## 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 received VDES message is decoded, and its conformity to the prescribed message format stored in the VDES original message database is verified for successful parsing and consistency, thus confirming adherence to the message format. Subsequent validation involves cross-referencing the AIS/VDES message distributed database to ascertain alignment between the parameters of the various message fields in the VDES message and those of legitimate messages previously transmitted by the base station and stored in the AIS/VDES message distributed database, thereby guaranteeing compliance with message parameter authenticity. Ultimately, the hash values of the received message and the legitimate message received by the base station in the VDES original message database are computed using the MD5 algorithm, with the results illustrated in Figure 5.

The simulation environments in Figure 5 employs the C# language within Visual Studio 2019 in Windows11 operating system environment. As depicted in Figure 5, the two message hash values highlighted by the red box differ, indicating that the messages did not pass the integrity check for message content. The discrepancies in the VDES messages are attributed to radio interference during the data link transmission process, resulting in transmission errors. This observation underscores the effectiveness of the MD5 algorithm in detecting data integrity issues within VDES messages. For instance, if a VDES base station receives the message \"IAVEM,1,1,0,A,403 sooQvN7 AgR'dfnF>BSW00D0c,0*67\", and a legitimate message previously sent by a ship station is \"IAVEM,1,1,0,A,403 sooQvN7 AgR'dfnF>BSW00D0c,0*60\", the hash values of both messages are computed using the MD5 algorithm after verifying compliance with the message format and the legitimacy of message parameters. The calculation results are illustrated in Figure 6.

Figure 5: Results of failed message content integrity verification detection. The simulation environments in Figure 6 employs the C# language within Visual Studio 2019 in Windows11 operating system environment. As shown in Figure 6, the hash values of the two messages are congruent, both represented as B286 DA103A03514 FOA6335 BF9A49 EC20. This congruence signifies the resilience of VDES message transmission against data link radio interference, validated through meticulous scrutiny of message integrity. This experimental endeavor affirms the efficacy of the MD5 algorithm in discerning the integrity of VDES message content. Moreover, within the realm of algorithmic evaluation, computational time serves as a pivotal benchmark. To further explore the processing efficiency advantage of the MD5 algorithm, this study compared the processing durations of the MD5 algorithm with three other prominent hashing algorithms, namely SHA-256, SHA-384 and SHA-512, in handling a single instance of the same VDES message. The VDES message under consideration for this comparative analysis is \"!ABTDB,1,1,0,A,403t?1 vLohh<JAnH=Tdp500D0G,0*6E\", with the outcomes of this verification detailed in Figure 7.

The simulation environment in Figure 7 runs using the Java language in the Windows 11 operating system under the IDEA2022 configuration environment with JDK17. According to Figure 7, the processing of the identical VDES message revealed that the MD5 algorithm demonstrated the shortest processing time, at 9.8928 ms. The SHA-256 algorithm followed with a processing time of 12.4144 ms, while the SHA-384 algorithm recorded the longest processing time of 13.6527 ms. The SHA-512 algorithm recorded the longest processing time of 16.4384 ms. This finding highlights that the MD5 algorithm, as applied in this investigation, effectively validates the integrity of VDES message content while also offering the swiftest computational efficiency. This proficiency facilitates the effective monitoring of potential radio interference threats to the VDES data link, thereby enhancing the overall robustness of the system.