Blockchain-based Multifactor Authentication for Future 6G Cellular Networks: A Systematic Review

: There are continued advances in the internet and communication ﬁelds regarding the deployment of 5G-based applications. It is expected that by 2030, 6G applications will emerge as a continued evolution of the mobile network. Blockchain technology is one of the leading supporting technologies predicted to provide a secure and unique network to 6G-enabled devices, transactions, and applications. It is anticipated that the 6G mobile networks will be virtualized, have cloud-based systems, and aim to be the foundation for the Internet of Everything. However, along with the development of communication technologies, threats from malicious parties have become more sophisticated, making security a signiﬁcant concern for the 6G era in the future. Despite enormous efforts by researchers to improve security and authentication protocols, systems still face novel intrusion and attacks. Recently, multifactor authentication techniques (MFA) have been deployed as potential solutions to attacks in blockchains. The 6G applications and the cellular network have speciﬁc vulnerabilities that need to be addressed using blockchain-based MFA technologies. The current paper is a systematic review that discusses the three technologies under consideration; then, several studies are reviewed that discuss MFA techniques in general and use blockchains as potential solutions to future security and authentication issues that may arise for 6G applications.


Introduction
Blockchain technology involves recording information that makes it hard to change, cheat, or hack the system [1,2].An exciting aspect of blockchain is the application with authentication, which formulates a system with multiple layers of security and authentications [3].Blockchain-based mechanisms solve authentication problems for distributed ledger technologies [4][5][6].Still, a significant amount of work is required to assist with machine learning models to predict incoming attacks or security threats [7].So, similarly, the 6G applications and cellular networks also have specific vulnerabilities, which need to be addressed through the application of blockchain-based multifactor authentication technologies [8].As the former applications are mainly based on artificial intelligence Visible light communication (VLC) technology, both are significantly vulnerable when it present information extracted from the reviewed articles published between 2015 and 2021; thus, the study provides more up-to-date and recent information.
The selected journal will be reviewed based on different aspects, including the studies' strengths, weaknesses, methodologies, data sets, and evaluation metrics.Furthermore, based on the review of the relevant articles and studies, this study provides details regarding the recent trends related to blockchain-based multifactor authentication and 6G cellular network strategies and integration.Based on these, the study includes the limitations and future indications.This study significantly contributes to explaining the multifactor authentication, blockchain and 6G-based phenomenon, strategies, integrations, and benefits and helping the users in decision-making processes regarding the adoption of the up-to-date systems of blockchain, multifactor authentication 6G mobile networks.Table 1 represents the comparison of the study with the other similar and relevant review articles.The table represents whether or not the study in comparison is a systematic study, blockchain-focused, MFA-focused, 6G-focused, and focused on future trends of these technologies.[9] 2021 × [14] 2020 × [13] 2021 × [20] 2020 × [22] 2020 × This study 2022 The rest of the paper is organized as follows: Section 2 details the research methodology used in this research.Section 3 discusses the 6G cellular concepts and security needs.Section 4 explains the Multifactor authentication schemes, followed by a blockchain discussion in Section 5. Section 6 provides details regarding recent studies that have listed some MFA techniques or applications of MFA to a blockchain.Section 7 details the observation based on this review study to highlight the current and future trends and the research challenges.Finally, Section 8 concludes this research study.

