## 4 Authentication Techniques in 6G Communication
### Multi-Factor Authentication  
Multi-Factor Authentication (MFA) is considered as a core element of a foolproof Identity and Access Management (IAM) policy scheme. MFA is used for enhanced security through multitude expansion in the key spectrum against brute force attacks and stolen third-party parameters. The additional factors used for MFA include SMS-based OTP, Email-based OTP, software token, smart cellular apps, biometric parameters, third-party certificates such as GMAIL, secret question, USB-based, smart cards, PIN, RFID, physical key, location-based, etc. Specifically, MFA-based biometric credentials have achieved technological diversity by introducing retina or iris scans, voice authentication, hand geometry, facial recognition, earlobe geometry and DNA specifications. Heterogenous connectivity of modern networks has provided several possibilities for cross-verifications in MFA, as shown in Figure 9. The prominent attacks against MFA include MitM, reverse brute force, credential stuffing, key loggers, spear-phishing and phishing. MFA is increasingly recommended for 6G-enabled massive and heterogenous communication scenarios to counter quantum computing-based cryptographic attacks [162].  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref.** & **Network Model** & **Technique** & \multicolumn{2}{c}{**Features**} & \multicolumn{2}{c}{**Limitations**} \\ \hline
[157] & Electronic & Biometric authentication & • & Undisclosed and untraceable & • & Finger print hacking \\  & healthcareLuo & & • & Unclonable one-way hash & • & vulnerability \\  & MiaoBiometric & & • & Semantic secure under the & • & Vulnerable to stolen \\  & parameters & & & real-or-random model. & & verifier attack \\ \hline
[158] & Smart Grid & Physically unclonable & • & Less resource consumption & • & Brute force and dictionary \\  & & functions & • & Tampering resistant & & attack issues \\  & & • & Low overhead & • & TA dependency \\ \hline
[160] & Subscriber authentication & Blockchain & • & Auditable & • & Scalability \\  & & • & Resistant to DoS attack & • & Standardization issues \\  & & • & Execution costs & & \\  & & • & Low overhead & & \\ \hline
[161] & Network In Box & Spatial crowdsourcing & • & Prevents leakage of sensing & • & Task assignment \\  & & • & node location & & results manipulation \\  & & • & Provides confidentiality and integrity & • & Data migration \\  & & & • & Processing requirements \\ \hline \hline \end{tabular}
\end{table}
Table 12: Emerging key agreement with privacy for authentication in 6G networks.  
Figure 9: MFA spectrum in modern communication.  
Similarly, the authors in [163] suggested a blockchain-based authentication for heterogenous devices to provide joint authentication and access control. Although this scheme provides minimum error, it has limitations in scalability, key management and consensus overhead. In [164], the authors proposed McEliece and Niederreiter crypto-code on elliptic codes for quantum-resistant authentication in a closed mobile internet channel. The scheme provides an offline mode for closing the voice channel; however, the employed algorithms are sensitive to fault injection attacks. Organized and structured CIA triad-compliant security policies are implemented through utilizing MFA in next-generation cloud computing-based architecture [165]. Due to a broad MFA spectrum, security policy management is becoming crucial and increasingly sophisticated. The authors in [166] suggested an MFA among distributed edge nodes and cloud nodes. The scheme utilizes various AI techniques for processed events-based authentication with human independence. Table 13 provides a summary of our analysis of MFA schemes in 6G mobile networks.