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European Frequency and Time Forum

A Tampering Risk of Fiber-Based Time-Frequency Synchronization and Its Countermeasure

Hongfei Dai, Wenlin Li, Dongqi Song, Bo Wang

Published Jun 25, 2024• 0 citations
Publisher

Abstract

Fiber networks are utilized globally and are highly valued as effective channels for transmitting time-frequency (TF) signals. Over recent decades, considerable research has been dedicated to fiber-based TF synchronization technologies. Instruments developed from these technologies have proven effective in various applications. As these instruments become more widespread, their security issues have increasingly come to the forefront. This paper introduces a method known as the “frequency expander” to address frequency tampering. On a 200 km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). Additionally, we propose a hybrid fiber-based time synchronization and vibration detection method as a countermeasure. This hybrid method is exploring the idle light within the time synchronization system, i.e., Rayleigh backscattering, which is usually considered harmful.

DOI
10.1109/eftf61992.2024.10722318

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Fiber networks are utilized globally and are highly valued as effective channels for transmitting time-frequency (TF) signals. Over recent decades, considerable research has been dedicated to fiber-based TF synchronization technologies. Instruments developed from these technologies have proven effective in various applications. As these instruments become more widespread, their security issues have increasingly come to the forefront. This paper introduces a method known as the “frequency expander” to address frequency tampering. On a 200 km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). Additionally, we propose a hybrid fiber-based time synchronization and vibration detection method as a countermeasure. This hybrid method is exploring the idle light within the time synchronization system, i.e., Rayleigh backscattering, which is usually considered harmful.
Fiber optic networks are used worldwide and have been regarded as excellent media for transmitting time–frequency (TF) signals. In the past decades, fiber-based TF synchronization techniques have been extensively studied. Instruments based on these techniques have been successfully applied. With the increasing application of TF synchronization instruments, their security has become an important issue. Unfortunately, the security risks of fiber-based frequency synchronization (FbFS) instruments have been overlooked. This article proposes a frequency tampering method called “frequency expander.” On a 200-km fiber link, we demonstrate a frequency tampering scenario using a frequency expander-enabled frequency tampering module (FTM). On the user side, the frequency value of the recovered 100-MHz signal can be stealthily altered within a range of 100 MHz ± 100 Hz, while the frequency dissemination stability of the system remains normal. Related to this tampering risk, potential hazards in two different application scenarios, which rely on precise frequency references, are analyzed. Two countermeasures are also proposed to solve this tampering risk.
Fiber optic networks are used worldwide and have been regarded as excellent media for transmitting time-frequency (TF) signals. In the past decades, fiber-based TF synchronization techniques have been extensively studied. Instruments based on these techniques have been successfully applied. With the increasing application of TF synchronization instruments, their security has become an important issue. Unfortunately, the security risks of fiber-based frequency synchronization (FbFS) instruments have been overlooked. This paper proposes a frequency tampering method called "frequency lens". On a 200 km fiber link, we demonstrate a frequency tampering scenario using a frequency lens-enabled frequency tampering module (FTM). On the user side, the frequency value of the recovered 100 MHz signal can be stealthily altered within a range of 100 MHz-100 Hz to 100 MHz+100 Hz, while the frequency dissemination stability of the system remains normal. Related to this tampering risk, potential hazards in three different application scenarios, which rely on precise frequency references, are analyzed. Two countermeasures are also proposed to solve this tampering risk.
Fiber optic communication links have long been touted as providing a secure communications channel for sensitive information. This reputation is largely based on the absence of electromagnetic emanations from the fiber. However, unauthorized access to data in optical fibers is a realistic possibility. Unfortunately it is not always possible to encrypt data transmitted via fiber optic links due to the lofty cost of high data rate encrypters. Therefore, on non-encrypted fiber communications channels, some form of tamper detection scheme is desirable to assure security. The limitations of conventional tamper detection techniques suggests that more sophisticated methods be used. Fiber optic links, like all communications channels, have a band-limited frequency response which produces a characteristic distortion of the signals that propagate down the fiber. A technique has been developed to perform a sensitive characterization of the fiber's frequency response function. This characterization can be exploited for detecting tamper in fiber optic communications links.>
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.
While it has been shown that backscattering induced phase noise can be suppressed by adopting acoustic-optic-modulators (AOMs) at the local and remote sites to break the frequency symmetry in both directions. However, this issue can not be avoided for conventional fiber-optic multiple-access coherent optical phase dissemination in which the interference of the signal light with the Rayleigh backscattered light will probably destroy the coherence of the stabilized optical signal. We suppress the backscattering effect by locally breaking the frequency symmetry at the extraction point by inserting an additional AOM. Here, we theoretically analyze and experimentally demonstrate an add-drop one more AOM approach for suppressing the Rayleigh backscattering within the fiber link. Near-complete suppression of backscattering noise is experimentally confirmed through the measurement the elimination of a common interference term of the signal light and the Rayleigh backscattered light. The results demonstrate that the Rayleigh backscattering light has a limited effect compared to the residual delay-limited fiber phase noise on the system's performance. Our results also provide new evidence that it is possible to largely suppress Rayleigh and other backscattering noise within a long optical fiber link, where the accumulated phase noise could be large, by using frequency symmetry breaking at each access node to achieve robust multiple-access coherent optical phase propagation in spite of scatters or defects.
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.