# A Survey of Secure Time Synchronization
## 4 Secure Time Synchronization
### Secure Time Synchronization Related Works  
Recently, many studies have addressed security issues in sensor networks [5; 10; 56] and have tried to provide solutions, however, it has been difficult to implement these solutions with the existing time synchronization techniques because they require high computational resources or provide low accuracy. Therefore, it proves that the existing time synchronization techniques were not designed with security in mind which leaves them vulnerable to many security attacks [6].  
[PERSON] et al. proposed a protocol for secure time synchronization which was resilient to outside attackers and inside compromised node attacks [57]. However, the main focus of the study was to detect the attack and abort the ongoing time synchronization rather than to prevent the attacks by taking actions against them. In addition, the secure technique proposed by [57] achieved the same accuracy as the TPSN during pairwise synchronization; however, for multi-hop synchronization, its accuracy decreased. In [5], the authors suggested a set of secure time synchronization protocols in which synchronization was addressed against a pulse-delay attack. However, in these protocols, nodes must go through a process of preloaded static key discovery which is performed through many communication messages, increasing the communication cost of the network and making them unsuitable for sensor networks.  
[PERSON] et al. published a theoretical work in which they described three major time synchronizing protocols: the RBS protocol, the TPSN, and the FTSP [58]. They outlined the attacks on each protocol in WSNs and proposed techniques to mitigate these attacks. However, compromise attacks were their main focus. After reviewing some of the current time synchronization algorithms, some of the possible attacks such as eavesdropping, pulse-delay attacks, and compromise attacks that could be performed on those algorithms, were presented by [59]. To protect the network against an outsider, they used authentic and cryptographic techniques.  
[PERSON] et al. proposed an attack-tolerant time synchronization protocol (ATSP), which is more desirable for WSNs [56]. Although the ATSP is able to provide synchronization in adistributed manner across a network and can accurately detect attacks, there is still a clock skew error. The above-mentioned studies show that it is important to consider security during the design of a protocol; it is complex and difficult to provide secure time synchronization to avoid cyber-physical attacks, if security is not considered in the designing stage. Possibly, it would be easier to design a new secure protocol than to modify and secure an existing protocol. In [10], the authors proposed a novel secure time synchronization protocol for WSN models based on bilinear pairing functions. The proposed protocol was applied to both heterogeneous and homogeneous WSN models. Moreover, in another study [60], the authors used a graph theoretical approach and proposed a robust and secure time synchronization protocol, named RTSP, against Sybil attacks.  
Many contributions have been made to the literature on secure time synchronization to provide security and to solve the privacy issues for routing. However, the existing time synchronization approaches require increased computational resources for routing which make it difficult to implement them for security purposes, especially in sensor networks. Thus, all the current schemes for time synchronization have not considered security in their designs which makes themselves vulnerable to security attacks [6]. Most of the efforts in the literature review have been devoted to provide secure time synchronization with a focus on updating and enhancing the existing protocols for WSNs by using authentication and fault detection mechanisms [61].