Realistic vulnerabilities of decoy-state quantum key distribution

TL;DR

The pseudo-photon-number resolution USD attack is introduced, which allows Eve to emulate all observable gains at Bob’s side so that she remains fully undetectable even with advanced statistical checks.

Abstract

We analyze realistic vulnerabilities of decoy-state quantum key distribution (QKD) arising from the combination of laser damage attack (LDA) and unambiguous state discrimination (USD). While decoy-state QKD is designed to protect against photon-number-splitting and beam-splitting attacks by accurately estimating the single-photon fraction, it relies on stable attenuation to prepare pulses with fixed mean-photon numbers. An eavesdropper (Eve) can exploit LDA to irreversibly alter the optical components on Alice's side, effectively increasing the mean-photon numbers beyond the decoy-state security regime. We show that once the alteration exceeds a critical threshold - on the order of 10--20 dB - Eve can implement an efficient USD-based intercept-resend strategy using current off-the-shelf technology, thus obtaining the entire secret key. Numerical simulations confirm that for sufficiently elevated mean-photon numbers, Eve's conclusive measurement outcomes skew the decoy-state statistics, yet remain undetected by standard security checks. We further demonstrate how a modified USD setup employing an additional beam splitter can reduce the required threshold, facilitating Eve's attack. Additionally, we introduce the pseudo-photon-number resolution (PPNR) USD attack, which allows Eve to emulate all observable gains at Bob's side so that she remains fully undetectable even with advanced statistical checks. Our findings emphasize the need for robust safeguards against high-power laser damage in QKD systems, including careful hardware selection, rigorous testing under high-power illumination, and real-time monitoring to ensure the integrity of the decoy-state protocol.

Figures (7)

• Figure 1: Eve's polarization-encoding setup for the USD attack. The depicted distribution of photons corresponds to a conclusive outcome for horizontally polarized state as an example. Det$_\text{H,V,D,A}$, detectors for horizontal, vertical, diagonal and anti-diagonal polarization; PBS, polarization beam-splitter; BS 50:50, symmetric beam-splitter; $\lambda/2$, half-wave plate for diagonal basis selection. The same setup can be designed for phase encoding.
• Figure 2: Eve's setup for the modified USD attack. Det, additional detector for "odd" photons monitoring; BS T:1-T, asymmetric beam-splitter. The fraction of the input radiation proportional to $T$ goes to the USD station, the remainder goes to Det; if Det fires, Eve forwards nothing to Bob.
• Figure 3: Dependence of the simulated single-photon gain estimation using Eq. (\ref{['Q1']}) on the attenuation alteration for different decoy-state parameters during the USD attack: $\mu=0.5$, $\nu=0.1$ (red); $\mu=0.5$, $\nu=0.01$ (purple); $\mu=0.1$, $\nu=0.01$ (blue). The threshold values (11.1 dB, 14.5 dB and 18.3 dB, respectively) are indicated by the grey lines.
• Figure 4: Simulated signal pulses gain dependence on the altered mean-photon number for modified USD attack with different beam-splitter transparency $T$: $T=1$ (blue); $T=0.5$ (purple); $T=0.3$ (red); $T=0.15$ (yellow).
• Figure 5: (a) Eve's PPNR USD measurement setup. (b) Eve's emitter: SPS, single photon source; IM, intensity modulator, PM, polarization modulator.