## 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
When the VDES ship station is located at (38 deg43.176' N, 121 deg10.02' E), the monitoring results are shown in Figure 39.

As shown in Figure 39, it was confirmed that the format of the message aligns with that stored in the VDES raw message database, enabling successful parsing. The ver

Figure 39: Results of testing for the absence of spoofing sources based on the Credibility Monitoring Software. ification 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 9C97229193B791768895B670295 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 dbd8397c6d967c2124167, 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 range calculated based on sea surface meteorological conditions is 27.1623 nautical miles. Concurrently, using the Haversine formula with the VDES base station coordinates, the ship-to-shore distance is determined to be 2.9418 nautical miles, which is less than the VDES signal coverage range. Considering a received signal power of \(-67\) dBm at the VDES ship station and a calculated VDES signal propagation distance of 3.02 nautical miles--based on a complex sea surface environment VHF signal propagation loss model--this ship-to-shore distance falls within the tolerance range of 97.3%, fitting the propagation distance. This alignment indicates that the signal emitted from the base station matches the received signal power at the receiving end, suggesting the absence of spoofing source interference in the VDES data link. At a specific moment, the coordinates of the VDES base station are (38\({}^{\circ}\)58.14\({}^{\prime}\) N, 121\({}^{\circ}\)2.46\({}^{\prime}\) E), with a sea surface wind speed of 12.23 m/s, a sea surface temperature of 17 \({}^{\circ}\)C, a tidal water level of 0.1 m, and a relative humidity of 69%. The VDES ship station is positioned at (38\({}^{\circ}\)43.62\({}^{\prime}\) N, 121\({}^{\circ}\)9.66\({}^{\prime}\) E), with a received power of \(-67\) dBm. The results of the ship-to-shore distance matching with the propagation distance test are depicted in Figure 40.

Figure 40: Results of spoofing source detection based on the Credibility Monitoring Software. As shown in Figure 40, 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 9C97229193B791768895B670295 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.