Methodology
The study has been conducted systematically in view of different research and experimental studies on the multifactor authentication techniques, preferably including the concepts of multifactor authentication, 6G, and blockchain.We focused on the published journal articles ranging from the year 2015 to the year 2021.A systematic approach is adopted for the review process, which is the type of methodology utilized and applied to identify, examine, and extract needful information from literature relevant to a specific research topic.The review has been done in two different phases.The first phase involves the identification of information resource that was the search engine and keywords that were utilized for the execution of the query.Whereas the second phase includes the application of specific criteria onto the initial generated list so that only the most relevant articles are shortlisted for the final list of the papers reviewed for the systematic literature review.Figure 1 depicts both phases of the review process adopted in this study.
We conducted comprehensive research in this review.In the first phase, the keywords and the search engine are identified for searching the articles.The researcher critically evaluated the research published in the English language between January 2015 and December 2021.The reason for focusing on these years is that the 6G technology research emerged in this era.For this study, "Scopus document search" and "Ebsco databases" are the chosen search engine because of their efficiency in searching through all authentic databases.The search keywords include "multifactor authentication in blockchains", "multifactor authentication in blockchains, and 6G cellular network".Once the search query is executed, the initial list of the articles is obtained.Retrieval run on both databases resulted in 299 papers, 191 papers collected from Scopus, and 108 from Ebsco.At the end of the process of overviewing and removing duplicated articles or whose full text could not be retrieved, 124 articles were left of the initial research of 299, 71 papers were collected from Scopus and 53 from Ebsco.After this, the abstracts were reviewed before the complete screening of material.In this stage, a further 41 articles were dropped.The final set of 73 papers was included in the review; 57 were retrieved from Scopus and 16 from Ebsco.All the retrieved references from the various databases included in the study were then added to the software used in this study for reference management, i.e., Endnote Version 9.2.Table 2 shows the papers included in this review in each domain.
x FOR PEER REVIEW 4 of 18 We conducted comprehensive research in this review.In the first phase, the keywords and the search engine are identified for searching the articles.The researcher critically evaluated the research published in the English language between January 2015 and December 2021.The reason for focusing on these years is that the 6G technology research emerged in this era.For this study, "Scopus document search "and "Ebsco databases" are the chosen search engine because of their efficiency in searching through all authentic databases.The search keywords include "multifactor authentication in blockchains", "multifactor authentication in blockchains, and 6G cellular network".Once the search query is executed, the initial list of the articles is obtained.Retrieval run on both databases resulted in 299 papers, 191 papers collected from Scopus, and 108 from Ebsco.At the end of the process of overviewing and removing duplicated articles or whose full text could not be retrieved, 124 articles were left of the initial research of 299, 71 papers were collected from Scopus and 53 from Ebsco.After this, the abstracts were reviewed before the complete screening of material.In this stage, a further 41 articles were dropped.The final set of 73 papers was included in the review; 57 were retrieved from Scopus and 16 from Ebsco.All the retrieved references from the various databases included in the study were then added to the software used in this study for reference management, i.e., Endnote Version 9.2.Table 2 shows the papers included in this review in each domain.In the second phase, we analyzed the initial list.We then defined a specific criterion for obtaining the articles that are significantly relevant and more focused on the multifactor authentication in blockchains and 6G cellular network for the final systematic review.Book chapters and survey/review-based articles are excluded to select more focused articles for this study.Additionally, articles are included that propose some multifactor authentication techniques and are written in English.

Concept and Development
With the research community involved in discussing possibilities and opportunities that may be opened up with the materialization of 6G technology, most countries in the world are still caught up in the deployment of the 5G technology.However, it is hypothesized by most researchers that 5G and Beyond 5G (B5G) technologies, once fully deployed, will be capable of enabling the Internet of Everything (IoE) to truly take off [23,24], leading to justifying the massive demands for 6G.The sixth generation of wireless technology (6G) will focus on communication between connected machines, i.e., thing-to-thing connection instead of people-to-people links in 1G-4G and people-to-thing communication as in the focus of 5G, as represented in Figure 2.
articles for this study.Additionally, articles are included that propose some multifact authentication techniques and are written in English.

