### Scope and Contributions  
Mainly, the focus of the literature review in this paper was based on a keyword search, namely, \"threats\",\" counter measures\", \"security schemes\", \"authentication techniques\", \"authentication system\" and \"authentication framework\". These terminologies were used to search the latest literature on various platforms such as SCOPUS, Web of Science, IEEE Xplore Digital Library and ACM Digital Library. Specifically, the proposed authentication schemes of 6G networks were shortlisted at the start. Thereby, each shortlisted work was reviewed with a focus on distinct criteria: (1) reputation, (2) relevance, (3) originality, (4) date of publication (between 2019 and 2022) and (5) most significant literature in the specific area. The explanatory portion of this survey primarily comprises papers in the field of 6G that specifically discuss security mechanisms as their main subject. We initialized our search on 11 July 2022 and proceeded until the submission for acceptance. The significant contributions of the survey are:  
1. We presented a threat model for the vulnerabilities in 6G cellular networks in five classifications: threats against Confidentiality, Integrity, Authentication, Availability and Access control (CIA3).
2. We presented a categorization for countermeasures used in 6G networks into three types based on asymmetries: (1) Cryptographic Methods (CM), (2) Entity Attributes (EA) and (3) Intrusion Detection Systems (IDS).
3. We suggested a taxonomy of authentication techniques for 6G networks in eight types: handover authentication, mutual authentication, physical layer authentication, deniable authentication, Token-based authentication, Certificate-based authentication, key agreement-based authentication and multi-factor authentication.
4. We specifically highlighted future research directions on the basis of overall discussion on the topic, including, (1) Privacy Preservation in 6G network-based 3D Fog Computing, (2) 6G-enabled secure smart Infrastructures and Augmented Reality, (3) SDN-based privacy-protected architecture in 6G networks, (4) Optimized secure routing in 6G networks, (5) 6G Physical Layer security and (Tera-Hertz) THz spectrum, (6) 6G security in Quantum computing and (7) Blockchain-based distributed security in 6G.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ref.** & **6G** & **Auth.** & **Observations** \\ \hline
[20] & \(\surd\) & 0 & Only specific authentication and privacy-preserving schemes are reviewed. It lacks a review concerning the threat spectrum. & None of the previous surveys cover the CIA3 triad-based Threat Model to analyze authentication and privacy in 6G Security. \\ \hline
[21] & \(\surd\) & 0 & Focus only on ML-based techniques. Moreover, it lacks threat model-based analysis. & None of the previous surveys cover the CIA3 triad-based Threat Model to analyze authentication and privacy in 6G Security. \\ \hline
[23] & \(\surd\) & X & privacy in 6G network-based security \\ \hline
[24] & \(\surd\) & X & Only privacy is covered with a general overview. Aspects related to various attacks and techniques require further review. \\ \hline
[25] & \(\surd\) & X & \\ \hline
[26] & \(\surd\) & X & Limited coverage to the security perspective. \\ \hline \end{tabular}
\end{table}
Table 2: Comparative analysis related to surveys in the previous literature.