# Reference Broadcast-Based Secure Time Synchronization for Industrial Wireless Sensor Networks
## 1 Introduction  
Industrial wireless sensor networks (IWSNs) [1, 2] connect sensors and actuators in a distributed manner, which is the important perception layer of the Industrial Internet of Things. Many fundamental applications in industrial networks, e.g., data collection [3], timely monitoring [4], time-division multiplexing communication [5], and other coordinated control operations [6], require the nodes in IWSNs to capture the production environment conditions and control the target device in a collaborative and synchronous manner, namely, time synchronization. Unfortunately, each sensor node records its time based on different crystal oscillators caused by manufacturing engineering and environmental influence [7, 8], which results in asynchronous time. As time synchronization is crucial for IWSNs to run normally, many algorithms and protocols are proposed to improve the synchronization accuracy, enhance the robustness, and reduce the energy consumption in IWSNs under no attacks [3, 4, 7, 8, 9, 10, 11]. However, it ignores the fact that secure information acquisition is a key aspect in achieving various time synchronization behaviors. The security problem is vital for the protocol design of IWSNs [12]. The openness of wireless communication and unattended deployment environment make IWSNs particularly vulnerable to intentional attacks [13, 14], such as the physical attack against network devices and the delay attack against communication links [15, 16]. In addition, time synchronization design involves two basic elements: timing and time information transmission, which happens to be closely related to the physical node attack and the delay attack onIWSNs. Consequently, time synchronization is prone to attacks against node identity, communication links, and packets [14]. Complex security mechanisms, such as encryption and authentication, can effectively improve the algorithms' security [17; 18] but are not suitable for IWSNs with severely limited resources including computing, communication, storage, and energy. Therefore, it is necessary to design a secure synchronization protocol in considering the characteristics of the IWSNs' time transfer system. To meet the security requirements of time synchronization, many security mechanisms are proposed. Information encryption [18], authentication technology [17], and threshold detection [15; 16; 19; 20] are the most commonly used defense methods. However, the research seldom considers intelligent attacks, such as the Sybil attacks [21; 22], or the situation in that attackers collude with each other. The Sybil attack is an intelligent attack mode, where the attackers can illegally pretend multiple identities [21; 22]. In time synchronization, a Sybil attacker injects incorrect time information into the network with a disguised legal identity, which produces more serious disruption. It may mislead the valid nodes out of synchronization and be isolated. In addition, most structures of the perceptual control networks in industrial circumstances are centralized. The action of the lower node depends on the command from the master node, namely, the lower node needs to synchronize with the master node, which is termed a reference time source in the time synchronization system. In conclusion, centralized time synchronization is better suited for industrial applications. Unfortunately, the serious single point of failure problem is an unavoidable defect for centralized protocols, and the problem will be magnified under Sybil attacks. Information encryption and decryption can defend well against multiple types of attacks including the Sybil attack, but produce additional communication, computing, and storage costs, which are not desirable in resource-constrained IWSNs. Consequently, we propose a reference broadcast-based secure time synchronization (RSTS) protocol to deal with intelligent attacks for centralized IWSNs. The core content of the RSTS protocol is to construct a novel time information transmission mechanism.