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International Conference on Information and Communication Technology Convergence (ICTC)

Side Channel Vulnerability in Parity Computation of Generic Key Reconciliation Process on QKD

Gyusang Kim, Dongjun Park, Hee Seok Kim, Seokhie Hong

Published Oct 20, 2021• 4 citations
Publisher

Abstract

Quantum key distribution (QKD) is a key exchange protocol based on quantum physics, and the key reconciliation process is a necessary process that should not be omitted from QKD in correcting discrepancies between the key sent and the one received. In this paper, we propose the single trace side channel attack on the parity sum, which is a common operation in general error-correcting algorithms. We show that one can fully recover the secret key using a power consumption trace captured when a processor computes the parity sum.

DOI
10.1109/ictc52510.2021.9620820

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Quantum key distribution (QKD) is a key exchange protocol based on quantum physics, and the key reconciliation process is a necessary process that should not be omitted from QKD in correcting discrepancies between the key sent and the one received. In this paper, we propose the single trace side channel attack on the parity sum, which is a common operation in general error-correcting algorithms. We show that one can fully recover the secret key using a power consumption trace captured when a processor computes the parity sum.
In this paper, we propose the first single trace side-channel attack on the key reconciliation process for the quantum key distribution (QKD) system. Through a single power consumption trace measured during the syndrome computation of the key reconciliation process on the sender's side, we were able to fully recover the sifted key and the syndrome. The shared secret key between the two users can be directly obtained from the sifted key and the syndrome. The result of our work shows that the QKD system must consider side-channel attacks on the key reconciliation process for its practical use.
Although quantum mechanics guarantees the security of the quantum key distribution system, it is crucial to examine whether the implementation flaws can lead to the disclosure of sensitive information. In this paper, we propose the side-channel attack on the key reconciliation in the quantum key distribution system. In this system, the sifted key and the syndrome are used to derive a shared secret key between two users. From our attack, these can be fully recovered through a single power consumption trace measured during the syndrome computation on the sender’s side. Additionally, we propose efficient countermeasures to thwart such side-channel attacks.
Quantum key distribution is often regarded as an unconditionally secure method to exchange a secret key by harnessing fundamental aspects of quantum mechanics. Despite the robustness of key exchange, classical post-processing reveals vulnerabilities that an eavesdropper could target. In particular, many reconciliation protocols correct errors by comparing the parities of subsets between both parties. These communications occur over insecure channels, leaking information that an eavesdropper could exploit. Currently there is no holistic threat model that addresses how parity-leakage during reconciliation might be actively manipulated. In this paper we introduce a new form of attack, namely the Manipulate-and-Observe attack in which the adversary (1) partially intercepts a fraction ρρρ of the qubits during key exchange, injecting the maximally tolerated amount of errors up to the 11 percent error threshold whilst remaining undetected and (2) probes the maximum amount of parity-leakage during reconciliation, and exploits it using a vectorised, parallel brute force filter to shrink the search space from 2n down to as few as a single candidate, for an n-bit reconciled key. We perform simulations of the attack, deploying it on the most widely used protocol, BB84, andthe benchmark reconciliation protocol, Cascade. Our simulation results demonstrate that the attack can significantly reduce the security below the theoretical bound and, in the worst case, fully recover the reconciled key material. The principles of the attack could threaten other parity-based reconciliation schemes, like Low Density Parity Check, which underscores the need for urgent consideration of the combined security of key exchange and post-processing.
The security of current key exchange protocols such as Diffie-Hellman key exchange is based on the hardness of number theoretic problems. However, these key exchange protocols are threatened by weak random number generators, advances to CPU power, a new attack from the eavesdropper, and the emergence of a quantum computer. Quantum Key Distribution (QKD) addresses these challenges by using quantum properties to exchange a secret key without the risk of being intercepted. Recent developments on the QKD system resulted in a stable key generation with fewer errors so that the QKD system is rapidly becoming a solid commercial proposition. However, although the security of the QKD system is guaranteed by quantum physics, its careless implementation could make the system vulnerable. In this paper, we proposed the first side-channel attack on plug-and-play QKD system. Through a single electromagnetic trace obtained from the phase modulator on Alice's side, we were able to classify the electromagnetic trace into four classes, which corresponds to the number of bit and basis combination in the BB84 protocol. We concluded that the plug-and-play QKD system is vulnerable to side-channel attack so that the countermeasure must be considered.
Over recent decades, quantum key distribution (QKD) has arisen as a promising solution for secure communications; however, like all cryptographic protocols, implementations of QKD can open security vulnerabilities. Until now, the study of physical vulnerabilities in QKD setups has focused primarily on the optical channel. In classical cryptoanalysis, power and electromagnetic side-channel analysis are powerful techniques that can be used to access secret information about the encryption key in symmetric-key algorithms. Such attacks have rarely been used in QKD, since they require an eavesdropper to have access to Alice’s or Bob’s setup; however, security proofs of QKD protocols generally assume that these setups are secure, making it crucial to understand the security measures required to ensure this protection. In this work, we propose and implement a power side-channel analysis of a QKD system by exploiting the power consumption of the electronic driver controlling the electro-optical components of the QKD transmitter. QKD modules typically require very precise electronic drivers, such as field-programmable gate arrays (FPGAs). Here, we show that the FPGA’s power consumption can leak information about the QKD operation and consequently the transmitted key. Our results are consistent and show critical information leakage, having reached a maximum accuracy of 73.35% in predicting transmitted qubits at a 100-MHz repetition frequency. We also discuss possible countermeasures to prevent such an attack.
Quantum key distribution (QKD) is a promising technique to share unconditionally secure keys between remote parties. As an essential part of a practical QKD system, reconciliation is responsible for correcting the errors due to the quantum channel noise by exchanging information through a public classical channel. In the present work, we propose a novel syndrome-based low-density parity-check (LDPC) reconciliation protocol to reduce the information leakage of reconciliation by fully utilizing the syndrome information that was previously wasted. Both theoretical analysis and simulation results show that our protocol can evidently reduce the information leakage as well as the number of communication rounds.