## 2 VDES Data Link Credibility Monitoring Approach
### Radio Interference Detection Based on Message Characteristics
#### 2.1.1 VDES Message Integrity Verification Method Based on the MD5 Algorithm
From Figure 9, it is evident that the HMAC value of the link management message broadcast by the VDES base station in TDB format is 972 cab20159 dbdb8397c6d967c2124167, while the HMAC value of the message received again by the base station is 010c6766 aeef9 fc5 fcaf4839f177837c. The inconsistency between these HMAC values indicates that the VDES messages encountered transmission errors due to radio interference in the data link. This demonstrates the effectiveness of the HMAC algorithm in verifying the consistency of message content exchanged by VDES base stations. At a particular moment, if the message received again by the VDES base station after being relayed by a ship station is \"!AIVEM,1,1,0,A,403 sooQvMSfR0W\"0B4@qWO400D0E,0\"33\", the HMAC value for this message can be calculated using the HMAC algorithm to yield the results of the consistency check for message exchanges, as shown in Figure 10.

The simulation environments in Figure 10 employs the C# language within Visual Studio 2019 in Windows 11 operating system environment. Illustrated in Figure 10, the congruence of HMAC values between link management messages originating from the VDES base station via TDB statements and those relayed by VDES ship stations using VEM statements--subsequently re-captured by the base station--underscores a uniformity represented by the value 972 cab20159 dbdb8397c6d967c2124167. This consistency serves as evidence that VDES communications endure unscathed amidst the challenges posed by wireless radio interference in the data link, a conclusion substantiated through meticulous assessments of message coherence. Consequently, this outcome affirms the efficacy of the HMAC algorithm in fortifying message integrity throughout VDES base station transmissions. In summary, employing a wireless radio interference detection mechanism grounded in message attributes facilitates a robust surveillance framework designed to pinpoint vulnerabilities associated with radio interference within the VDES data network. By rigorously scrutinizing message format adherence, parameter validity, content integrity, and the uniformity of messages during transit and reception, latent risks stemming from radio interference within the VDES data conduit can be efficiently exposed and vigilantly monitored. Figure 6.0 Results of successful verification of message consistency between VDES base stations. Figure 6.1 Results of failed verification of message consistency between VDES base stations. ### Spoofing Source Identification and Localization Method Based on Distance Matching Analysis

Due to the lack of encryption in the messages broadcast by VDES base stations and the absence of an identity verification mechanism within VDES itself, malicious entities could deceive vessels within the signal coverage area by illicitly sending false VDES messages using the MMSI code stolen from a legitimate base station. This deceptive practice can mislead vessel navigation decisions, posing a significant threat to maritime safety. Therefore, to ensure the safety of vessel operations, it is imperative to identify and locate any spoofing sources present in the VDES data link. This paper employs a method for spoofing source detection based on distance matching analysis. Specifically, it involves calculating the distances between VDES base stations and vessels by analyzing the message parsing of vessel position coordinates and base station location information. Simultaneously, it utilizes a model for VHF signal propagation loss and receiver power calculations under complex sea surface conditions to determine the propagation distance of VDES signals. By analyzing whether the ship-to-shore distance falls within the tolerance range of the propagation distance, a matching analysis is conducted. If the ship-to-shore distance is within this range, no spoofing source is present; if it exceeds this range, the presence of a spoofing source is inferred.