# Toward a Trust Evaluation Mechanism in the Social Internet of Things
## 4 REK Trust Evaluation Model in the SIoT  
### REK Trust Evaluation Model  
We propose a trust evaluation model that comprises of triad of Reputation, Experience and Knowledge TIs so-called REK Trust Evaluation Model (Figure 5). The reason to come up with the three TIs is that in social science, people normally base their determination of trust on three main sources: (i) public opinion on a trustee (as Reputation); (ii) previous transaction with a trustee (as Experience); and (iii) understandings on a trustee (as Knowledge). We believe this social cognitive process could be applied to the SIoT environment.  
Knowledge TI is the _direct trust_ mentioned in Section 3 that renders trustor's perspective on trustee's trustworthiness in a respective environment. Knowledge TI can be obtained based on limited available information about characteristics of the trustee and the environment under the trustor's observation. Knowledge TI can reveal a portion of trust which is illustrated in Figure 5. It indicates more about trustworthiness of the trustee and trustor's propensity but not much about the environmental vulnerabilities, threats and risks.  
Experience and Reputation TIs are social features and attained by accumulating previous interactions among entities in the SIoT over time. Experience TI is a personal perception of the  
Figure 4: Trust evaluation and risk management in comparison.  
Figure 5: Reputation, experience and knowledge as the three indicators in the REK trust evaluation model.  
trustee's trustworthiness by analyzing previous interactions from a specific trustor to a particular trustee in various contexts. As the personal perception, Experience TI indicates more about trustor's propensity but not trustee's trustworthiness and environmental factors due to limited knowledge obtained. Reputation TI, instead, reflects global perception about a trustee by aggregating all previous experiences from entities (in a society) with this trustee. Thus, Reputation TI is able to effectively exhibit about the trustee's trustworthiness and the environment characteristics; but not about the trustor's propensity (Figure 5). In SIoT scenarios with billions of entities, there is very high possibility that there are no prior interactions between two any entities, resulting in no Experience. Therefore, Reputation TI is a necessary indicator for trust, especially in case there are no previous interactions between a trustor and a trustee. Reputation is taken into account when evaluating trust because of the propagation characteristic of trust: Each entity (a trustor) has previous interactions with a specific entity (as the trustee) has its own opinions; and a reputation model (or a recommendation model) let it share the opinions (as its recommendations) to others. Entities, then, can refer the opinions as one of the cues of trust to personally judge trust. By doing so, trust is propagated throughout the network.  
By synthesizing the three TIs, REK Trust Evaluation Model consolidates the _computational trust_ so that it can be used on behalf of the _complete trust_ in most of cases in the SIoT environment with high accuracy.