# Social Internet of Vehicles for Smart Cities
## 5 SIoV Security Issues
### SIoV Trust Issues
#### 5.2.1 Trust Characteristics of SIoV  
Trust in the SIoV exhibits five characteristics that are different from trust in traditional social settings.  
Uncertainty  
Trust in the SIoV is uncertain and dynamic. Because the SIoV is inherently dynamic, information can change rapidly, and trust may only be valid for a short period of time. When establishing trust, the SIoV needs to take into account three uncertainties caused by the SIoV's dynamic nature: the uncertainty of an entity's identity, the uncertainty of an interaction entity's behaviour (intentional or unintentional) and the uncertainty in observations. Existing work in social networks addressed this issue by expressing trust using a continuous variable instead of a discrete-valued variable, as the former can better capture the dynamic feature of the context [105].  
Subjectivity  
Trust in the SIoV is subjective. Our level of trust depends on how our own actions are affected by the context [106]. Trust decision-making thus exhibits a level of subjective probability that is difficult to predict and monitor. We face an inherent risk in vehicular networks when we make trust decisions within the SIoV. Existing work in social trust addressed this issue by proposing recommendation systems. For example, [PERSON] proposed a recommendation system for online social networks based on Bayesian inference [107]. Users can share content ratings with their friends, allowing them to make informed decisions. This provides an opportunity to choose the option with the smallest level of perceived risk.  
Intransitivity  
Trust in the SIoV is not always transitive. Trust is transitive when it can be extended outside the scope of the two entities who established it [108]. However, this is not always the case. If A trusts B and B trusts C, this does not necessarily mean that [PERSON] trusts C. Trust transitivity depends on the extent to which you trust the trustee and the trustee's recommendations. As with subjectivity, research in social networks has developed recommendation systems to make trust-based recommendations by allowing users to share trust ratings with peers.  
Context Dependence  
Trust in the SIoV is context-dependent. For example, [PERSON] may trust [PERSON] as a physician to perform medical checks (in the context of a medical situation), while [PERSON] would not trust [PERSON] to perform surgery (in the context of an accident). Similarly, different types of trust need to be established in an SIoV, depending on the given context. In social networks, [PERSON]'s subjective logic [109] has been widely used to model trust networks. [PERSON] uses subjective logic in a decision-making approach that considers the notion of confidence. The approach determines weights associated with trust ratings and outputs a trust degree that depends on the current interaction context [110].  
Non-Cooperativeness  
Trust in the SIoV is not always cooperative. Trust decisions are usually made cooperatively between different entities. However, not all entities in an SIoV are cooperative. Entities may refuse to cooperate for selfish or malicious reasons. Existing work in social networks aims to separate selfish entities once they are detected and encourages altruistic behaviours with incentive mechanisms. [PERSON] [111] researched the trade-off between selfish and altruistic behaviours in terms of a node's individual welfare, for example saving energy, and global welfare, for example achieving a goal with adequate service availability.