Secure Combination of Untrusted Time Information Based on Optimized Dempster–Shafer Theory

Secure precision time synchronization is important for applications of cyber-physical systems (CPSs). However, several attacks, especially the time delay attack (TDA), deteriorate the performance of the time synchronization system seriously. The multiple paths scheme is thought as an effective security countermeasure to decrease the influence of TDA. However, the effective secure combination algorithm is still missed for precision time synchronization. In this article, a secure combination algorithm based on the Dempster-Shafer (D-S) theory is proposed for the multiple paths method. Special optimizations are done for the combination algorithm to solve the potential problems due to untrusted evidence. Theoretical simulation shows that the proposed algorithm works much better than the fault-tolerant algorithm (FTA) and the attack detection method based on a single path. An experimental demonstration proves the feasibility and superiority of the proposed algorithm, where the time stability with 27.97, 1.57, and 1.12 ps at average time 1, 10, and 100 s is achieved under TDAs and local clock jumps. The proposed algorithm can be used to improve the security and resilience of many important synchronization protocols, such as network time protocol (NTP), precision time protocol (PTP), and two-way fiber-optic time transfer (TWFTT).