## 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
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Explain** \\ \hline MMSI Code & 004132804 \\ \hline Longitude & \(106^{\circ}36.3714^{\prime}\) E \\ \hline Latitude & \(29^{\circ}32.1212^{\prime}\) N \\ \hline Repeat & 0 \\ \hline Position accuracy & 1 \\ \hline Types of electronic positioning devices & 5 \\ \hline Transmission control of long-range broadcast & 0 \\ messages & 0 \\ \hline Spare & 0 \\ \hline Sync State & 0 \\ \hline RAIM flag & 0 \\ \hline Time slot timeout & 5 \\ \hline Received Stations & 23 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameter table for VDES message content integrity verification testing. Figure 36: Results of VDES message content integrity verification testing. the consistency check of the VDES transmission and reception messages failed, suggesting the presence of radio interference within the VDES data link at that time. ### Testing of Radio Interference Vulnerability and Threats

Assuming that the VDES base station broadcasts a link management message in TDB format as follows, \"IABTDB,1,1,0,A,403 sooQvMSfR0W\"0B4@qW0400D0E,0\"34\", the message received by the VDES shipborne unit--after automatic forwarding by VEM and then re-received by the VDES base station--is \"AIVEM,1,1,0,A,403 sooQvMSfR0W\"0B4@qW0400D0 E,0\"33\". Upon querying the VDES raw message database to parse the received message and compare parameters in the AIS/VDES message distributed database, the test results are depicted in Figure 38.

As shown in Figure 38, 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 38: Results of radio interference vulnerability and threat testing. Figure 37: Results of VDES transmission and reception messages consistency check test. 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 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. ### Testing of Spoofing Source Vulnerability and Threats

Building upon the completion of radio interference detection in the VDES data link, this section employs a method that matches ship-to-shore distance with propagation distance to validate the effectiveness of the VDES data link integrity monitoring system in detecting spoofing sources within the data link. Testing of Ship-to-Shore Distance and Propagation Distance Matching

To validate the effectiveness of the ship-to-shore distance matching analysis method in detecting spoofing sources within the VDES data link, this section simulates various interference scenarios to verify the functionality of the algorithm. (a) Match Test Passes the Experiment

Assuming the coordinates of the VDES base station are (38 deg46.0398' N, 121 deg9.1872' E), with a sea surface wind speed of 10.88 m/s, a sea surface temperature of 16 degC, a tidal water level of 0.2 m, and a relative humidity of 75%.