This paper presents a survey of state-of-the-art trust models for Vehicular Ad Hoc Networks (VANETs). Trust management plays an essential role in isolating malicious insider attacks in VANETs which traditional security approaches fail to thwart. To this end, many trust models are presented; some of them only address trust management, while others address security and privacy aspects besides trust management. This paper first reviews, classifies, and summarizes state-of-the-art trust models, and then compares their achievements. From this literature survey, our reader will easily identify two broad classes of trust models that exist in literature, differing primarily in their evaluation point. For example, most trust models follow receiver-side trust evaluation and to the best of our knowledge, there is only one trust model for VANETs which evaluates trust at the sender-side unless a dispute arises. In the presence of a dispute, a Roadside Unit (RSU) rules on the validity of an event. In receiver-side trust models, each receiver becomes busy while computing the trust of a sender and its messages upon the messages' arrival. Conversely, in the sender-side class, receivers are free from any kind of computation as the trust is verified at the time the message is announced. Also, vehicles can quickly act on the information, such as taking a detour to an alternate route, as it supports fast decision-making. We provide a comparison between these two evaluation techniques using a sequence diagram. We then conclude the survey by suggesting future work for sender-side evaluation of trust in VANETs. Additionally, the challenges (real-time constraints and efficiency) are emphasized whilst considering the deployment of a trust model in VANETs

翻译：本文对车载自组织网络（VANETs）中的前沿信任模型进行了系统性综述。信任管理在隔离传统安全方法难以抵御的恶意内部攻击方面发挥着至关重要的作用。为此，学界提出了多种信任模型：部分模型仅关注信任管理，另一些则在信任管理之外同时兼顾安全与隐私维度。本文首先回顾、分类并总结了当前最先进的信任模型，进而对其成果进行了比较分析。通过文献梳理，读者可清晰识别出文献中存在的两大类信任模型，其主要差异在于评估节点的选择。例如，绝大多数信任模型采用接收方信任评估机制；据我们所知，目前仅有一种VANET信任模型采用发送方信任评估（除非发生争议）。当出现争议时，由路侧单元（RSU）裁定事件的有效性。在接收方评估模型中，每个接收方需在消息到达时实时计算发送方及其消息的可信度，这会消耗计算资源。相反，在发送方评估模型中，由于消息发布时已完成信任验证，接收方可免于任何计算负担。同时，该模型支持快速决策，使车辆能及时响应信息（例如立即绕行至替代路线）。我们通过序列图对这两种评估技术进行了对比分析。最后，本文为VANET中发送方信任评估的未来研究方向提出建议，并特别强调了在VANET中部署信任模型时面临的实时性约束与效率挑战。