Concept and Development
With the research community involved in discussing possibilities and opportuniti that may be opened up with the materialization of 6G technology, most countries in t world are still caught up in the deployment of the 5G technology.However, it is hypot esized by most researchers that 5G and Beyond 5G (B5G) technologies, once fully d ployed, will be capable of enabling the Internet of Everything (IoE) to truly take off [23,24 leading to justifying the massive demands for 6G.The sixth generation of wireless tec nology (6G) will focus on communication between connected machines, i.e., thing-t thing connection instead of people-to-people links in 1G-4G and people-to-thing comm nication as in the focus of 5G, as represented in Figure 2. In the past, a new wireless communication standard emerged after around a decad and given this trend, it is expected that we would be witnessing 6G around 2030 [25][26][27] As more and more users are connecting to the internet and using a large number of d vices connected to the Internet, big changes and challenges are coming up for intern research.The research communities are already looking towards solutions to the cha lenges posed by 5G mobile communication.It is expected that many of these challeng will be addressed by the time 6G materializes [20,28].

Security Needs
While 6G applications and communication technologies will be powerful and a re olution, there will be many specific vulnerabilities [20].Communication, access contro malicious behavior, authentication, and encryption-related issues will be faced in the applications (see Figure 3 below).It can be seen in the figure that 6G will support auto omous systems powered by A.I. and ML, multi-sensory X.R. applications built upon m lecular communication technology, the THz technology, and the quantum communic tion technology and distributed ledger technologies that will be mainly developed usin blockchains, etc. A.I. technologies and multi-sensory X.R. applications will be susceptib to issues with malicious behavior, encryption, and communication due to heavy da transmissions; however, the blockchains and DLT will be relatively safe as they alread implement techniques of multistep or multifactor authentication.In the past, a new wireless communication standard emerged after around a decade, and given this trend, it is expected that we would be witnessing 6G around 2030 [25][26][27].As more and more users are connecting to the internet and using a large number of devices connected to the Internet, big changes and challenges are coming up for internet research.The research communities are already looking towards solutions to the challenges posed by 5G mobile communication.It is expected that many of these challenges will be addressed by the time 6G materializes [20,28].

Security Needs
While 6G applications and communication technologies will be powerful and a revolution, there will be many specific vulnerabilities [20].Communication, access control, malicious behavior, authentication, and encryption-related issues will be faced in these applications (see Figure 3 below).It can be seen in the figure that 6G will support autonomous systems powered by A.I. and ML, multi-sensory X.R. applications built upon molecular communication technology, the THz technology, and the quantum communication technology and distributed ledger technologies that will be mainly developed using blockchains, etc. A.I. technologies and multi-sensory X.R. applications will be susceptible to issues with malicious behavior, encryption, and communication due to heavy data transmissions; however, the blockchains and DLT will be relatively safe as they already implement techniques of multistep or multifactor authentication.

Concept and the Main Goal
Multifactor authentication is a technology developed for security purposes and requires multiple kinds of authentication from multiple independent sources and through varying credentials to verify the user's identity [16,29].The MFA is mainly developed for logins and transactional purposes [30,31].The primary goal of the MFA is to design a layered defense that makes the accessibility of an unauthorized person to the device, location, network, and database difficult [32].If one of the barriers or layers is accessed, the attacker, hacker, or simply the unauthorized person has to get through one or more security layers to completely breach the applications and systems through software and hardware.The main goal behind the development of multiple authentications was to increase the security and also to increase the integrity of the digitized transactions [33,34].Figure 4 presents the basic framework for the architecture of the MFA system.

Multifactor Authentication 4.1. Concept and the Main Goal
Multifactor authentication is a technology developed for security purposes and requires multiple kinds of authentication from multiple independent sources and through varying credentials to verify the user's identity [16,29].The MFA is mainly developed for logins and transactional purposes [30,31].The primary goal of the MFA is to design a layered defense that makes the accessibility of an unauthorized person to the device, location, network, and database difficult [32].If one of the barriers or layers is accessed, the attacker, hacker, or simply the unauthorized person has to get through one or more security layers to completely breach the applications and systems through software and hardware.The main goal behind the development of multiple authentications was to increase the security and also to increase the integrity of the digitized transactions [33,34].Figure 4 presents the basic framework for the architecture of the MFA system.

