# A Bidirectional Trust Model for Service Delegation in Social Internet of Things
## 1 Introduction
Moreover, they analyzed the various attack scenarios, such as slandering/bad-mouthing attack, Sybil attack, self-promoting attack, and ballot stuffing attack to demonstrate the effectiveness of the proposed model. [PERSON] et al. proposed an integrated trust evaluation model which combines direct and indirect trustworthiness [32; 33]. Moreover, they further proposed a series of new metrics, such as friendship similarity, social contact similarity, and community of interest similarity to quantify the indirect trust evaluation. They also applied the typical application scenarios, including air pollution detection and augmented map travel assistance, to illustrate the feasibility of the proposed model. In order to comprehensively compare the recent studies along with advantages and disadvantages, we presented detailed comparison of various works in the literature on the SIoT trust model in our previous work [34]. However, the current research on trust model in SIoT still faces three important challenges. First, most works focus on the unidirectional trust evaluation from the SR to the SP. The evaluation of the trustworthiness of SR is ignored, which may cause the trust crisis from the SPs to the SR. The SPs may gradually lose enthusiasm if they suffer prejudiced treatment of the malicious SR. Second, the trust model and service delegation are context- or environment dependent. The properties of the same task are different in different contexts or environments. Third, the decision of service delegation should not only consider the trust of SPs, but also the utility of the SR. In addition, the correlation between trust and utility is ambiguous. To address the above challenges, we propose a bidirectional trust model and trust-based service delegation approach by comprehensively considering the trust and utility of service requesters and providers. We combine the social trust theory and characteristics of IoT tasks to formalize the trust evaluation and service delegation model. The main contributions of this paper are as follows:  
* In order to improve the quality of the IoT service, we propose the bidirectional evaluation and selection model between the SRs and SPs to formulate the process of service or task in SIoT, thus preventing the malicious behaviors of SRs and SPs. * A context-aware trust model which comprehensively considers the task properties in the specific environment is presented. We employ the subjective logic to construct the opinion-based and evidence-based trust quantification method. * We present a trust-based service delegation approach that optimizes the utility of the SR while effectively isolating the malicious SPs. Since the service delegation problem in SIoT seldom considers the trust and utility issue at the same time, this paper explores the correlation of trust and utility and their impacts on the service delegation. * In order to validate the feasibility of our proposed trust model and service delegation method, we present a series of vital experiments to explain the operation of our model. Our results show that the proposed model can effectively assist the IoT object to make the decision of the service delegation. The remainder of the paper is organized as follows: the system overview and problem statement are presented in Section 2. On this basis, we present the trust and service delegation model, including the trust quantification method and integrated service delegation mechanism in Section 3. In Section 4, in order to demonstrate the feasibility of proposed trust model, we present a series of experiments. In Section 5, we conclude the paper and summarize the contributions. Moreover, some pending research issues are discussed for further research.