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European Frequency and Time Forum

An Asymmetric Channel Attack Monitoring Method for Round-Trip Fiber Time Synchronization System

Xuesong Xu, Yichen Zhang, Hailong Xu, Yiming Bian, Yang Li, Bingjie Xu, Song Yu

Published Jun 25, 2024• 0 citations
Publisher

Abstract

We present a method for monitoring potential asymmetric channel attacks within a round-trip fiber time synchronization system. This is achieved by observing and tracking the time difference at the remote end. Experimental results show that the attacking operation causes unavoidably obvious fluctuations in time difference, with the time difference measured by our monitoring module changing by nanoseconds when an attack occurs, which can be used for attack recognition. Our work provides a reference for further improving the security of fiber time synchronization systems and lays the foundation for the development of more advanced time synchronization attack identification techniques.

DOI
10.1109/eftf61992.2024.10722051

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We present a method for monitoring potential asymmetric channel attacks within a round-trip fiber time synchronization system. This is achieved by observing and tracking the time difference at the remote end. Experimental results show that the attacking operation causes unavoidably obvious fluctuations in time difference, with the time difference measured by our monitoring module changing by nanoseconds when an attack occurs, which can be used for attack recognition. Our work provides a reference for further improving the security of fiber time synchronization systems and lays the foundation for the development of more advanced time synchronization attack identification techniques.
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.
Using optical fiber links to transmit time information is widely regarded as an accurate and secure time synchronization scheme. However, the accuracy of the round-trip time synchronization system can be manipulated arbitrarily by attack schemes. In this paper, a controllable asymmetric attack scheme against round-trip time synchronization system is proposed and experimentally demonstrated. By adjusting the asymmetry of the link, the time synchronization accuracy can be controlled down from 23 ps to 0.91 ns, 2.13 ns and 5.20 ns. Our work reveals system vulnerability caused by asymmetry, and points out that current systems can be attacked unwittingly by adversaries.
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.
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.
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.
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.