# Toward a Trust Evaluation Mechanism in the Social Internet of Things
## 2 Augmentation of Trust Concept in the SIoT
### Conceptual Trust Model in SIoT Environment  
It is important to clarify that trust is neither a property of a trustor (e.g., trustor's preferences) nor a property of a trustee (e.g., trustee's trustworthiness and trustee's reputation). It is a relationship between the trustor and the trustee that is subjective and asymmetric which is derived from the triad of trustee's trustworthiness, trustor's propensity and environment's characteristics. Based on the clarification of the trust concept, a conceptual trust model in the SIoT is proposed as illustrated in Figure 2. Then, a more specific trust definition in the SIoT associated with the conceptual trust model is proposed as follows:  
Trust is the _perception of a trustor_ on _trustee's trustworthiness_ under a _particular environment_ (within a _period of time_) so-called _perceived trustworthiness_.  
According to the proposed model illustrated in Figure 2, trust will be obtained by harmonizing the trustor's propensity and environment conditions into the trustee's trustworthiness. The harmonization is accomplished by aggregating both the observation of a trustor toward a trustee and the interactions between the two. It is worth to note that the environment conditions are reflected as risks taken during the observations and interactions. The trustor's propensity includes both requirements for the trust goal and the trustor's preferences about the trustee's trustworthiness whereas the environment conditions are the considerations for some factors such as vulnerabilities, threats and risks. The trust goal requirements with the environmental factors helps determining the set of TAs for deriving the perceived trustworthiness whereas the trustor's preferences is to help combining these TAs to obtain an overall trust value for making a decision. For example, trustor's preferences could be represented in forms of weights of TAs, indicate the levels of importance of the TAs when constructing trust. Trust as perceived trustworthiness is as an instance of trustee's trustworthiness respecting to a particular trustor and an environment, thus, even same a trustee and same an environment, different trustors might  
Figure 2: Conceptual Trust Model in the SIoT environment.  
have different propensities of the trustee's trustworthiness. This illustrates the subjective characteristic of trust. Another important characteristic of trust is the context-dependence that can also be illustrated using this conceptual model as follows: with the same trustor and trustee, different environments might result in different TAs and different trustor's propensities.  
Based on the conceptual model, the goal of any trust model is two-fold: (i) to specify and evaluate TAs of the trustworthiness of a trustee respecting to the trustor's propensity and the environment conditions; (ii) to combine the TAs to finalize the perceived trustworthiness as the trust value. From now on in this article, the term \"trust\" is referred to this conceptual model and it is exchangeably used with the term \"perceived trustworthiness\".