Protecting fiber-optic quantum key distribution sources against light-injection attacks

Abstract

A well-protected and characterised source in a quantum key distribution system is needed for its security. Unfortunately, the source is vulnerable to light-injection attacks, such as Trojan-horse, laser-seeding, and laser-damage attacks. In particular, the latter attack can modify properties of components inside the source and open it up for the other attacks. Here we propose a countermeasure against the laser-damage attack, consisting of an additional sacrificial component placed at the exit of the source. This component should either withstand high-power incoming light while attenuating it to a safe level that cannot modify the rest of the source, or get destroyed into a permanent highattenuation state that breaks up the line. We demonstrate experimentally that off-the-shelf fiberoptic isolators and circulators have these desired properties, at least under attack by a continuouswave high-power laser.

Improved finite-key security analysis of quantum key distribution against Trojan-horse attacks

“…To see this, note, for instance, that if the power of Eve's injected light is limited to, say, 1 dBm [35][36][37], then to obtain a lower bound δ δ L ≈ 1-4 × 10 −8 (see appendix C)-which is already a relatively high value for long-distance QKD-it is necessary to consider about 140 dB of total isolation, which includes the effect of optical isolators, attenuators, the reflectivity of the optical elements, and other internal losses. Besides, note that since Eve's light has to make a round trip, the isolation provided by certain elements-e.g.…”

“…Indeed, some experiments with continuous-wave lasers have shown that single-mode optical fibers are severely damaged if they are exposed to an average power of 2-5 W [12,33,34]. Moreover, this power limit may be lowered by using on-purpose devices as optical fuses [35], optical circulators and isolators [36], or other power limiting devices based on the thermo-optical defocusing effect [37]. In addition, one could also put a limit on the intensity of Eve's injected light by placing a monitor detector to abort the protocol if the input intensity surpass a predefined threshold.…”

“…In this appendix we illustrate how to relate the crucial quantity δ with the amount of isolation at the transmitter by means of a specific example. In particular, we assume that |ñ a,μ BE = |n a B ⊗ |ξ a,μ E in equation ( 4), being |ξ a,μ E the state of Eve's back-reflected light, and Alice places at the input port of the transmitter a device that limits the power of Eve's injected light to, say, 1 dBm [35][36][37]. This constraints the power of the back-reflected light to 1 dBm − I tot , where I tot (measured in dB) is the total amount of isolation at Alice's transmitter.…”

Characterisation of state-preparation uncertainty in quantum key distribution

“…Fortunately, a measurementdevice-independent (MDI) QKD protocol [13] is immune to all measurement-side secure loopholes, because this protocol does not make assumption on the measurement devices. Thus, the loopholes in the source unit are the last obstacle to achieve security of QKD in reality [8,9,[14][15][16]. The most effective solution to eliminate such security threat is to consider the practical imperfections of the source in the security model.…”