Study of BB84 Information Leakage via APD Backflash

Abstract

Quantum key distribution (QKD) promises a theoretically unbreakable cryptosystem by employing the probabilistic nature of quantum measurement over mutually unbiased bases. However, it has been shown that QKD systems are vulnerable to side channel attacks due to engineering and technical imperfections in practical implementations. There is a particular vulnerability in QKD systems that use avalanche photodiodes that has not been well investigated. Those avalanche photodiodes are known to emit secondary photons upon photon detection. If intercepted by an eavesdropper, those backflash photons could constitute a source of information leakage by providing clues regarding the original quantum states measured by the legitimate receiver without affecting the quantum bit error rate. We investigate a practical means for an eavesdropper to exploit backflashes from a BB84 QKD system, hypothesizing that an eavesdropper would be able to determine the original BB84 state by analyzing the polarization of the backflashes. The data shows that there is a direct correlation between backflash polarization and the original state, allowing an eavesdropper to gain information regarding the original QKD states without revealing their presence.