## 4 Authentication Techniques in 6G Communication
### Deniable Authentication  
Pre-sharing of system parameters before security verification of the communication paradigm is an essential part of the authentication process. This exposes some of the vulnerable characteristics of devices, resulting in serious security incidents, especially in wireless network communication scenarios. Therefore, Deniable Authentication (DA) protocols are designed to empower the sender with the ability to reject the authentication process to any third party. DA protocols are mostly recommended to avoid the \"Encryption-then-MAC\" paradigm by disallowing entities from initiation of the MAC-based authentication process [134]. In [135], the authors suggested a source-hiding scheme with projective hash functions. The scheme secures WIFi authentication by rejection to third party linkages.  
Similarly, Mobile Ad hoc Network (MANET) is the foundational element in modern communication [136; 137]. In [138], the authors suggested an identity-based DA protocol for MANET with formal security verification through a random oracle model. DA protocols are developed to maintain possible heterogeneity due to the highly diverse nature of the emerging communication environment in wireless networks. The authors in [139] proposed a bilinear pairing-based formal security proof of DA though identity-based cryptography. Group Key Agreement (GKA) protocols also utilize DA in fog computing-enabled vehicular networks, as this highly mobile wireless environment makes key parameters vulnerable. The authors in [140] proposed a random oracle model for authentication in social media network communication. This scheme executes through a single logical step and fairly reduces cipher text length and computational cost. It is pertinent to highlight that limited available works in the literature directly relate the implementation, solutions and challenges for deniable authentication in 6G networks. Table 9 summarizes the analysis on deniable authentication techniques for 6G mobile networks.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Ref.** & **Network Model** & **Technique** & & **Features** & **Limitations** \\ \hline
[121] & Spread spectrum & Secret modulation & \(\bullet\) & Robustness to interference & \(\bullet\) & Compatibility issues \\  & & & \(\bullet\) & Negligible bandwidth dependence & \(\bullet\) & Complexity \\ \hline
[123] & Channel estimation & ML with kernel least mean square & \(\bullet\) & Improved reliability & \(\bullet\) & Limited to low power devices \\  & & & \(\bullet\) & Robust & \(\bullet\) & Combability issues with 3 rd \\  & & & \(\bullet\) & Compatible with time-varying & & party devices \\ \hline
[124] & CSI & Clustering and & \(\bullet\) & Secure and simple & \(\bullet\) & Limited to MIMO-OFDM \\  & & lightweight symmetric & \(\bullet\) & No trusted party & \(\bullet\) & systems \\  & & cipher with channel state & \(\bullet\) & Resist small integer attacks & \(\bullet\) & Combability issues with 3 rd \\  & & information & \(\bullet\) & Reduced data loss & \(\bullet\) & party devices \\ \hline
[129] & 6G transmission & Mailbox theory, distributed learning and & \(\bullet\) & Self-organization & \(\bullet\) & Hardware complexity \\  & & blockchain & \(\bullet\) & Self-learning & \(\bullet\) & High processing \\  & & & \(\bullet\) & Reduced transmissions quantity & \(\bullet\) & Complex implementation \\  & & & \(\bullet\) & Reduced transmission error & \(\bullet\) & Backward compatibility issues \\ \hline \end{tabular}
\end{table}
Table 8: Emerging physical layer authentication techniques in 6G networks.