By interacting, synchronizing, and cooperating with its physical counterpart in real time, digital twin is promised to promote an intelligent, predictive, and optimized modern city. Via interconnecting massive physical entities and their virtual twins with inter-twin and intra-twin communications, the Internet of digital twins (IoDT) enables free data exchange, dynamic mission cooperation, and efficient information aggregation for composite insights across vast physical/virtual entities. However, as IoDT incorporates various cutting-edge technologies to spawn the new ecology, severe known/unknown security flaws and privacy invasions of IoDT hinders its wide deployment. Besides, the intrinsic characteristics of IoDT such as \emph{decentralized structure}, \emph{information-centric routing} and \emph{semantic communications} entail critical challenges for security service provisioning in IoDT. To this end, this paper presents an in-depth review of the IoDT with respect to system architecture, enabling technologies, and security/privacy issues. Specifically, we first explore a novel distributed IoDT architecture with cyber-physical interactions and discuss its key characteristics and communication modes. Afterward, we investigate the taxonomy of security and privacy threats in IoDT, discuss the key research challenges, and review the state-of-the-art defense approaches. Finally, we point out the new trends and open research directions related to IoDT.

翻译：通过与其物理实体实时交互、同步和协作，数字孪生有望推动现代城市实现智能化、预测性和优化化发展。通过大规模物理实体及其虚拟孪生体之间的孪间通讯与孪内通讯，数字孪生互联网（IoDT）能够实现跨海量物理/虚拟实体的自由数据交换、动态任务协作与高效信息聚合，从而形成复合洞察力。然而，IoDT整合多种前沿技术以构建新生态的同时，其已知和未知的严重安全缺陷及隐私侵犯问题也阻碍了其广泛部署。此外，IoDT的固有特性（如去中心化结构、信息中心路由和语义通信）给其安全服务提供带来了严峻挑战。为此，本文从系统架构、使能技术与安全/隐私问题三个维度对IoDT进行深度综述。具体而言，我们首先提出一种具有信息物理交互特性的新型分布式IoDT架构，并探讨其关键特征与通信模式；继而分析IoDT中安全与隐私威胁的分类体系，讨论核心研究挑战，并综述当前最先进的防御方法；最后，指出IoDT相关的新兴趋势与开放研究方向。