Concept and the Main Goal
Multifactor authentication is a technology developed for security purposes and requires multiple kinds of authentication from multiple independent sources and through varying credentials to verify the user's identity [16,29].The MFA is mainly developed for logins and transactional purposes [30,31].The primary goal of the MFA is to design a layered defense that makes the accessibility of an unauthorized person to the device, location, network, and database difficult [32].If one of the barriers or layers is accessed, the attacker, hacker, or simply the unauthorized person has to get through one or more security layers to completely breach the applications and systems through software and hardware.The main goal behind the development of multiple authentications was to increase the security and also to increase the integrity of the digitized transactions [33,34].Figure 4 presents the basic framework for the architecture of the MFA system.

Use of MFA
MFA is a development that has been aimed at increasing the level of security of the authorized access to personal information, resources, and important data.In accordance with the Commission Implementing Regulation (E.U.) 2015/1502 [35], a secure and strong authentication process is based on at least two authorization factors.The MFA uses the shortcomings of the single and double authentication and the traditional I.D. and password mechanisms, and the developers use these to develop a more secure method.The main problem with less sturdy mechanisms of authentication is that the users become susceptible to hacking, security breaches by compromising the passwords and, in the case of financial transactions, costing large sums of money to the users [16].Brute force attacks are also an actual hazard, as hackers and other bad actors can use the automated passwords, thereby cracking the safety protocols by trial and error until they arise on the correct sequence.However, some accounts and application developers enable locks after multiple incorrect password attempts, helping the organizations track and protect their users.Thus, one of the primary uses of the MFA is its characteristic of reducing the security risks [30,[36][37][38][39][40].There are different ways through which the MFA is enabled.Table 3 illustrates the methods through which MFA is enabled.

Advantages of MFA Implementation
First, the primary benefit of MFA is the fact that it provides a superior level of security as it adds an extra layer of security to reduce access to sensitive data and protected systems [41][42][43][44].Many employees in today's organizations work from their homes, leading to increased exposure to cyber threats and data breaches.MFA allows 99.9% of automated password breaks to be blocked [40,41,[45][46][47][48][49].MFA can also protect against credential and device theft as it ensures that a password alone cannot authenticate a login attempt [50][51][52].Another advantage to MFA is that it is one of the easiest and least costly cybersecurity solutions.MFA is non-intrusive and does not cause downtime, so it is easy to use and implement [44,45,53,54].It also allows for Single Sign-On (SSO) compatibility by providing options of a one-time password (OTP) so that an active login session is not exploited to unlock any additional accounts [55,56].MFA also provides organizations with an opportunity to meet all the security-related regulatory requirements without extra effort [57].

What Is Blockchain Technology?
Blockchain technology can be claimed to be the most-hyped innovation of the 21st century that was designed to support bitcoin but now powers many business applications and is hyped to be the leading technology for the support of 6G technologies [22,58,59].Advancements in blockchains are still young and hold the promise of a bright future [60][61][62].The blockchains can be defined as a digital ledger of transactions (DLT), a database that can store encrypted transaction data in chronological order and chain the data together in the form of blocks [63,64].Blockchain is used to define a structure of data that can be described as an ordered arrangement of blocks, where each of the blocks contains a small list of transactions and each of the blocks is chained together [65][66][67].Through these chains, each component of the data can be traced to its source; however, the blockchains cannot be altered, deleted, or replaced without invalidation of the hash chain [65].Blockchain technology has extensive applications in payment systems and other digital financial or Fintech solutions.Thus, this technology requires strict authentication protocol for managing the safety of the users and transactions [15,32].

How does Blockchain Work?
This technology, therefore, enables safe transactions between individuals without the fear of government, bank, or other third-party software snooping and stealing the data [68,69].Figure 5 maps out roughly how a blockchain works; first, an individual node in a peer network requests a transaction, which is then broadcasted to a P2P network of nodes that authenticates it by verification technique and combines it to other transactions to form an encrypted block of data which is added to an existing blockchain [70,71].Blockchains can be considered, therefore, as a promising and revolutionary technology for future developments of applications in the 6G network era as it can help in reducing the risks, stamps out frauds, and brings multilayer transparency in any transactions between two nodes in a scalable way, opening up a myriad of application scope and uses.Blockchain technology has extensive applications in payment systems and other digital financial or Fintech solutions.Thus, this technology requires strict authentication protocol for managing the safety of the users and transactions [15,32].

