  Two-way fiber-optic time transfer is a promising precise time synchronization
technique with sub-nanosecond accuracy. However, asymmetric delay attack is a
serious threat which cannot be prevent by any encryption method. In this paper,
a dynamic model based scheme is proposed to defense the sub-nanosecond
asymmetric delay attack. A threshold is set according to the estimated time
difference by a two-state clock model where the fixed frequency difference is
excluded from the time difference to detect the asymmetric delay attack which
is smaller than the time difference induced by the fixed frequency difference.
Theoretical simulation and experimental demonstration are implemented to prove
the feasibility of the scheme. A two-way fiber-optic time transfer system with
time stability with 24.5ps, 3.98ps, and 2.95ps at 1s, 10s, and 100s averaging
time is shown under sub-ns asymmetric time delay attack experimentally. The
proposed method provides a promising secure sub-ns precise time synchronization
technique against asymmetric delay attack.
