Abstract
Internet of Things (IoT) is a rapidly growing field which provides seamless connectivity to physical objects to make them part of smart environment. In order to fully utilize the potential power of these connected objects of IoT, trust existence among these objects is essential. Traditional security measures are not enough to provide the comprehensive security to this smart world. Trust is used to mitigate the risk of uncertainty while connecting nodes to the intemet. Different trust models for IoT environment have been proposed. However, these have not completely mapped with the uncertain and dynamic environment of smart IoT. This paper presents a comprehensive overview of existing surveys on trust models of IoT. It provides classification of Trust Related Attacks (TRA) and comparison of existing trust models with respect to TRA and Function Requirements (FR) of IoT. The aim of this comparison is to summarize the FR of IoT which must be considered while designing Trust Management System (TMS). Furthermore, this survey categorizes and compares existing trust models with respect to their resiliency against TRA, including: attack on node, attack on service, and attack on communication path. The paper expands on current open issues and identifies possible future research directions to address them. Finally, a conceptual framework has been proposed which shows the minimum requirements to make a successful trust model for smart IoT environment. Precisely, this study will help the reader to understand the vulnerabilities in existing IoT trust models and will direct towards future work to propose new models which can cater all possible and highlighted threats.