Backflash Attack on Coherent One-Way Quantum Key Distribution

In this article, we experimentally demonstrate an eavesdropper's (Eve's) information gain by exploiting the breakdown flash generated by the single photon avalanche detector (SPAD) used in coherent one-way quantum key distribution (COW-QKD) setup. Unlike prior studies focusing on the device-level characterization of backflash photons, this work quantifies Eve's learning with a QKD system that includes a key distillation engine (KDE). Eve's learning is quantified using the backflash photons emitted by SPAD and the information available on the classical channel. Experimentally observed data are in good agreement with theoretical simulations. Some mitigation strategies against the backflash attack are also discussed.