Abstract
The growing importance of Underwater Networks (UNs) in mission-critical
activities at sea enforces the need for secure Underwater Communications
(UCs). Classical encryption techniques can be used to achieve secure
data exchange in UNs. However, the advent of Quantum Computing will pose
threats to classical cryptography, thus challenging also UCs. Currently,
underwater crypto-systems mostly adopt symmetric ciphers, which are
considered computationally quantum-robust, but pose the challenge of
distributing the secret key upfront. Post-Quantum Public-Key (PQPK)
protocols promise to overcome the key distribution problem. The security
of PQPK protocols, however, only relies on the assumed computational
complexity of some underlying mathematical problems. Moreover, the use
of resource hungry PQPK algorithms in resource-constrained environments
such as UNs can require non-trivial hardware/software optimization
efforts. An alternative approach is underwater Quantum Key Distribution
(QKD), which promises unconditional security built upon the physical
principles of Quantum Mechanics. This tutorial provides a basic
introduction to free-space Underwater QKD (UQKD). At first, the basic
concepts of QKD are presented, based on a fully worked out QKD example.
A thorough state-of-the-art analysis of UQKD is carried out. The paper
subsequently provides a theoretical analysis of the QKD performance
through free-space underwater channels, and its dependence on the key
optical parameters of the system and seawater. Finally, open challenges,
points of strength and perspectives of UQKD are identified and
discussed.