## 1 Introduction
PLKG aims to exploit shared physical randomness as a high-entropy source. In wireless communication, physical randomness from channel characteristics--such as the fading caused by multipath effects--can be used to generate symmetric keys. Key generation based on the physical properties of the wireless channel is primarily grounded in the following theories [6]:  
* Randomness of wireless channels: Wireless channels are influenced by multipath effects, reflections, scattering, and fading during signal propagation, making their characteristics highly unpredictable. This is particularly true in the marine environment, where rough sea surfaces, waves, and objects such as ships contribute to complex multipath effects. Furthermore, due to the mobility of ships as well as the presence of noise and environmental factors, channel parameters--such as amplitude, phase, and received signal strength--become even more challenging to forecast or replicate. This inherent randomness in the channel can serve as a natural source of entropy, making it ideal for generating secure cryptographic keys. * Reciprocity of wireless channels: Under identical carrier conditions, the forward and reverse channels experience the same wireless propagation environment over a short period, resulting in symmetrical characteristics. Under ideal conditions, the channel characteristics observed by the communicating parties are identical. * Spatial diversity of wireless channels: Channel characteristics change rapidly with spatial position. As a result, even if an attacker attempts to eavesdrop, the channel characteristics observed by the attacker are usually inconsistent with those observed by the legitimate communicating parties. Numerous studies have shown that wireless channels can be considered independent if the distance between the third-party node and the legitimate communication node exceeds half a wavelength [7, 8, 9]. This spatial separation prevents eavesdroppers at different locations from obtaining relevant information about the legitimate channel. This characteristic makes passive attacks on PLKG particularly challenging. The inherent properties of wireless channels allow PLKG to establish symmetric keys between communication nodes in a plug-and-play manner, allowing for real-time key generation and rapid updates without relying on a key distribution server. Additionally, PLKG does not require the calculation of complex mathematical problems, further reducing the computational overhead associated with modern cryptography. Studies have shown that the energy consumption for key negotiation using the ECDH protocol is 98 times higher than that of PLKG [10]. This makes PLKG particularly attractive for resource-constrained nodes. These characteristics make PLKG highly suitable for use in maritime environments with limited resources and communication capabilities. However, there has been no research on applying PLKG to enhance AIS security. This paper aims to propose a novel solution to address this issue. The main contributions are as follows:  
* Considering the limitations and unique characteristics of ship communication scenarios, a physical-layer key generation protocol tailored for AISs is proposed based on the ITU-R M.1371-5 standard [11] to enhance the unicast communication security of AISs. This protocol serves as a standalone technical solution while offering novel insights into the broader challenges in ship communication security. * The protocol dynamically adapts key processes, including channel probing and quantization, to mitigate signal fading, multipath variations due to vessel movement, and malicious interference in maritime environments. * The security of the scheme against common passive and active attacks is analyzed, and the protocol's performance and overhead are evaluated. The rest of this paper is organized as follows: Section 2 provides an overview of the current state of AIS security and PLKG, along with the related work. Section 3 delves into the model and detailed process of the proposed PLKG protocol.