Stronger Attacks on Causality-Based Key Agreement

TL;DR

This paper introduces stronger attacks on causality-based quantum key distribution protocols, challenging their security assumptions and questioning the feasibility of practical schemes with single devices.

Contribution

It presents a novel, more powerful attack construction that surpasses previous methods, impacting the security analysis of practical causality-based QKD schemes.

Findings

New attack construction outperforms previous attacks

Potential to disprove the feasibility of practical CKD schemes

Highlights vulnerabilities in single-device, consecutive-measurement protocols

Abstract

Remarkably, it has been shown that in principle, security proofs for quantum key-distribution (QKD) protocols can be independent of assumptions on the devices used and even of the fact that the adversary is limited by quantum theory. All that is required instead is the absence of any hidden information flow between the laboratories, a condition that can be enforced either by shielding or by space-time causality. All known schemes for such Causal Key Distribution (CKD) that offer noise-tolerance (and, hence, must use privacy amplification as a crucial step) require multiple devices carrying out measurements in parallel on each end of the protocol, where the number of devices grows with the desired level of security. We investigate the power of the adversary for more practical schemes, where both parties each use a single device carrying out measurements consecutively. We provide a novel construction of attacks that is strictly more powerful than the best known attacks and has the potential to decide the question whether such practical CKD schemes are possible in the negative.