## 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
In conclusion, at this point, the VDES messages were not vulnerable to threats of radio interference in the data link, allowing for subsequent spoofing source identification. The VDES signal coverage is calculated to be 27.1623 nautical miles based on sea surface meteorological conditions. Simultaneously, the ship-to-shore distance computed using Equation (9) from the VDES ship station position is 15.5931 nautical miles, which is less than the VDES signal coverage range, indicating that the information broadcast by the base station can be received by the ship station. Considering the received power, the VDES signal propagation distance calculated using Equation (8) is 3.02 nautical miles. At this point, the ship-to-shore distance exceeds the tolerance range of 97.3% fitting the propagation distance, suggesting that the received VDES message is not from the base station. It is inferred that spoofing source interference exists in the VDES data link at this instance, leading to subsequent engagement with the spoofing source localization module. As shown in Figure 41, it was confirmed that the format of the message aligns with that stored in the VDES raw message database, enabling successful parsing. The verification of message format compliance was successful. Following this, a query to the AIS/VDES message distributed database revealed that all parameters of the message matched those of previously sent legitimate messages, validating the legitimacy of the message parameters. By utilizing the MD5 algorithm to compute the hash value of the received message and comparing it to the hash value of the legitimate message previously received by this base station, the computed results were consistent; both hashes were 9C972291938791768895B670295 ACD2F, confirming the integrity of the message content. Additionally, by applying the HMAC algorithm to compute the HMAC values of the TDB statement message and the VEM statement message separately, the results were also consistent, both being 972 cab20159 dbd8397 cd967c2124167, thereby passing the consistency check of the transmission and reception messages. In conclusion, at this point, the VDES messages were not vulnerable to threats of radio interference in the data link, allowing for subsequent spoofing source identification. The VDES signal coverage is calculated to be 27.1623 nautical miles based on sea surface meteorological conditions. Additionally, the distances between VDES monitoring station A and the ship station, calculated using Equation (9), is 2.9513 nautical miles, while the distance between VDES monitoring station B and the ship station is 3.2272 nautical miles. Both ship-to-shore distances are less than the VDES signal coverage range, indicating that the information broadcast by both monitoring stations can be received by the VDES ship station at this time. However, when considering the received power and the VDES signal propagation distance calculated using a complex VHF signal propagation loss model in the intricate sea surface environment, the distance between monitoring station A and the ship station falls within the tolerance range of 97.3%, fitting the propagation distance. Conversely, the distance between monitoring station B and the ship station exceeds this tolerance range, suggesting that the received VDES message does not originate from monitoring station B. This inference indicates the presence of a spoofing source in the VDES data link. The spoofing source localization algorithm determines the coordinates of the spoofing source to be (38 deg42.2063' N, 121 deg8.1238' E), validating the capability of the VDES data link integrity monitoring software system to successfully detect and locate spoofing sources within the data link. ## 5 Conclusions

This paper proposes a credibility monitoring approach based on radio interference detection incorporating spoofing source identification and localization, in the context of the two major classes of data link vulnerabilities outlined in the IALA G1181 VDES VDL Integrity Guide.