# Toward a Trust Evaluation Mechanism in the Social Internet of Things
## 4 REK Trust Evaluation Model in the SIoT
### Knowledge TI Evaluation Model
#### 4.2.1 A General Set of TAs for Knowledge TI  
For the first task, we specify six important attributes introduced in the system dependability concept namely Serviceability, Safety, Reliability, Confidentiality, Availability, and Integrity as six TAs for the _Ability dimension_ of the trustworthiness illustrated as D1 to D6 in Figure 6. These six TAs could precisely indicate capability of a trustee to dependably accomplish a trust goal. Besides, the _Ability dimension_ might contain other TAs according to a specific scenario. For instance, in the User Recruitment in MCS use-case, the spatial distance between a trustor and a trustee is considered as a TA (see Section 4.2.2). The meanings of the six TAs in quantifying trustworthiness are as following:  
* _Availability:_ Probability of an entity in operation in a given period of time. * _Confidentiality:_ Preserving the authorized restriction on access and disclosure on data, information or system. * _Integrity:_ Ability to guard against improper modifications and destruction. * _Safety:_ A property to guarantee that an entity will not fail in a manner that would cause a great amount damage in a period of time. * _Reliability:_ Probability that a component correctly performs a required job in a specified period of time under stated conditions. * _Serviceability:_ Property indicating how easy and simply a system can be repaired or maintained. Generally, combination of the TAs is a measure of a system's capability to accomplish a given task that can be defensibly trusted within a period of time [37]. However, it is not necessary to include all of the six TAs which could require huge effort. Instead, only some of them are necessarily taken into consideration according to a specific trust goal and environmental factors. The TAs are quantitatively or qualitatively measured based on different types of information and methodologies, which have been intensively explored over time [38]. Each TA can be slightly interpreted and attained differently depending on particular use-cases due to the variations and ambiguity of its linguistic meaning. Details of dependability models can be found on a large number of articles such as Cyber-Physical System (CPS) Framework [39] and Managing Information Security Risk [36] by National Institute of Standards and Technologies (NIST). As SIoT environment, we characterize two major TAs constituted the _Benevolence dimension_ for Knowledge TI as _Cooperativeness_ and _Community-Interest_ illustrated as B1 and B2; and two TAs constituted the _Integrity dimension_ as _Honesty_ and _Similarity_, illustrated as I1 and I2 in Figure 6, respectively. * _Cooperativeness_: this property indicates the level of cooperativeness between a trustor and a trustee based on the following hypothesis: \"the more cooperative between the two entities in a social network, the more trustworthy they are\". Cooperativeness can be calculated by considering the common features between the two entities such as mutual friends and same locations. * _Community-Interest_: Due to the integration of social networks in SIoT, concept of community (of SIoT entities) is also introduced that refers to a group of entities sharing same characteristics (e.g., physical areas, a same goal, and same required tasks). This property indicates the level of community relationship between two entities based on the following hypothesis: \"the more similar among communities that entities belong to, the more trustworthy they get\". * _Honesty_: a property indicates the level of honesty of an entity based on observation toward an entity that whether it conducts some suspicious interactions or it breaks social etiquette using a set of anomaly detection rules. * _Similarity_: a property indicates the level of similarity between two entities (in terms of their features) using similarity measurement mechanisms between two profiles of entities [40]. This TA is taken into account because of the following hypothesis: \"a trustor tends to trust a trustee if they are similar\".