## 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
As illustrated in Figure 35, querying the VDES raw message database confirmed that the format of the message aligns with the stored format, allowing for successful parsing. The verification of message format compliance passed. However, upon consulting the AIS/VDES message distributed database, it was revealed that previously sent legitimate messages from this base station identified the Electronic Positioning Device type as 5, with an RAIM flag of 0. In contrast, the Electronic Positioning Device type in the received VDES message, as listed in Table 1, was 7, with an RAIM flag of 1. This presents inconsistencies with the parameters of legitimate messages. Consequently, the validation of the legitimacy of VDES message parameters failed, indicating the presence of radio interference within the VDES data link at that time. ## 3 Verification of VDES Message Content Integrity

Assuming the message received by the VDES base station is \"!AIVEM,1,1,0,A,403t?i1v Lohh!<'dD=LF7M3U00D0G,0*0A\", upon querying the VDES raw message database, it was found that a legitimate message previously sent by the ship station received by this base station was \"!AIVEM,1,1,0,A,403t?i1v Lohh!<'W0B4@qWO500D0G,0*04\". The parameters of this message retrieved from the VDES raw message database are detailed in Table 2. Following the parsing and parameter comparison of the received message using the VDES raw message database, the test results are depicted in Figure 36.

As shown in Figure 36, querying the VDES raw message database confirmed that the format of the message aligns with the stored format, enabling successful parsing. The verification of message format compliance passed. Following parsing, a query to the AIS/VDES message distributed database revealed that all parameters of this message matched those of previously sent legitimate messages, thereby passing the validation of message parameter legitimacy. However, upon computing the hash values of the received message and the hash value of a legitimate message previously received by this base station using the MD5 algorithm, the software interface indicated a discrepancy between the two hash values. This integrity check failure of the message content suggests the presence of radio interference within the VDES data link at that time. ## 4 VDES Transmission and Reception Messages Consistency Check Test

Assuming the link management message broadcast by the VDES base station in TDB format is \"!ABTDB,1,1,0,A,403t?i1v Lohh!<'JAhH=Tdp50D0 DG,0*6E\", the message received by the VDES shipborne unit after automatic forwarding by VEM, and subsequently received by the VDES base station, is \"!AIVEM,1,1,0,A,403 sooQvMSfR0W'0B4@qWO40D0D E,0*33\". Upon querying the VDES raw message database for parsing the received message

Figure 35: Results and figures of VDES message parameter legitimacy verification testing. and comparing parameters in the AIS/VDES message distributed database, the test results are depicted in Figure 37.

As depicted in Figure 37, querying the VDES raw message database confirmed that the message format aligns with that stored in the 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, thus 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 software interface indicated a discrepancy between the HMAC values of the two messages. Consequently,