## 4 Authentication Techniques in 6G Communication
### Key Agreement with Privacy  
Key agreement is considered the backbone of most of the authentication schemes. Various hierarchical key agreement frameworks have evolved for the privacy-protected authentication among gateways, sensors and users. Key agreement is also considered a primary driving factor in various cryptographic algorithm-based security protocols. Various vulnerabilities associated with the critical agreement process include protocol replay, key reuse, deniability, signature tempering, key compromise, server trust issues, identity manipulation, etc. However, a major challenge is a secret-key agreement over unauthen \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** & \multicolumn{2}{c}{**Technique**} & \multicolumn{2}{c}{**Features**} & \multicolumn{2}{c}{**Limitations**} \\ \hline
[152] & Smart Homes & ECC & \(\bullet\) & Lightweight authentication & \(\bullet\) & Fixed curve vulnerability in ECC & Fixed curve vulnerability in Secular to \\  & & & & \(\bullet\) & MitM attack & \\  & & & & \(\bullet\) & DoS attack & \\  & & & & \(\bullet\) & Impersonation attack & \\  & & & & \(\bullet\) & Replay attack & \\ \hline
[153] & Phasor measurement unit & Explicit certificate & \(\bullet\) & Designed with a focus on MitM attacks & \(\bullet\) & Overheads \\  & & & & \(\bullet\) & Real-time solution & \(\bullet\) & Vulnerable to brute force attacks \\  & & & & \(\bullet\) & Covers IEC 61850-90-5 & \\  & & & & \(\bullet\) & communication standard & \\ \hline
[154] & Cross-domain identity management & Self-sovereign Identity & \(\bullet\) & Self-sovereign certification & \(\bullet\) & Scalability \\  & & & & \(\bullet\) & Decentralized architecture & \(\bullet\) & Key management \\ \hline
[155] & CyberTwin & Diffused Practical Byzantine Fault Tolerance & \(\bullet\) & Non-dependence on centralized CA & \(\bullet\) & Scalability \\  & & & & \(\bullet\) & Blockchain empowered & \(\bullet\) & Standardization issues \\  & & & & \(\bullet\) & Improved latency, overhead and storage cost & \(\bullet\) & Processing limitation in IoTs \\ \hline
[156] & Cross-domain authentication model & Blockchain & \(\bullet\) & Unconventional, decentralized blockchain certificates & \(\bullet\) & Scalability \\  & & & & \(\bullet\) & Tomper-resistant & \(\bullet\) & Vulnerable to quantum computing attacks \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Emerging certificate-based authentication techniques in 6G networks. ticated public channels, where massive attacks rise exponentially, such as the health care system facing challenges in establishing a secure session key among medical servers and patients. Moreover, biometric credential misuse and privacy exploitation can result in irreparable damages. Therefore, dynamic privacy-protected key agreements are required for the intractability of biometric parameters in authentication servers [157]. Similarly, smart grids and smart cities are low processing, dynamic and massively expanded scenarios in next-generation communication. Smart grids are usually targeted by malicious entities for monetary gains related to billing manipulation; however, large-scale sophisticated attacks on smart grid systems can paralyze the whole governance and management system. In [158], the authors proposed novel privacy-aware authenticated and unclonable one-way hash-based key agreement scheme for secure communication between resource-constrained smart meters and the grid management server. Due to dense interconnectivity and massive networking in modern communication, key management is emerging as a challenge, as depicted in Figure 8.  
Physically Unclonable Functions (PUF) are being designed for local security to digital devices and biometric credentials are distributed 6G-enabled networks. The PUF-related security implementations are focused on low processing power scenarios such as signal processing, information theory, coding theory and hardware complexity limitations [159].