsoft Excel to systematically code the content of the studies and to relate the studies to each other. The interpretation was idiomatic, and focused on the meaning content rather than the literal equivalents . The stuThe reconceptualization achieved by the reciprocal analysis resulted in two categories: the fluid concepts of user involvement and key practices.Three studies ,27,28 exIt was evident in many of the primary studies that user involvement is understood as professionals engaging in meaning production, together with the user of the welfare services. In other words, user involvement was associated with the professionals arranging conversations with service users, wherein they gave them relevant information and assistance in reflecting upon what they wanted. User involvement understood as a joint meaning production may include decision making, but was not limited to making decisions. Fischer and Neale  and FiscUser involvement was associated with the informant’s subjective feeling that their humanity was recognised and acknowledge, and that their story was heard and respected ,32,33. AAnother understanding of user involvement evident in the primary studies was that the service users could make their own decisions and were equally involved in decisions about the service offer relevant to them. This understanding highlighted that people with lived experiences of substance use have knowledge that others do not have and, as a result, the best expertise . TherefoIn relation to service users having their views heard, some studies indicated that service users were in different places in the recovery process, which had implications for how much their opinions should be emphasised . For exaA third understanding of user involvement visible in the data material was the engagement of people with lived experiences of substance use to develop or participate in welfare services. The focus was on how they could use their knowledge and skills to deliver help and support services. Different variants of ex-users participating in the welfare system were highlighted, such as the employment of ex-drug users , and peeWe then examined the practices that were highlighted as being significant to user involvement. We investigated the different understandings of service user involvement to identify the key elements of a practice to be considered as involving service users, and the crucial components of a practice to not be considered as involving service users. This analysis resulted in three essential practices. A common feature of the primary studies was the identification of a gap between the ideals of user involvement and reality. The three essential practices were highlighted, both in terms of challenges in achieving user involvement and conditions for promoting user involvement. A consistent feature of the primary studies was an emphasis on the importance of being seen, understood, respected, and met as a unique human being. This was expressed by service users through statements such as “having their ‘humanness’ recognised and acknowledged”  (p. 33);The importance of seeing the service user as a person and not as a category was relevant for user involvement at the individual and system levels. It was also relevant to how the professional was seen by the service user, and emphasised the importance of understanding how professionals thought and the reasoning behind their actions. This was expressed by a service user in Rance and Treloar’s  investigThe quality of the interaction between people with lived experiences of substance use and the staff they met was fundamental to the feeling of being seen and respected as a unique person. The included studies presented a wide range of accounts and descriptions about the quality of the interactions. Descriptions such as “everyone’s working together”  p. 33);;33] (p. In studies that explored projects aiming to enhance user involvement, the characteristics used to describe the positive changes were closely linked to changes in how the participants interacted. Rance and Treloar  describeSimilarly, challenges related to user involvement were also related to challenges in the interaction between clients and staff. Patterson  noted a Another practice that was highlighted as necessary for user involvement was that the participants had a real room for manoeuvre. The need for flexibility applies both to the person with lived experience of substance use when meeting with professionals, and the professionals within their own system. The analysis of the primary studies showed that the participants appreciated floating and flexible services . HoweverLarsen and Sagvaag  observedThe need for room for manoeuvre was also linked to the fact that people with experience of SUPs find that they do not have the time or availability to engage in user involvement. Several studies ,27,35,36Several primary studies pointed to the significance of changing the scope of action in order to improve user involvement. Achieving increased freedom of choice for the service users and their helpers strengthened user involvement ,34. FurtThe findings of this review raised two main topics: the fluid understanding of user involvement and key practices. The fluid understanding consisted of three different understandings of user involvement: joint meaning production, points of view, and representation in welfare services. Key practices included being seen and respected as a unique person, the quality of the interactional process, and the scope of action, including issues of stigma, power, and fatalism. The fact that our review points to ambiguous and fluid understandings of user involvement confirms previous studies that have described a lack of consensus on the definition of user involvement ,2,3. We The key practices that we found point towards essential areas that should receive attention in research and clinical practice to improve user involvement. Many of the participants in the primary studies highlighted the gap between the ideals of user involvement and the actual outcomes in practice, and several of the studies that were included highlight the societal marginalisation of people with SUPs, the illegality of substance use, and the hierarchical and sometimes conspicuous tradition of many of the therapeutic approaches in the field. These might be regarded as significant institutional and cultural barriers to implementing a practice that addresses user involvement. Our findings also indicate that stigma is an essential theme. This is in line with Ti et al.’s  narrativOur findings also indicate that there is a need for a dialogical turn and for a greater research focus on processes rather than a limited focus on specific decisions. The analysis of key practices points to the relevance of the conversations between the service user and the professional. Based on the review, we argue that these conversations themselves should be the subject of close examination and that it is important to see conversation as an important professional tool for enhancing user involvement.For clinicians, the findings of this review should be promising, as the service users highlighted understandings and practices related to the professional role, values, and skills, which could feasibly be used to improve user involvement. However, the analysis also poses a significant challenge on a more systemic level to address the power differences and stigma within the services, and enhance the possibilities for staff to manoeuvre in ways that make involvement worth the effort, and organise the services accordingly.The strengths of this study are evident in its transparency of the study, as the inclusion of studies was determined through a detailed and transparent analytical approach by three independent researchers, and the review was conducted according to PRISMA guidelines . HoweverThe results of this metasynthesis provide a consolidated picture of three different understandings of user involvement. These results suggest that while the term “user involvement” is commonly used, it is a fluid and ambiguous concept. Since understandings of the concept have consequences for expectations of user involvement, practices, and focus areas, these findings have implications for improving both the research on user involvement and future clinical practice to ensure user involvement. Due to the ambiguity of the concept, the analysis highlights the importance of continuously examining how user involvement is understood and including the service users’ perspectives when investigating and defining user involvement. Furthermore, the study synthesises key practices due to user involvement. The metasynthesis underscores that there are significant shortcomings linked to the realisation of user involvement, and the findings suggest a focus on relational and contextual factors as stigma, power and fatalism to encourage user involvement.