How does Blockchain Work?
This technology, therefore, enables safe transactions between individuals without the fear of government, bank, or other third-party software snooping and stealing the data [68,69].Figure 5 maps out roughly how a blockchain works; first, an individual node in a peer network requests a transaction, which is then broadcasted to a P2P network of nodes that authenticates it by verification technique and combines it to other transactions to form an encrypted block of data which is added to an existing blockchain [70,71].Blockchains can be considered, therefore, as a promising and revolutionary technology for future developments of applications in the 6G network era as it can help in reducing the risks, stamps out frauds, and brings multilayer transparency in any transactions between two nodes in a scalable way, opening up a myriad of application scope and uses.

Benefits of Blockchain
Blockchains are gaining rapid fame due to the importance of record-keeping and storage of transactions and their crucial status for various kinds of businesses [72][73][74].Blockchains allow efficient processing and faster transactions, saving both time and money [75,76].Blockchain technology uses a highly secure digital signature feature to ensure that transactions are fraud-free [77].Blockchains enable decentralized transactions and ensure smoother, safer, and faster transactions as they are carried out with a mutual consensus of users [78,79].Moreover, these systems enable systematic actions, events, and payments.Transactions using blockchains are immutable, i.e., they cannot be changed,

Benefits of Blockchain
Blockchains are gaining rapid fame due to the importance of record-keeping and storage of transactions and their crucial status for various kinds of businesses [72][73][74].Blockchains allow efficient processing and faster transactions, saving both time and money [75,76].Blockchain technology uses a highly secure digital signature feature to ensure that transactions are fraud-free [77].Blockchains enable decentralized transactions and ensure smoother, safer, and faster transactions as they are carried out with a mutual consensus of users [78,79].Moreover, these systems enable systematic actions, events, and payments.Transactions using blockchains are immutable, i.e., they cannot be changed, edited, or altered in any way after being recorded once [80].Hence, in addition to speed and security, blockchains also enable the establishment of trust between users and parties in a transaction [81][82][83].

