Can Two-Way Direct Communication Protocols Be Considered Secure?

TL;DR

This paper demonstrates that certain undetectable eavesdropping attacks on two-way quantum key distribution protocols can compromise security, revealing gaps in existing security proofs.

Contribution

It introduces specific undetectable attack strategies showing that current security proofs for two-way protocols are incomplete.

Findings

Mutual information remains constant at one under the attack

Attacks cause no disturbance in message mode

Existing security proofs do not cover these attacks

Abstract

We consider attacks on two-way quantum key distribution protocols in which an undetectable eavesdropper copies all messages in the message mode. We show that under the attacks there is no disturbance in the message mode and that the mutual information between the sender and the receiver is always constant and equal to one. It follows that recent proofs of security for two-way protocols cannot be considered complete since they do not cover the considered attacks.