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Security Loophole Induced by Photorefractive Effect in Continous-variable Quantum Key Distribution System

Zehao Zhou, Peng Huang, Tao Wang, Guihua Zeng

Abstract

Modulators based on the Mach-Zehnder interferometer (MZI) structure are widely used in continuous-variable quantum key distribution (CVQKD) systems. MZI-based variable optical attenuator (VOA) and amplitude modulator can reshape the waveform and control the intensity of coherent state signal to realize secret key information modulation in CVQKD system. However, these devices are not ideal, internal and external effects like non-linear effect and temperature may degrade their performance. In this paper, we analyzed the security loophole of CVQKD under the photorefractive effect (PE), which originates from the crystal characteristic of lithium niobate (LN). It is found that the refractive index change of modulators because of PE may lead to an overestimate or underestimate of the final secret key rate. This allows Eve to perform further attacks like intercept-resend to get more secret key information. To solve this problem, several countermeasures are proposed, which can effectively eliminate potential risks.

Security Loophole Induced by Photorefractive Effect in Continous-variable Quantum Key Distribution System

Abstract

Modulators based on the Mach-Zehnder interferometer (MZI) structure are widely used in continuous-variable quantum key distribution (CVQKD) systems. MZI-based variable optical attenuator (VOA) and amplitude modulator can reshape the waveform and control the intensity of coherent state signal to realize secret key information modulation in CVQKD system. However, these devices are not ideal, internal and external effects like non-linear effect and temperature may degrade their performance. In this paper, we analyzed the security loophole of CVQKD under the photorefractive effect (PE), which originates from the crystal characteristic of lithium niobate (LN). It is found that the refractive index change of modulators because of PE may lead to an overestimate or underestimate of the final secret key rate. This allows Eve to perform further attacks like intercept-resend to get more secret key information. To solve this problem, several countermeasures are proposed, which can effectively eliminate potential risks.
Paper Structure (13 sections, 33 equations, 5 figures)

This paper contains 13 sections, 33 equations, 5 figures.

Table of Contents

  1. INTRODUCTION
  2. PRINCIPLE OF THE LOOPHOLE FROM PHOTOREFRACTIVE EFFECT
  3. Mechanism of photorefractive effect
  4. Impact on Mach-Zehnder modulator
  5. PARAMETER ESTIMATION UNDER PHOTOREFRACTIVE EFFECT LOOPHOLE
  6. Gaussian-modulated coherent state protocol
  7. Process of parameter estimation
  8. SECURITY ANALYSIS AND KEY RATE UNDER PHOTOREFRACTIVE EFFECT
  9. COUNTERMEASURES
  10. Attack pattern
  11. Defense strategy
  12. CONCLUSION
  13. ACKNOWLEDGMENTS

Figures (5)

  • Figure 1: Transfer function deviations of modulator
  • Figure 2: Entanglement-based model of GMCS-CVQKD protocol.
  • Figure 3: Practical and estimated secret key rate versus transmittance and PE index with 0.2 dB/km fiber loss, $f=300MHz$, $\eta=0.6$, $v_{el}=0.01$, $\varepsilon=0.05$, $V_A=4$, $\beta=0.95$.
  • Figure 4: Simulation results of secret key rate versus transmittance. Estimated result presented in solid line, practical result presented in dot line, parameters except excess noise keep unchanged.
  • Figure 5: System diagram of traditional attack strategy