Recent Exploration of MFA Applications and Blockchain Authentication
Blockchain authentication refers to the system developed for increasing the security for the users and it verifies the users and connects it to the resources found on the technologies of digital currency, digital payments, transactions, and cryptocurrencies.The blockchain mainly uses the PKC, public-key cryptography, for the encryption of wallets and also the other places or links of the blockchain where the work of value has been stored.Thus, the authentication requirements for the blockchain increase similarities between the technology itself and the measures taken for securing it [16].Multifactor authentication is referred to as a process of verification of users through at least two authentication factors [29].The users can make use of an additional password, flash drive, special software, some particular files, and a flash drive containing important software.The MFA works in such a way that all layers of the verification need to be cleared, i.e., the user needs to provide proof of identity through OTPs, passwords, etc.Although this method is not preferred by most internet applications due to their security risks, blockchain applications use private keys for the identification of users [84].
There are different methods and techniques for ensuring the safety of the private key.Most of the time, there is a trade-off between security and usability.One of the methods proposes the security key be placed in a device without the addition of extra security [85].However, anyone can access the unprotected device and obtain the keys.Users may use the encryptions on these devices as they need to enable them themselves.Digital wallets are commonly used for storage and access to blockchains [86][87][88][89][90].They work by encrypting the security key/s by setting up a password.However, the security and key recovery are all challenges as there are numerous attacks on the passwords and security layers applied on the wallets [85].Another method used for increasing security is using passwords to derive a combination of keys for accessing the information on the blockchains.When following this method, the private key needs to be unlocked so that previously defined passwords can be accessed during the creation of the key.This method is also prone to some disadvantages; one of the main disadvantages is that the user will be unable to change the password.Some devices possess computing capabilities that may be used with blockchain.Although these devices do not possess or support storage and they require zero understanding of the mechanism, it is highly susceptible to malware attacks [91].
Shin et al. [92] proposed a multifactor authentication procedure for WSNs in recent network applications that act in real-time but is found to be vulnerable to the collision of users and desynchronization attacks.Ni et al. [93] presented a service for authentication mechanism for 5G enabled IoT networks using key agreement mechanism by use of an anonymous key.However, after rigorous testing, it was revealed to be a single-level authentication, despite using a complex public key cryptosystem and a single authentication method is not suitable for 5G multiservice systems [94].A robust MFA protocol was proposed by Huang et al. [95] for systems that use fragile communications based on two standalone schemes for authentication; however, these systems are not comprehensible, making them vulnerable to access attacks.Luo et al. [96] presented a flexible 3-factor authentication mechanism for various applications based on 5G multiservice systems.In this system, four different kinds of schemes can be provided for a user to ensure identity authentication and safety.The technique was slightly costly in terms of time efficiency.These issues are addressed by Luo et al. [97], who presented a Service-Based Architecture for 5G multiservice systems that build upon the work of Luo et al. [96] and uses an adaptable and decomposable 3-factor authentication system that can be applied simultaneously to ensure efficiency and speed.The tests on this system showed that it could provide an ideal efficiency in terms of transparency of action, security, and speed.However, further testing and real-time application are required.Wong et al. [98] also presented a 3-factor authentication scheme to ensure a high-security environment for communicating parties and integrates biometrics, password, and smart card authentication into a single system.Multiple server technologies are used to ensure that performance is quick and transparent.However, the technique still poses a communication cost.
There is limited research on the integration of multifactor analysis in blockchainbased applications.However, some papers were reviewed that can be valuable to indicate the benefits of these integrations.Antonio et al. [32] presented a multifactor analysis named 2FA for WordPress pages by using Hydro Raindrop multifactor authentication technology.In this paper, the researcher summarized the use of a blockchain-based twofactor authentication solution by a page on WordPress that contributes to securing user information.The study is not experimental, however, and the entire proposal is based on other theoretical and practical evidence.Overall, it suggested that the use of a decentralized technique provided by the integration of blockchain can enable multifactor and transparent user authentication, strengthening the security of information and the assets of individuals.Several studies also indicate the use of blockchain-based authentication procedures for autonomous vehicles [99][100][101].In the study by Feng et al. [10], Blockchain-assisted privacypreserving authentication system (BPAS) is presented to secure the data generated by vehicular ad hoc networks.This technique was proposed for ensuring accuracy as well as trust in the systems.However, there was a lack of support for batch verification in this paper that could provide an optimized verification in the form of blocks of data and hence reduce load on the resource consumption.Kebande et al. [15] also proposed an MFA based on Blockchain technology that proposed the use of an embedded Digital Signature (MFBC_eDS) that was found to be a suitable technique for countering the adversarial attacks on the Internet of vehicles in the past.Alharbi et al. [8] proposed a framework for authentication based on Blockchain technology as they claimed it could add more security to the authentication process.In this framework, the one-time password (OTP) is encrypted and sent to the application/website to complete the authentication process.This system was claimed to be safer than SMS-based authentication mechanisms, but it was less efficient in terms of time efficiency.Wu et al. [11] proposed an out-of-band 2factor authentication mechanism for IoT devices by use of blockchain to enable flexible, secure, and reliable authentication [102][103][104][105][106]. The overheads of blockchain use, however, were high.Table 4 summarizes all the studies related to MFA and Blockchain.A robust and effective system for fragile communications between two nodes.Vulnerable to access attacks.
[93] The authentication mechanism for 5G enabled IoT networks in the form of a service.Not an MFA-based technique. [96] The flexible 3-factor authentication mechanism for various kinds of applications that are based on 5G multiservice systems.
Costly in terms of time efficiency. [92] Provides a multifactor authentication procedure for WSNs in recent network applications and may be extendible to future network advancements such as 6G.
Vulnerable to the collision of users and desynchronization attacks.Technique still poses a communication cost as it integrates biometrics, password, and smart card authentication. [97] An adaptable and decomposable 3-factor authentication system that can be applied simultaneously to ensure efficiency and speed.Further testing and real-time application are required.
[32] Hydro Raindrop multifactor authentication technology to conduct 2FA for WordPress.Not experimental in nature.
[10] BPAS for ensuring accuracy as well as trust in the systems.
A lack of support for batch verification for an optimized verification in form of blocks of data and hence reduce the load on the resource consumption.
[15] Embedded Digital Signature-based MFA suitable technique for countering the adversarial attacks.Overheads are high.
[8] More security to the authentication process as compared to SMS-based authentication protocols.
This system was claimed to be safer than SMS-based authentication mechanisms, but it was less efficient in terms of time efficiency.[11] Flexible, secure, and reliable authentication.The overheads of blockchain use, however, were high.

