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Entropy

Underwater Wavelength Attack on Discrete Modulated Continuous-Variable Quantum Key Distribution

Kangyi Feng, Yijun Wang, Yin Qiang Li, Yuang Wang, Zhiyue Zuo, Ying Guo

Published Jun 14, 2024• 4 citations
Publisher

Abstract

The wavelength attack utilizes the dependence of beam splitters (BSs) on wavelength to cause legitimate users Alice and Bob to underestimate their excess noise so that Eve can steal more secret keys without being detected. Recently, the wavelength attack on Gaussian-modulated continuous-variable quantum key distribution (CV-QKD) has been researched in both fiber and atmospheric channels. However, the wavelength attack may also pose a threat to the case of ocean turbulent channels, which are vital for the secure communication of both ocean sensor networks and submarines. In this work, we propose two wavelength attack schemes on underwater discrete modulated (DM) CV-QKD protocol, which is effective for the case with and without local oscillator (LO) intensity monitor, respectively. In terms of the transmittance properties of the fused biconical taper (FBT) BS, two sets of wavelengths are determined for Eve's pulse manipulation, which are all located in the so-called blue-green band. The derived successful criterion shows that both attack schemes can control the estimated excess noise of Alice and Bob close to zero by selecting the corresponding condition parameters based on channel transmittance. Additionally, our numerical analysis shows that Eve can steal more bits when the wavelength attack controls the value of the estimated excess noise closer to zero.

DOI
10.3390/e26060515

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The wavelength attack utilizes the dependence of beam splitters (BSs) on wavelength to cause legitimate users Alice and Bob to underestimate their excess noise so that Eve can steal more secret keys without being detected. Recently, the wavelength attack on Gaussian-modulated continuous-variable quantum key distribution (CV-QKD) has been researched in both fiber and atmospheric channels. However, the wavelength attack may also pose a threat to the case of ocean turbulent channels, which are vital for the secure communication of both ocean sensor networks and submarines. In this work, we propose two wavelength attack schemes on underwater discrete modulated (DM) CV-QKD protocol, which is effective for the case with and without local oscillator (LO) intensity monitor, respectively. In terms of the transmittance properties of the fused biconical taper (FBT) BS, two sets of wavelengths are determined for Eve's pulse manipulation, which are all located in the so-called blue-green band. The derived successful criterion shows that both attack schemes can control the estimated excess noise of Alice and Bob close to zero by selecting the corresponding condition parameters based on channel transmittance. Additionally, our numerical analysis shows that Eve can steal more bits when the wavelength attack controls the value of the estimated excess noise closer to zero.
Integrated continuous-variable quantum key distribution (CV-QKD) systems based on photonic chips offer a promising avenue for establishing large-scale and low-cost global quantum networks. In such systems, the grating coupler (GC) serves as an interface for optical coupling, but its strongly wavelength-dependent efficiency creates a severe security loophole in practice. In this paper, we propose a wavelength-shifting attack targeting the wavelength-dependent GC in chip-based CV-QKD systems. By intercepting Alice’s quantum states and resending forged quantum states together with a local oscillator (LO), both at a shifted wavelength, this attack manipulates the receiver’s excess-noise estimation, effectively masking the excess noise introduced by Eve. Numerical simulations show that the attack can keep Bob’s estimated excess noise below the security threshold, leading Alice and Bob to infer a positive secret key rate while the actual secure key rate is zero. This research reveals how the integrated components can be exploited to mount side-channel attacks. To mitigate this vulnerability, we propose practical defenses, including on-chip wavelength monitoring and randomized shot-noise calibration. These findings provide important guidance for the secure development of next-generation integrated quantum communication systems.
The security proof of continuous variable quantum key distribution(CV QKD) based on two assumptions that the eavesdropper can neither act on the local oscillator nor control Bob's beam splitter. These assumptions maybe invalid in practice due to some potential imperfections of the implementations of the protocol. In [Phys.Rev.A, 84.062308], we found that one can control the intensity transmission of the beam splitter by switching the wavelength of the input light. In this paper, we revisit the problem of transmitting the local oscillator in public channel, and propose a wavelength attack targeting the CV QKD systems using heterodyne protocol. Our attack is proved to be feasible and renders all final key shared between the legitimate parties insecure even they have monitored the intensity. To prevent our attack, a wavelength filter should be added before performing the heterodyne detection.
Recently, several practical wavelength attacks on continuous-variable quantum key distribution (CVQKD) were proposed based on the wavelength-dependent property of a beam splitter in Bob's station. These attack schemes are limited to the fiber channel systems with a relatively stable transmittance. However, unlike the fiber channel, the transmittance of the atmospheric channel is complex and fluctuates dramatically in time, which may invalidate the attack equations that must be satisfied in the optical fiber channel. In this paper, we propose two wavelength attack schemes against complex atmospheric CVQKD systems and formulize the two schemes based on the parameter estimation method. To verify the feasibility and availability of this attack, we deduce the success criteria for the two attack schemes. By numerical analysis, our attack schemes are proved to be feasible in the permitted parameter regimes. The secret key rate and propagation distance of CVQKD can be further overestimated by selecting the condition parameters effectively. Moreover, we discuss the possible prevention countermeasures to resist these attack schemes. The proposed method will fill the gap of wavelength attack for the implementation of practical atmospheric CVQKD.
We report here a new side channel attack on a practical continuous-variable (CV) quantum key distribution (QKD) system. Inspired by blinding attack in discrete-variable QKD, we formalize an attack strategy by inserting an external light into a CV QKD system implemented Gaussian-modulated coherent state protocol and show that our attack can compromise its practical security. In this attack, we concern imperfections of a balanced homodyne detector used in CV QKD. According to our analysis, if one inserts an external light into Bob’s signal port, due to the imperfect subtraction from the homodyne detector, the leakage of the external light contributes a displacement on the homodyne signal which causes detector electronics saturation. In consequence, Bob’s quadrature measurement is not linear with the quadrature sent by Alice. By considering such vulnerability, a potential Eve can launch a full intercept-resend attack meanwhile she inserts an external light into Bob’s signal port. By selecting proper properties of the external light, Eve actively controls the induced displacement value from the inserted light which results saturation of homodyne detection. In consequence, Eve can bias the excess noise due to the intercept-resend attack and the external light, such that Alice and Bob believe their excess noise estimation is below the null key threshold and they can still share a secret key. Our attack shows that the detector loopholes also exist in CV QKD, and it seems influence all the CV QKD systems using homodyne detection, since all the practical detectors have finite detection range.
We experimentally demonstrated the feasibility of an underwater continuous-variable quantum key distribution (CV-QKD) system based on four-state protocol, which is promising to guarantee the unconditionally secure underwater wireless optical communication. CV-QKD parameter estimation is performed after transmitting quantum coherent signal from Alice to Bob through a water tank. The secure key rate under collective attack of the demonstrated CV-QKD system is estimated as 22.9 kbits/s at a channel loss of 12.4 dB. In addition, the performance is also investigated with various water types and the maximum underwater transmission distance of the demonstrated CV-QKD system is estimated as 148.7 m in the pure sea water.
The security proofs of continuous-variable quantum key distribution are based on the assumptions that the eavesdropper can neither act on the local oscillator nor control Bob's beam splitter. These assumptions may be invalid in practice due to potential imperfections in the implementations of such protocols. In this paper, we consider the problem of transmitting the local oscillator in a public channel and propose a wavelength attack which allows the eavesdropper to control the intensity transmission of Bob's beam splitter by switching the wavelength of the input light. Specifically we target continuous-variable quantum key distribution systems that use the heterodyne detection protocol using either direct or reverse reconciliation. Our attack is proved to be feasible and renders all of the final keys shared between the legitimate parties insecure, even if they have monitored the intensity of the local oscillator. To prevent our attack on commercial systems, a simple wavelength filter should be randomly added before performing monitoring detection.