Practical Security Bounds Against the Trojan-Horse Attack in Quantum Key Distribution

Abstract

In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices. To stop the Trojan photons, the use of passive optical components has been suggested. However, to date, there is no quantitative bound that specifies such components in relation to the security of the system. Here, we turn the Trojan-horse attack into an information leakage problem. This allows us quantify the system security and relate it to the specification of the optical elements. The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.

Extended analysis of the Trojan-horse attack in quantum key distribution

“…Attenuation-based defense systems, which aim to muddy the phase information on Eve's state by blocking most of the incoming attack state, have been previously analysed by Lucamarini et al [18]. In their analysis, it was assumed that Eve would send in a pure coherent state, which is by necessity separable.…”

“…This may be done by way of some defense such as an optical fuse [25], which melts when sufficiently many photons pass through it, or by identifying some other component which will be irreversibly damaged when subject to a bright enough light [26]. A more detailed examination of the numbers and figures behind such defenses may be found in [18], but for our purposes we may simply assume that there does exist some bound N such that n ρ ≡ Tr [nρ] < N , wheren =â †â . Another relevant property may be the purity of the state ρ η after passing through the attenuator.…”

“…One such SCA that has recently received theoretical [16][17][18] and experimen- * svinay1@sheffield.ac.uk † p.kok@sheffield.ac.uk tal [19][20][21] attention is the so-called Trojan Horse Attack (THA). Here, Eve will tap into the optical channel that Alice and Bob use to communicate.…”

“…This attack has previously been analysed by Lucamarini et al [18,22]. They assume that Eve uses a coherent state to probe the system, and describe using a one-way attenuating filter at the entry-point of Alice's apparatus as a defense.…”

Upconversion-based receivers for quantum hacking-resistant quantum key distribution

“…In that sense, our work significantly adds to the advantages of upconversion technology for practical QKD systems. It also joins the growing body of countermeasures [41,42] and novel schemes, such as MDI-QKD [23], conceived in order to heal the Achilles' heel of QKD [43]: the (vulnerable) photon detection system, without compromising on performance.…”