## 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
Initially, the system parses received messages using its VDES message database to verify their compliance with the VDES message format. Next, the parameters extracted from these parsed VDES messages are compared with the legitimate parameters previously sent by the VDES base station and stored in the AIS/VDES message distributed database to validate their authenticity. The integrity of the message content is then verified using the MD5 algorithm, which is more efficient for ensuring message integrity compared to SHA1 and SHA256 algorithms. Further, the consistency of the link management messages sent by the VDES base station in TDB statements and the VEM message statements received by VDES via shipborne automatic retransmission is checked using the HMAC algorithm to detect radio interference threats. Upon detecting radio interference, the VDES signal propagation distance is calculated using a VHF signal transmission loss model in complex sea surface environments. The ship-to-shore distance is then computed based on the location information in the messages, and a comparative analysis is conducted between this ship-to-shore distance and the propagation distance to identify potential spoofing sources. If spoofing sources are detected, a spoofing source identification algorithm is employed to accurately locate these sources, thereby ensuring the integrity of the VDES data link. To support automated monitoring of the VDES data link, we have developed VDES data link monitoring software based on the C/S model. This software includes a VDES communication module, a VDES message processing module, and a link integrity monitoring module. It facilitates the visualization of the VDES link monitoring status, alerts, and records of spoofing source locations. However, this paper has not taken into account other VHF data link vulnerability threats mentioned in the IALA G1181 guide, including unauthorized signaling, misbehaving devices, incorrect device configuration and installation, DOS attack and protocol attack. In the future, the VDES data link trustworthiness monitoring methodology will further enhance the monitoring of unauthorized signaling, misbehaving devices, incorrect device configuration and installation, DOS attack and protocol attack, and the VDES data link vulnerability monitoring methodology will further intensify the monitoring of VHF data link vulnerability threats. The research findings are significant for enhancing maritime traffic safety, improving shipping efficiency, and preventing maritime accidents. [PERSON] and [PERSON] supervised the work, arranged the architecture, and contributed to the writing of the paper; [PERSON] and [PERSON] designed the measurement scheme, carried out the simulations, and wrote the paper; [PERSON] analyzed and compiled the data. All authors have read and agreed to the published version of the manuscript. This research was funded by the National Natural Science Foundation of China (No. 62301106) and the National Key Research and Development Program of China (No. 2021 YFB3901502). Not applicable. All data have been provided in this paper. Further inquiries can be directed to the corresponding authors. The authors declare no conflicts of interest. The following is a comprehensive list of all abbreviations utilized in the text: