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IEEE Access

Security Analysis of Gaussian and Discrete Modulations in FSO/CV-QKD Systems Employing LLO Under Phase and Amplitude Attacks

Sara Ahmed, Nancy Alshaer, Khaled Abdulaziz Alaghbari, Tawfik Ismail

Published Jan 1, 2022• 9 citations
Publisher

Abstract

Continuous variable quantum key distribution (CV-QKD) using a local-local oscillator (LLO) is recently proposed to overcome security issues in conventional CV-QKD systems. However, Eve can manipulate the phase or amplitude of the phase reference pulse (PRP) transmitted over the insecure quantum channel, which has opened the door to new security issues. Maintaining optimal performance and preventing Eve’s activities on the quantum channel depends on such a choice of modulation technique. In this paper, the performance of CV-QKD employing LLO over the free space optical (FSO) channel under weak turbulence conditions is investigated. Channel transmittance is introduced into the system model according to the log-negative Weibull distribution. We have adopted the trusted noise model and included the channel phase distortion in the excess noise calculations. We have also evaluated the secret key rate (SKR) using Gaussian modulation (GM) and discrete modulation (DM) protocols. We have reported the superiority of GM in achieving the highest SKR over long distances with the optimum choice of the modulation variance under Eve’s attack on the PRP. Moreover, we analyzed phase and amplitude attacks on the PRP and showed that phase attacks are more severe and deteriorate the communication link more rapidly than amplitude attacks.

DOI
10.1109/access.2022.3208132

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Continuous variable quantum key distribution (CV-QKD) using a local-local oscillator (LLO) is recently proposed to overcome security issues in conventional CV-QKD systems. However, Eve can manipulate the phase or amplitude of the phase reference pulse (PRP) transmitted over the insecure quantum channel, which has opened the door to new security issues. Maintaining optimal performance and preventing Eve’s activities on the quantum channel depends on such a choice of modulation technique. In this paper, the performance of CV-QKD employing LLO over the free space optical (FSO) channel under weak turbulence conditions is investigated. Channel transmittance is introduced into the system model according to the log-negative Weibull distribution. We have adopted the trusted noise model and included the channel phase distortion in the excess noise calculations. We have also evaluated the secret key rate (SKR) using Gaussian modulation (GM) and discrete modulation (DM) protocols. We have reported the superiority of GM in achieving the highest SKR over long distances with the optimum choice of the modulation variance under Eve’s attack on the PRP. Moreover, we analyzed phase and amplitude attacks on the PRP and showed that phase attacks are more severe and deteriorate the communication link more rapidly than amplitude attacks.
Continuous-variable quantum key distribution (CV-QKD) with a real local oscillator (LO) experiences a phase attack security challenge due to the reference pulses transmitted along with quantum signals over an unsecure quantum channel. In this paper, a phase attack on reference pulses of CV-QKD with a real LO is investigated. The performance of the discrete-modulated (DM) CV-QKD under this attack is analyzed analytically and by simulation, and compared with the Gaussian-modulated (GM) CV-QKD. It is found that such an attack by an eavesdropper's, significantly reduce the transmission distance for the GM based CV-QKD, whereas, the DM based CV-QKD shows a high tolerance to high phase noise. Based on phase attack monitoring, a new adaptive modulation scheme is proposed for CV-QKD to adaptively switch between GM and DM so as to maintain the optimum secret key rate.
In practical continuous-variable quantum key distribution system using local local oscillator (LLO CV-QKD), the phase noise related to coherent detection and phase-reference pulse intensity can be locally calibrated at the receiver side. Therefore, it can be regarded as trusted noise in order to obtain a higher secret key rate and transmission distance. However, security loopholes will be opened for the eavesdropper to launch attacks without appropriate countermeasures. To this end, we propose two practical phase-reference pulse intensity attack strategies, where Eve can reduce the trusted phase noise by manipulating the intensity of the phase-reference pulse, thereby hiding her attack on the signal pulse. The performance of the LLO CV-QKD system under these attacks has been analyzed. We show that precisely monitoring the intensity of the phase-reference pulse in real time is an essential countermeasure to prevent the proposed attacks. Moreover, an intensity-monitoring scheme for the phase-reference pulse is proposed to strengthen the security of the practical LLO CV-QKD system and make the trusted phase noise model more robust.
In practical continuous-variable quantum key distribution system using local local oscillator (LLO CV-QKD), the phase noise related to coherent detection and phase-reference pulse intensity that can be locally calibrated at the receiver side is considered to be trusted noise to improve the performance. However, if the intensity of the phase-reference pulse is not monitored precisely in real-time, a security loophole will be opened for the eavesdropper to launch attacks. To this end, we propose two practical phase-reference pulse intensity attack strategies, where Eve can reduce the trusted phase noise by manipulating the intensity of the phase-reference pulse, thereby hiding her attack on the signal pulse if the total excess noise remains unchanged. The performance of the LLO CV-QKD system under these attacks has been analyzed. We show that precisely monitoring the intensity of the phase-reference pulse in real-time is an essential countermeasure to prevent the proposed attacks. Moreover, an intensity-monitoring scheme for the phase-reference pulse is proposed to strengthen the security of the practical LLO CV-QKD system and make the trusted phase noise model more robust.
We study the validity of partially trusting the phase noise associated with estimation uncertainty in a continuous variable quantum key distribution system with a local local oscillator (LLO-CVQKD) and investigate optimal system parameters. We find that the trusted phase noise model enhances the key rate and transmission distance performance but opens a security loophole by manipulating the reference pulse amplitude. Therefore, we propose a “reference pulse attack” on the trusted phase noise LLO-CVQKD system where Eve achieves 100% information hacking without being detected at 23.8km and 32.0km while using lossless and 0.14dB/km loss channels, respectively. Nevertheless, by re-analyzing the attack, countermeasures are also proposed to keep the performance advantages of the trusted phase noise model. As a result, the trusted phase noise LLO-CVQKD can still provide a superior secure key rate (e.g., 9 times higher key rate at 20km) and an increased transmission distance by 45%, compared to that of the untrusted phase noise system.
Unmanned aerial vehicles (UAV) based on free-space optical (FSO) systems combine the benefits of both the high data rate of the FSO and the mobility of UAV. However, cumulative effects of laser beam divergence and turbulence-induced fading on the received irradiance in the FSO link might allow an external eavesdropper located near the authorized receiver to break the transmission under certain conditions. Quantum key distribution (QKD) is a technique of secure communication that employs quantum mechanics-based cryptography principles. A combination of a continuous-variable quantum key distribution (CV-QKD) and Gaussian modulation of quantum coherent states (GMCS) enables a relatively secure communication system against collective attacks over FSO transmission links. However, the performance is restricted and degraded due to the influence of atmospheric turbulence and excess noise. This paper aims to evaluate the performance of a secured UAV-based FSO system utilizing prepare and measure CV-QKD protocol based on GMCS under collective attack in terms of quantum bit error rate (QBER), outage probability, and secret key rate (SKR) considering the impairments of the FSO channel, link budget, deviations in the position and attitude angles of the UAV, channel fluctuating transmittance, and excess noise. The system parameters, including transmit power, receiver’s field-of-view, and beam divergence angle, are optimized to satisfy the system’s design criteria of threshold-QBER=10−3=10^{-3}=10−3and threshold outage probability=10−2=10^{-2}=10−2. The proposed system tolerates boresight displacement up to 7 cm with the optimized values. The Closed-form expressions and analytical results are confirmed by Monte-Carlo simulations.
In continuous-variable quantum key distribution system with a true local oscillator (LLO CV-QKD), part of the phase noise associated with the coherent detector and the phase-reference intensity can be considered as trusted because which can be locally calibrated at the receiver’s side. The trusted phase noise model can significantly improve the noise tolerance of the system since the phase noise is the major excess noise. However, the transmission of the phase-reference pulse through the insecure quantum channel in the LLO CV-QKD system may leave rooms for the eavesdropper to mount attacks. Here, we propose a practical and flexible phase-reference intensity attack scheme using a phaseinsensitive amplifier to amplify the intensity of the phase-reference pulse. In this case, the eavesdropper can compromise the security of the LLO CV-QKD system severely by lowering the trusted part of the phase noise to compensate her increased attack on the signal pulse while the total excess noise is unchanged. We simulate the secure key rate with respect to the transmission distance to show that precisely monitoring the instantaneous intensity of the phase-reference pulse in real time is of great importance to guarantee the security of the LLO CV-QKD system.