Quantifying the Upper Limit of Backflash Attack in Quantum Key Distribution

TL;DR

This work tackles the practical security risk of backflash, a passive side-channel from SPAD detectors in fiber-based QKD. By combining an experimental backflash attack with a broadband spectral model, the authors bound Eve’s ability to distinguish backflash states and translate this leakage into secure-key-rate implications for decoy-state BB84. They find that Eve can decode a substantial portion of backflash information (up to ER ≈ 22, corresponding to about 95.7 percent of backflash photons), but the overall impact on key rates is limited under realistic detector parameters. The study provides a quantitative framework to assess and mitigate backflash risk, offering a general methodology for practical QKD security evaluations and highlighting avenues for wavelength-resolved improvements.

Abstract

Quantum key distribution (QKD) provides information-theoretic security grounded in the fundamental laws of physics. Nevertheless, practical imperfections can introduce side channels that expose QKD systems to quantum hacking, especially passive attacks that are inherently difficult to detect. In this study, we experimentally and theoretically investigate the upper limit of the backflash attack-a representative passive side-channel threat. Using a fully equipped fiber-based QKD receiver, we demonstrate the feasibility of the attack and reveal its limited capability in distinguishing quantum states. We further develop a theoretical framework to quantify the maximum distinguishability achievable by an eavesdropper, taking into account the broadband spectral nature of backflash photons. The analysis shows that Eve can extract effective key information from at most 95.7% of the backflash photons. Based on these findings, we evaluate the secure key rate of a decoy-state BB84 QKD system under backflash attack. Our results provide a quantitative assessment of the vulnerability of QKD systems to backflash emissions and offer a general methodology to evaluate the practical security of QKD systems.

Figures (10)

• Figure 1: Experimental setup. The black solid lines represent optical fibers, the blue solid lines indicate cables used for triggering devices, and the blue dashed arrows depict the electrical signals transmitted to the time-correlated single photon counting. LD, laser diode; ATT, optical attenuator; PC, polarization controller; CIR, optical circulator; PBS, polarization beam splitter; SPAD, single photon avalanche detector; TCSPC, time-correlated single photon counting.
• Figure 2: Histogram of the time interval between Alice's laser pulse emission and Eve's detection. When the QKD system transmits the $\ket{H}$ state, the click events of Eve's $\ket{H}$ state detector (SPAD3, red) and $\ket{V}$ state detector (SPAD4, blue) are recorded in the histogram.
• Figure 3: The spectrum of backflash photons by using Ansys Luminescent FDTD simulation platform to simulate the secondary photon emission of avalanche photodiode.
• Figure 4: The backflash photons with $\ket{H}$ polarization state undergo measurement of their polarization state by Eve after passing through PBS1, PC2, PC3, PBS2 and reaching Eve's SPAD. The blue solid line represents the number of photons decoded as $\ket{H}$ state at different wavelengths, while the red dashed line represents the number of photons decoded as $\ket{V}$ state at different wavelengths.
• Figure 5: Simulation results of the decoy-state BB84-QKD protocol. The blue solid line shows the key rate without a backflash attack. The green dashed and yellow dotted lines show the key rates in the presence of a backflash attack, with backflash probabilities of 4.05% and 9.2%, respectively.