1 Introduction
2 Related work
3 Multi-level security certification of VANETs
3.1 3.1 Security authentication based on direct trust evaluation
3.1.1 3.1.1 The security vector model
3.1.2 3.1.2 Trust evaluation based on the historical security events
3.2 3.2 Security authentication based on indirect trust evaluation
3.2.1 3.2.1 The recommendation trust vector model
3.2.2 3.2.2 Trust evaluation based on the recommendation trust value
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When the correlation coefficient ρ ns of T_V a and T_V n belongs to (1−Δ ρ 1,1), T_V a and T_V n are close to linear correlation. Therefore, we can consider that the node recommendation trust consists with the average recommendation trust. We think the recommendation trust of the node is reasonable. The recommendation node is not a malicious node.
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When the correlation coefficient ρ ns of T_V a and T_V n belongs to (1−Δ ρ 1−Δ ρ 2,1−Δ ρ 1], the linear relationship between T_V a and T_V n is not so obvious. Therefore, the reasonableness of the recommended trust value needs a further discussion. We introduce a norm method to discuss reasonableness of the recommendation trust. The single recommendation trust vector and average recommendation trust vector are normalized by using the following equations:$$ {}\|T\_V_{n}\|=\sqrt{T\_V_{n},T\_V_{n}} =\sqrt{P_{S1}^{XB}(t)^{2}+P_{S2}^{XB}(t)^{2},\ldots,P_{Sm}^{XB}(t)^{2}} $$(9)$$ {}\|T\_V_{a}\|=\sqrt{T\_V_{a},T\_V_{a}} =\sqrt{P_{S1}^{B}(t)^{2}+P_{S2}^{B}(t)^{2},\ldots,P_{Sm}^{B}(t)^{2}} $$(10)The deviation between ∥T_V n ∥ and ∥T_V a ∥ can be calculated by the following:$$ \Delta\|T\_V\|=|\|T\_V_{n}\|-\|T\_V_{a}\|| $$(11)We set the minimum deviation of normalized trust vector as \(\|T\_V\|_{\text {min}}^{'}\). When \(\Delta \|T\_V\|>\|T\_V\|_{\text {min}}^{'}\), we think the recommendation node is not trusted. Otherwise, the deviation is small. We think that the recommendation node is in a temporary trusted state.
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When the correlation coefficient ρ ns of T_V a and T_V n belongs to (0,1−Δ ρ 1−Δ ρ 2], T_V a and T_V n do not show a linear relationship. Therefore, we can consider that the deviation is very large. We think the recommendation trust value of the node is unreasonable. The recommendation node is a malicious node.