# Invisible Trojan-horse attack

## TL;DR

This paper demonstrates an experimentally feasible Trojan-horse attack on practical quantum key distribution systems at a specific wavelength, revealing a security vulnerability that was previously considered ineffective at that wavelength.

## Contribution

The study provides the first experimental demonstration of an invisible Trojan-horse attack at 1924nm, showing its effectiveness against certain QKD protocols.

## Key findings

- Attack remains nearly invisible at 1924nm due to reduced detector noise.
- Numerical comparison shows attack success at 1924nm but not at 1536nm.
- Highlights security risks in QKD systems at specific wavelengths.

## Abstract

We demonstrate the experimental feasibility of a Trojan-horse attack that remains nearly invisible to the single-photon detectors employed in practical quantum key distribution (QKD) systems, such as Clavis2 from ID Quantique. We perform a detailed numerical comparison of the attack performance against Scarani-Acin-Ribordy-Gisin (SARG04) QKD protocol at 1924nm versus that at 1536nm. The attack strategy was proposed earlier but found to be unsuccessful at the latter wavelength, as reported in N.~Jain et al., New J. Phys. 16, 123030 (2014). However at 1924nm, we show experimentally that the noise response of the detectors to bright pulses is greatly reduced, and show by modeling that the same attack will succeed. The invisible nature of the attack poses a threat to the security of practical QKD if proper countermeasures are not adopted.