## 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
The complexity of the MD5 algorithm can be classified into birthday search, near-collision, and computation time. Among these, birthday search represents a birthday attack, which is the application of probabilistic modeling to lower the complexity of identifying MD5 hash function conflicts, as well as to assess the approximate risk of a hash conflict within an MD5 hash of a given overall scale. The complexity of birthday search \(C_{b}(r,k,w)\) is dictated by two principal factors, namely the search space \(|V|=2^{64+k}\) and the probability \(P_{r,k,w}\). The probability \(P_{r,k,w}\) refers to the probability of attaining a birthday collision that is not a pseudo-collision within the aforementioned search space and that the collision can be materialized given the selected

Figure 6: Results of successful message content integrity verification detection. Figure 7: Comparison chart of computational speed of message digest algorithms. parameter \(w\) as well as the requirement of approximating the number of collisions up to \(r\) times [37]. Hence, the complexity of the birthday search is \(C_{b}(r,k,w)=\sqrt{2^{64+k}\pi/2P_{r,k,w}}\). In 2009, [PERSON] et al. provided a theoretically optimal attack complexity of \(2^{39}\)[38]. Near-collision denotes the presence of two distinct input data having the same MD5 hash value, for the differential path construction and finding the near-collision block two components. Regarding near-collision complexity \(C_{nc}(r,k,w)\), it consists of differential path construction complexity C1 and finding near-collision block complexity C2. The computation of differential path construction complexity C1 is relatively independent, and the average complexity of constructing such differential paths is \(\overline{C}_{1}\approx 2^{35}\)[38]. The complexity C2 of identifying each pair of collision blocks, conversely, relies predominantly on the parameter w, the strength of the tunnel, and the number of conditions in the last 2.5 rounds of the differential path [39]. For \(w=0\), 1, 2 and the aforementioned differential path constructed above, the average complexity of identifying each pair of collision blocks is \(\overline{C}_{2}\approx 2^{34}\)[38]. Based on the foregoing analysis, the collision complexity of the MD5 algorithm when \(w=2\), \(k=0\) and \(r=9\) is \(2^{39.1}\)[38]. The complexity of the birthday search and r near-collisions at this juncture are \(2^{37}\) and \(2^{38.8}=2^{38.8}=9\times(2^{34}+2^{35})\)[38]. This complexity is the optimal outcome obtained by weighing the parameters and the equilibrium relationship between the two components of the complexity, whereas in practice, the number of near-collision blocks amounts to 3; thus, the actual attack complexity is approximately \(2^{49.9}\) when \(w=5\), \(k=8\) and \(r=3\)[38]. Regarding the analysis of the computational time complexity, the paper conducts the time complexity analysis of the MD5 algorithm on page 13, that is, it computes the time for the MD5 algorithm and the other three typical hash algorithms, SHA256, SHA384 and SHA512 algorithms, to process the same VDES message once, and the VDES message utilized is \"IABTDB,1,1,0,A,403t?j1 vLohh<JAhH=Tdp500D0G,0*6E\". For the processing of the same VDES message, the MD5 algorithm consumed the least time at 9.8928 ms, followed by the SHA256 algorithm at 12.4144 ms, the SHA384 algorithm at 13.6527 ms, and the SHA512 algorithm, which consumed the most time, at 16.4384 ms.

#### 2.1.2 VDES Message Consistency Detection Method Based on the HMAC Algorithm

The VDES base station broadcasts link management messages in TDB format, which are automatically relayed by VDES ship stations using VEM statements. However, due to radio interference in the data link, the content of messages relayed by VDES ship stations and subsequently received again by the VDES base station may be tampered with, resulting in inconsistencies and posing safety risks to maritime navigation.