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Journal of Experimental and Theoretical Physics Letters

Measurement-Device-Independent Quantum Key Distribution

S. P. Kulik, S. N. Molotkov

Published Jul 1, 2023• 4 citations
Publisher

Abstract

The security of the measurement-device-independent quantum key distribution protocol through untrusted nodes has been directly proven using fundamental entropy uncertainty relations. Reasons for the amazing coincidence of the expressions for the secrete key length in the BB84 and MDI protocols are presented.
DOI
10.1134/s002136402360177x

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The aim of the research is to analyze the secrecy of quantum key distribution through untrusted nodes in quantum networks. Research method: the use of entropy uncertainty relations. Result(s) of the study: the secrecy of quantum key distribution through untrusted nodes in quantum networks is proved. The use of entropy uncertainty relations makes it possible to obtain an accurate solution for the length of the secret key in the single-photon case. A comparison with the exact solution for the BB84 protocol is made and the fundamental difference in the logical structure of proof of the secrecy of keys in these protocols is clearly shown, which, in our opinion, is important for the development of quantum cryptography systems. Scientific novelty: the article proves the secrecy of quantum key distribution through untrusted nodes in quantum networks.
The security of practical quantum key distribution (QKD) systems is fundamentally constrained by vulnerabilities of single-photon detectors. Measurement-device-independent quantum key distribution (MDI-QKD) was proposed to remove this limitation by allowing all measurements to be performed by a completely untrusted party, under the assumption that the measurement node can be treated as adversarial but does not compromise the security guarantees of the protocol. Here we show that this assumption is insufficient under realistic adversarial control of the measurement device. We present an attack in which an adversary exploits active control of the measurement node (Charlie) to obtain significant information about the secret key. The attack enables recovery of up to 70\% of the sifted key while introducing only 5.6\% quantum bit error rate. Unlike previously reported attacks targeting specific implementations of MDI-QKD, our results demonstrate a limitation of the standard security model underlying the protocol. These findings indicate that additional constraints on the measurement-device independence assumption, or refined security analyses incorporating stronger adversarial capabilities, are required to ensure the security of MDI-QKD in realistic scenarios.
Abstract A proof of the secrecy of the quantum key distribution through an untrusted node is given. The use of entropy uncertainty relations allows us to obtain an exact solution for the length of the secret key in the single-photon case. Comparison with the exact solution for the BB84 protocol is made and the principal difference in the logical structure of the key secrecy proofs in these protocols is clearly shown, which, in our opinion, is important for the development of quantum cryptography systems.
Quantum key distribution (QKD) can serve large numbers of users with unconditional secure communication, in which measurement-device-independent (MDI) QKD protocol can conveniently enhance realistic security by eliminating all detector side channels. In multi-user scenarios, MDI-QKD networks should meet the requirements of reliability and robustness. Around the issues, we complete a series of work on performance improvement, low-cost deployment, and robust networking. First is to answer how to change the trusted node to the untrusted node with low cost, here we propose and demonstrate an MDIQKD which is compatible with phase encoding BB84 networks, allowing for economical deployment of MDI-QKD networks and reconciliation between high key rate and high realistic security in the same network. Then we make the MDI networks more robust. We design an MDI-QKD networking scheme for multi-user access where more than two users can generate keys synchronously regardless of aligning reference frames and compensating channel disturbance on polarization. Third, we find a general method to distil secure keys from quantum discord, which is different to the previous entanglement-based frameworks of MDI-QKD, allowing imperfect preparation and measurement of quantum states. The series of work can help the practical application of MDI-QKD networks, and promote the development of quantum networks.
This report reviews two quantum key distribution (QKD) protocols: the BB84 protocol and the measurement device independent (MDI) QKD protocol. The goal of this report is to recreate the security proof of the BB84 protocol, to generate the secret key rate for a practical application of the BB84 protocol, both with and without decoy states, and to review the MDI-QKD protocol and look at the advantages it has for practical QKD. The proof security of the BB84 protocol is done by designing an equivalent theoretical protocol and proving its security by bounding the information of the eavesdropper to an exponentially small number. The best distance over which secret key rate can be generated with the practical model of the BB84 protocol that is presented in this report, is 165 km. This is achieved by employing decoy states and is more than three times as far as the model can achieve without decoy states, which gave a distance of 52 km at best. The zero distance key rate of the model with decoy states is R = 1.21 · 10-2 bits per pulse (bpp), at best. This is an order of magnitude larger than R =1.02 · 10-3 bpp, which is what the model without decoy states could produce. The decoy state method is therefore an improvement for practical QKD. By reviewing the MDI-QKD protocol it can be concluded that because it eliminates the measurement device side channels, it is very useful for practical QKD, since it makes security analysis simpler and more precise. It also necessarily employs decoy states, which improves the secret key rate. MDI-QKD therefore is a promising protocol to use for future quantum communications.