  Secure precision time synchronization is important for applications of
Cyber-Physical Systems. However, several attacks, especially the Time Delay
Attack (TDA), deteriorates the performance of time synchronization system
seriously. 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 paper, a secure combination algorithm based on Dempster-Shafer theory is
proposed for 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 Fault Tolerant Algorithm (FTA) and the attack detection method
based on single path. And experimental demonstration proves the feasibility and
superiority of the proposed algorithm, where the time stability with 27.97 ps,
1.57 ps, and 1.12 ps at average time 1s, 10s, 100s is achieved under TDA and
local clock jump. The proposed algorithm can be used to improve the security
and resilience of many importance synchronization protocol, such as NTP, PTP,
and TWFTT.
