Can Two-Way Direct Communication Protocols Be Considered Secure?
This work highlights a critical security gap in quantum cryptography protocols, which is incremental as it builds on prior proofs by exposing an overlooked attack scenario.
The paper identifies a vulnerability in two-way quantum key distribution protocols where an eavesdropper can copy messages without detection, showing that mutual information remains constant at one, and concludes that existing security proofs are incomplete as they do not address this attack.
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.