Joint eavesdropping on the BB84 decoy state protocol with an arbitrary passive light-source side channel

TL;DR

This paper investigates a joint eavesdropping strategy on the BB84 decoy-state quantum key distribution protocol that exploits passive light-source side channels, using optimal cloning and collective measurement to assess security vulnerabilities.

Contribution

It introduces a comprehensive attack model accounting for joint eavesdropping on both the operational degree of freedom and passive side channels in QKD.

Findings

Joint eavesdropping significantly affects QKD security.

Optimal phase-covariant cloning is the most effective attack without side channels.

The developed error estimation method applies to the BB84 decoy-state protocol.

Abstract

Passive light-source side channel in quantum key distribution (QKD) makes the quantum signals more distinguishable thus provides additional information about the quantum signal to an eavesdropper. The explicit eavesdropping strategies aimed at the passive side channel known to date were limited to the separate measurement of the passive side channel in addition to the operational degree of freedom. Here we show how to account for the joint eavesdropping on both operational degree of freedom and the passive side channel of the generic form. In particular, we use the optimal phase-covariant cloning of the signal photon state, which is the most effective attack on the BB84 protocol without side channels, followed by a joint collective measurement of the side channel and the operational degree of freedom. To estimate QKD security under this attack, we develop an effective error method and show its applicability to the BB84 decoy-state protocol.