# Reference-Beam Attacks against Twin-Field Quantum Key Distribution using Optical Injection Locking

## TL;DR

This paper identifies potential side-channel attacks on twin-field quantum key distribution systems using optical injection locking and proposes practical countermeasures to enhance security.

## Contribution

It experimentally demonstrates two realistic attack scenarios on TF-QKD and introduces effective countermeasures to mitigate these vulnerabilities.

## Key findings

- Attack scenarios can increase photon number or bypass decoy states.
- Experimental validation of intensity modulation and wavelength embedding attacks.
- Proposed countermeasures effectively secure TF-QKD without performance loss.

## Abstract

Twin-Field Quantum Key Distribution (TF-QKD) has become a leading protocol to bring quantum communications to the national scale. The protocol requires the establishment of a shared phase and frequency reference between distant parties, which is commonly achieved by using an external reference laser in an Optical Injection Locking (OIL) architecture. In this work, we analyze the side channels in OIL-based TF-QKD that may arise from adversarial manipulation of the various degrees of freedom of this untrusted reference beam. We experimentally demonstrate two realistic attack scenarios: fast intensity modulation of the reference laser, and additional signals embedded in the reference light exploiting wavelengths undetectable by conventional monitoring techniques. These attacks can allow a potential eavesdropper to deterministically increase the mean photon number of the sources, or circumvent the decoy-state technique, respectively. To counter these vulnerabilities, we propose practical and highly effective countermeasures that reinforce the security of TF-QKD systems without significant additional complexity or performance degradation.