## 8 Security and Privacy in Underwater VLC Communication  
Underwater VLC could present a very promising solution for aquatic high-speed, secure data transmission. However, various properties of the channels, including scattering, absorption, and turbulence, present serious security and privacy risks in underwater systems. Handling these concerns requires innovative encryption techniques along with key management and channel estimation specially tailored to the underwater environment.  
One remarkable development in enhancing security for UVLC systems is the integration of chaotic phase scrambling along with conjugate frequency hopping. Chaotic PS uses a logistic map to generate a unique scrambling key for every single communication session, imbuing the encryption process with unpredictability. This is complemented by CFH [49], which spreads the signal across different frequencies by introducing zero-frequency points in the spectrum, making it hard for unauthorized entities to intercept or decode data in transmission. The system, validated with an 8-level PAM over a 1.2 m long underwater link, was able to achieve a record data transmission rate of 2.1 Gbps, with a BER below the HD-FEC threshold, hence guaranteeing reliable and secure communication even under constrained conditions.  
Another approach takes advantage of the intrinsic randomness of the underwater channel for generating keys securely. The variability in the conditions underwater, such as turbulence and scattering, is utilized by [52] in extracting symmetric encryption keys dynamically. This technique rules out the requirement for any predefined keys, thus reducing the chances of key compromise. Ref. [52] adapts to the changing underwater environment to ensure integrity and confidentiality in the data being transmitted, hence assuring strong defenses against potential eavesdroppers.  
Ref. [82] also introduces opportunities that can be exploited for enhancing security. Employing physical layer information, like channel reciprocity, in generating keys alleviates the drawback of open-channel communication in aquatic environments. Synchronous probing techniques reduce discrepancies in measurement time between the forward and backward channels, further improving the accuracy of key generation. In this way, these approaches allow for secure communication without using the traditional key distribution systems and, therefore, are particularly useful in resource-constrained underwater scenarios.  
The security performance in NOMA-assisted UVLC systems [83] in conditions of imprecise channel state information (CSI) was analyzed. Precise CSI estimation is critically important for maintaining secure communication links; however, underwater conditions often introduce uncertainties into the system. Hence, to solve it, a minimum mean square error approach was first employed for CSI estimation. The study further proposed a mechanism of LED selection for optimizing the secrecy rate even in the presence of eavesdroppers. The system will ensure that, even though the SOP is reduced, the conventional single-LED system outperforms for higher resilience against interception.  
These latter developments point to the multi-faceted approach that needs to be applied in ensuring both the security and privacy of UVLC systems. Methods of dynamic key generation, sophisticated encryption mechanisms, and optimized system architectures come together to address vulnerabilities introduced by the underwater environment. As UVLC systems find their practical implementation for military communications, environmental monitoring, and AUVs, integration of these solutions will be key to pervasive adoption. Future research should be directed at refining those methods to adapt to diverse underwater conditions and explore novel approaches that allow for even greater enhancement of system resilience against emerging threats.