Recent Trends and Observations
Based on the reviewed articles in this study, we observe that the MFA scheme provides an immersed level of improvement to secure sensitive data by preventing the systems from security threats.Figure 6 depicts the researchers' percentage of usage of different MFA enable mechanisms based on research studies.Almost 30% of the MFA methods were hardware tokens to highlight their importance.The soft token and OTP based on the SMS were the next important MFA enable mechanism, with almost 23% of studies employing it.While the hardware-based OTP and the authenticator applications were the less popular mechanism as adopted by almost 12% of studies only.

Recent Trends and Observations
Based on the reviewed articles in this study, we observe that the MFA scheme provides an immersed level of improvement to secure sensitive data by preventing the systems from security threats.Figure 6 depicts the researchers' percentage of usage of different MFA enable mechanisms based on research studies.Almost 30% of the MFA methods were hardware tokens to highlight their importance.The soft token and OTP based on the SMS were the next important MFA enable mechanism, with almost 23% of studies employing it.While the hardware-based OTP and the authenticator applications were the less popular mechanism as adopted by almost 12% of studies only.Figure 7 highlights the usage of different security mechanisms for the 3G, 4G, or 5G communication technologies based on the reviewed articles.It is also observed that 67% of the reviewed papers focused on the MFA-based schemes only to provide security to different networks considering the 3G, 4G, or 5G communication networks.Additionally, only 33% of studies focused on integrating BC along with the MFA authentication schemes for securing communication networks.We observed that mostly BC-based MFA studies were conducted from 2019 onward, which is one of the main reasons for its lower popularity compared to MFA schemes.This observation also highlights that BC-based MFA schemes are less explored, and there is more room for research in this emerging field.It can be observed that most of the studies conducted to date about the BC integration with the MFA are focused on the 5G networks.Since 5G networks are only deployed in very few countries globally, there is still a lack of research into how blockchain integration improves the MFA schemes.Security remains the prime issue in this digital world due to advancements in communication and network technologies such as 5G and internet of things networks.So, BC is a robust technology that can be integrated with the MFA schemes to provide much-needed security for future networks.

Future Research Trends and Challenges
This subsection highlights the future research trends and challenges based on the review conducted in this study.It can be observed that there is a lack of research into the application of how blockchain integration can lead to multifactor authentication in the 6G network era.However, multifactor authentication technologies implemented for 5G and prior networks can be enhanced in the 6G network in the future.
Therefore, one of the major future research trends is to examine strategies to reduce the time efficiency issues in multifactor authentication techniques proposed in the past and to increase overall effectiveness and trust in the networks.Additionally, BC-based schemes are more complex and require immense computational power and resources, which highlights one of the future research challenges for the researchers to improve the BC's computation and complexity issues to make it possible to integrate with the MFAbased security authentication schemes.
One major challenge that may be faced in this domain is that the 6G network technology is still very hypothetical and theoretical.It will be years before its implementation is materialized.Hence, researchers will need to test the proposed MFA techniques on current networks and simulation-based environments.

