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IEEE Transactions on Instrumentation and Measurement

Secure Two-Way Fiber-Optic Time Transfer Against Sub-ns Asymmetric Delay Attack With Clock Model-Based Detection and Mitigation Scheme

Yang Li, Jinlong Hu, Yan Pan, Wei Huang, Li Ma, Jie Yang, Shuai Zhang, Yujie Luo, Chuang Zhou, Chenlin Zhang, Heng Wang, Yun Shao, Yichen Zhang, Xing Chen, Ziyang Chen, Song Yu, Hong Mei Guo, Bingjie Xu

Published Jan 1, 2023• 14 citations
Publisher

Abstract

Two-way fiber-optic time transfer (TWFTT) is a promising precise time synchronization technique with subnanosecond stability. However, the asymmetric delay attack is a severe threat, which can deteriorate the performance of the TWFTT system. In this article, a clock model-based scheme is used to defend the subnanosecond asymmetric delay attack. For the scheme, a security threshold is set according to a two-state clock model, and the estimated frequency difference is excluded from the measured time difference to detect the subnanosecond asymmetric delay attack. Systematic detection and mitigation scheme for asymmetric delay attack is developed in this article. Theoretical simulation and experimental demonstration are implemented to explore the feasibility of the method. A TWFTT system of time stability with 24.5, 3.98, and 2.95 ps at average times of 1, 10, and 100 s is shown under subnanosecond asymmetric time delay attack experimentally for the first time. The proposed method provides a promising secure subnanosecond precise time synchronization technique against asymmetric delay attacks.

DOI
10.1109/tim.2023.3268476

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Two-way fiber-optic time transfer (TWFTT) is a promising precise time synchronization technique with subnanosecond stability. However, the asymmetric delay attack is a severe threat, which can deteriorate the performance of the TWFTT system. In this article, a clock model-based scheme is used to defend the subnanosecond asymmetric delay attack. For the scheme, a security threshold is set according to a two-state clock model, and the estimated frequency difference is excluded from the measured time difference to detect the subnanosecond asymmetric delay attack. Systematic detection and mitigation scheme for asymmetric delay attack is developed in this article. Theoretical simulation and experimental demonstration are implemented to explore the feasibility of the method. A TWFTT system of time stability with 24.5, 3.98, and 2.95 ps at average times of 1, 10, and 100 s is shown under subnanosecond asymmetric time delay attack experimentally for the first time. The proposed method provides a promising secure subnanosecond precise time synchronization technique against asymmetric delay attacks.
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.
Precise and secure time synchronization between two remote sites is necessary in many practical scenarios. Recently, the two-way fiber time transfer (TWFTT) system has become an attractive option for high-precision time synchronization. However, TWFTT is implemented based on symmetric link assumption, and such assumption may be maliciously utilized to cause significant time asynchronization to TWFTT. In this paper, we propose two novel controllable asymmetry attack schemes for TWFTT, namely, the link asymmetry attack and the attenuation asymmetry attack. We theoretically analyze the controllability and the effectiveness of these two attacks. Experimental results show that our link asymmetry attack can introduce 2.49 ns time offset per asymmetric link length (in meters), and our attenuation asymmetry attack can introduce at most 36.6 times time offset (compared to normal situation) when the attenuation is 45.3%. Our work can provide instructive insights for future studies of protecting TWFTT from delay asymmetry attacks.
We build a new two-way fiber time transfer technique (TWFTT) simulation model, and simulate controllable asymmetric attack of delay and attenuation. Both asymmetric attacks can make the clock in the remote module slower by attacking the 1pps signal from local to remote side. Otherwise, the clock will be faster. In this paper, the asymmetric delay attack can linearly control the synchronization error from 0 to 300 ps. The asymmetric attenuation attack can adjust the synchronization error from 0 ~ 302.7 ps with the controllable attenuation from 0 to 2.8 dB. Moreover, we find that the time interval counter change greatly when the system is attacked. The research has a significant meaning in defense of such asymmetric attacks.
The round-trip fiber time synchronization system is preferred for high-precision time synchronization due to its easier implementation compared to bidirectional systems. However, its link-symmetry assumption renders it susceptible to asymmetric delay attacks, posing a serious threat to the system that current encryption methods cannot effectively prevent. This letter proposes a controllable asymmetric channel attack scheme for the round-trip fiber time synchronization system. The objective is to assess the impact of asymmetry on time synchronization accuracy by measuring the time difference between the local and remote ends of the system. Experimental results demonstrate that by employing asymmetric channels, the synchronization accuracy of the system can be reduced to 1.01 ns, 2.09 ns, and 5.06 ns, respectively, from the initial accuracy of 52.40 ps. This work highlights the critical security issue in round-trip systems, emphasizing the urgent need for effective countermeasures against asymmetric delay attacks.
The two-way quantum time transfer method has been proposed and experimentally demonstrated for its potential enhancements in precision and better guarantee of security. To further testify its advantage in practical applications, the applicable direct transmission distance as well as the achievable synchronization precision between independent time scales is of great interest. In this paper, an experiment on two-way quantum time transfer has been carried out over a 50 km long fiber link. With the common clock reference, a short-term stability of 2.6 ps at an averaging time of 7 s and a long-term stability of 54.6 fs at 57300 s were obtained. With independent clock references, assisted by microwave frequency transfer technology, the achieved synchronization showed almost equal performance and reached a stability of 89.5 fs at 57300 s. Furthermore, the spectral consistency of the utilized entangled photon pair sources has been studied concerning its effect on the transfer accuracy and long-term stability. The results obtained have promised a bright future of the two-way quantum time transfer for realizing high-precision time synchronization on metropolitan area fiber links.
The two-way quantum clock synchronization has been shown to provide femtosecond-level synchronization capability and security against symmetric delay attacks, thus becoming a prospective method to compare and synchronize distant clocks with enhanced precision and safety. In this letter, a field test of two-way quantum synchronization between a H-maser and a Rb clock linked by a 7 km-long deployed fiber is implemented by using time-energy entangled photon-pair sources. Limited by the intrinsic frequency stability of the Rb clock, the achieved time stability at 30 s is measured as 32 ps. By applying a fiber-optic microwave frequency transfer technology to build frequency syntonization between the separated clocks, the limit set by the intrinsic frequency stability of the Rb clock is overcome. A significantly improved time stability of 1.9 ps at 30 s is achieved, which is mainly restrained by the low number of acquired photon pairs due to the low sampling rate of the utilized coincidence measurement system. Such implementation demonstrates the high practicability of the two-way quantum clock synchronization method for promoting field applications.