# A Survey of Secure Time Synchronization
## 1 Introduction
The time synchronization protocol should be able to detect several attacks and should be able to enable fast recovery from these attacks [11]. Clocks need to be resynchronized regularly because computers' internal clocks can differ from each other, even when initially set accurately, and therefore, over time computer clocks will be different because clocks in different computers count time at slightly different rates. In recent years, many time synchronization protocols for multi-hop networks have been proposed and their performances have been evaluated using simulation or hardware platforms. In [12], the PISync algorithm was proposed and multi-hop performance was evaluated on the MICAz hardware experimental platform using the flooding time synchronization protocol (FTSP). In this case, the clock frequency of the MICAz node was 1 MHz. The delay compensation-based time synchronization (DCBTS) algorithm was presented in [13] and the time synchronization performance was evaluated using the PISync algorithm on the FPGA hardware platform. Specifically, the hardware clock frequency of the FPGA testbed was 50 MHz. Recently, a series of time synchronization protocols for packet-coupled oscillator (PkCO) networks have been proposed [14; 15; 16; 17; 18]. The packet-coupled oscillators time synchronization protocol (with an accuracy of around 30.5 us) was first proposed in [14], and the effects of packet exchange and processing delay were modeled in hardware experiments and also theoretically analyzed. A proportional-integral control scheme was used to fully remove the impacts of processing delay [14]. The packet-coupled oscillators protocol using a dynamic controller (D-PkCOs) was proposed by [15], and a \(H_{\infty}\) design solution [15; 16] was also proposed to choose the controller parameters. In these two works, synchronization performances (of less than 1 us in a single-hop network [15] and around 6 us in a 21-node spanning tree network [16]) were evaluated on both simulation and hardware platforms. The authors of [17] showed that time synchronization precision of around 1 ms could be realized on low-accuracy RC oscillator clocks (with around 4 \(\times\) 10\({}^{5}\) ppm) in 24-h hardware experiments, and they also demonstrated implementation of the PkCOs algorithm. Moreover, this paper aims to find solutions for the following three research questions (RQs):  
**RQ1** What are the current time synchronization protocols? **RQ2** What are the security issues of time synchronization protocols and the research progress on secure time synchronization? **RQ3** What are the future research trends of secure time synchronization?The contributions of this paper are twofold. First, we provide the recent research progress on time synchronization and secure time synchronization. Second, we suggest some future directions in this domain. The remainder of this paper is organized as follows: In Section 2, we describe the basic concepts, important challenges, and requirements of time synchronization. In Section 3, we analyze the current time synchronization protocols. Then, in Section 4, we review the research progress on secure time synchronization and, in Section 5, we discuss future research work directions. The conclusions for this paper are presented in Section 6.