## 4 Authentication Techniques in 6G Communication
### Certificate-Based Authentication  
Foundationally, a certificate-based authentication scheme is a joint implementation of various cryptographic algorithms and handshaking protocols. Certificate-based authentication is widely used in current world applications and smart infrastructures. In [152], the authors proposed a lightweight cryptography-based certificate authentication scheme with anonymity and untraceability. The scheme provides resistance against several attacks such as MitM, DoS, impersonation and replay attacks. Although certificate-based security mechanisms are considered impracticable in resource-constrained IoTs, several overhead \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Ref.** & **Network Model** & **Technique** & \multicolumn{2}{c}{**Features**} & \multicolumn{2}{c}{**Limitations**} \\ \hline
[141] & MEC & JSON Web Token & • & Compliance with the standard requirements & • & JSON security failures \\  & & & • & Credentials transfer between parties & • & Vulnerable to confusion attacks \\ \hline
[143] & Conventional client-server & On-chip physically unclonable functions & • & Reduced energy consumption • & • & IoT specific \\  & & & • & Token-based data transfer & • & ECC fixed curve issues \\  & & & • & Energy-quality scaling & • & \\ \hline
[145] & Geographically distributed networking & Lightweight security module & • & Reduces delay & • & IoT specific \\ \hline
[147] & Edge level & Gait information aggregation & • & Acurate for abnormal gait diagnosis & • & Specific application \\  & & aggregation & • & Latency of 335 ms & • & Scalability issues \\  & & & • & High SDN coverage Ratio & • & \\ \hline
[148] & SDN & Blockchain & • & Auditable orchestration & • & Scalability issues \\  & & & • & Zero-trust security model & • & Key management issues \\  & & & • & End-to-end encryption & • & Consensus overheads \\ \hline
[150] & Narrow-band internet of things & SHA-256 cyclic keys & • & Dynamic key-based security & • & Vulnerable to \\  & & & • & D2D communication & • & Prediction attacks \\  & & & • & Machine-to-machine & • & Stealing attacks \\ \hline
[151] & Cross-domain secure data sharing & Blockchain with ECC & • & Secure data movement & • & Scalability issues \\  & & & • & Trust among the industries & • & Fixed curve vulnerability \\  & & & • & Collaborate on manufacturers & & in ECC \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Emerging token-based authentication techniques in 6G networks. reduction modifications are being performed for the suitability of certificate-based authentication in next-generation networks based on distributed IoTs. These modifications include pre-validation, session resumption and handshake delegation. Certificate-based authentication is designed with a focus on MitM attacks in wireless network communication, unlike attribute-based mechanisms such as biometric or One Time Password (OTP) mechanisms [153]. Certificate-based authentication provides many solutions with public key-based unique credentials. 6G communication is bringing huge cross-domain communication along with state legislative obligations of centralized certification authorities, which is normally an unsecured global phenomenon. Therefore, massive heterogenous communication in 6G is shifting from centralized certification to decentralized self-sovereign identity-based certification [154]. Lightweight cryptography-based certification is widely suggested for key agreement in 6G-enabled Internet of Drones (IoDs), where the edge computing environment provides processing and implementation of authentication protocols. In 6G communication security, the implementation of blockchain technology is expanding rapidly due to no dependency on a centralized Certification Authority (CA) [155]. Therefore, CA-based schemes will become outdated due to unconventional decentralized blockchain certificates (Bcert).