Conclusions
Conduction of a secure and reliable authentication of a large-scale transaction in the context of Internet-of-things devices is not trivial, and current network frameworks and It can be observed that most of the studies conducted to date about the BC integration with the MFA are focused on the 5G networks.Since 5G networks are only deployed in very few countries globally, there is still a lack of research into how blockchain integration improves the MFA schemes.Security remains the prime issue in this digital world due to advancements in communication and network technologies such as 5G and internet of things networks.So, BC is a robust technology that can be integrated with the MFA schemes to provide much-needed security for future networks.

Future Research Trends and Challenges
This subsection highlights the future research trends and challenges based on the review conducted in this study.It can be observed that there is a lack of research into the application of how blockchain integration can lead to multifactor authentication in the 6G network era.However, multifactor authentication technologies implemented for 5G and prior networks can be enhanced in the 6G network in the future.
Therefore, one of the major future research trends is to examine strategies to reduce the time efficiency issues in multifactor authentication techniques proposed in the past and to increase overall effectiveness and trust in the networks.Additionally, BC-based schemes are more complex and require immense computational power and resources, which highlights one of the future research challenges for the researchers to improve the BC's computation and complexity issues to make it possible to integrate with the MFA-based security authentication schemes.
One major challenge that may be faced in this domain is that the 6G network technology is still very hypothetical and theoretical.It will be years before its implementation is materialized.Hence, researchers will need to test the proposed MFA techniques on current networks and simulation-based environments.

Conclusions
Conduction of a secure and reliable authentication of a large-scale transaction in the context of Internet-of-things devices is not trivial, and current network frameworks and existing security mechanisms often fall short.The MFA technology was introduced for integration in the blockchains recently.It may present a viable set of protocols and possible applications that can lead to secure transactions and efficient authentication in the 6G era.The present study reviewed the research carried out between 2015-2021 for defining and exploring the processes and strategies of blockchain, multifactor authentication, 6G, and the overall benefits through the integration of blockchain in 6G and MFA in the blockchain.The results indicate room for improvement in the 5G technologies based on the review.Additionally, the results suggest that the MFA techniques need to be integrated within the blockchain to develop new protocols and test the effectiveness of the authentication processes so that by the time the 6G technologies are developed, there are adequate authentication techniques for security.This is an essential provision as the intricacy and connectedness of the data nodes will be high in 6G technologies as they will be based on IoT.
The present review was subject to some limitations as well.First, the review was fairly generic and was performed to compile the present research on the emerging topics and technologies of MFA, blockchain, and 6G.Moreover, the current research in this domain is limited; thus, there was not a significant body of research to go through.For future researchers, it is recommended that a meta-analysis be carried out on the same topic and the applications of the MFA techniques within the 5G technologies to configure the most secure and performing strategies.The historical information can be used to test the response in the 6G technologies.

Figure 3 .
Figure 3. Security and Privacy issues in 6G networks.

Figure 4 .
Figure 4. Basic Architecture of MFA system.

Figure 3 .
Figure 3. Security and Privacy issues in 6G networks.

18 Figure 3 .
Figure 3. Security and Privacy issues in 6G networks.

Figure 4 .
Figure 4. Basic Architecture of MFA system.

Figure 4 .
Figure 4. Basic Architecture of MFA system.

Figure 6 .
Figure 6.Trends in different authentication types to enable MFA.

Figure 7 Figure 6 .
Figure 7 highlights the usage of different security mechanisms for the 3G, 4G, or 5G communication technologies based on the reviewed articles.It is also observed that 67% of the reviewed papers focused on the MFA-based schemes only to provide security to different networks considering the 3G, 4G, or 5G communication networks.Additionally,

Table 2 .
Summary of included research papers.

Table 3 .
Various Types of MFA and its Pros and Cons.

Table 4 .
Summary of Reviewed MFA and Blockchain Technology Papers.