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IEEE Journal of Selected Topics in Quantum Electronics

Spatial Mode Side Channels in Free-Space QKD Implementations

Markus Rau, Tobias Vogl, Giacomo Corrielli, Gwenaëlle Vest, Lukas Fuchs, Sebastian Nauerth, Harald Weinfurter

Published Nov 20, 2014• 34 citations
Publisher

Abstract

The quantum key distribution protocol uses one degree of freedom of a single quantum system to encode information. If this information has correlations with the system's other degrees of freedom, or if the measurement efficiencies on the receiver side depend on them, a security loophole called side channel is created. An eavesdropper can exploit it to gain information without disturbing the system, and thus, without revealing the attack. Here, we analyze side channels in a free-space QKD sender and receiver implementation and focus especially on the dependencies and side channels for the spatial degree of freedom.

DOI
10.1109/jstqe.2014.2372008

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The quantum key distribution protocol uses one degree of freedom of a single quantum system to encode information. If this information has correlations with the system's other degrees of freedom, or if the measurement efficiencies on the receiver side depend on them, a security loophole called side channel is created. An eavesdropper can exploit it to gain information without disturbing the system, and thus, without revealing the attack. Here, we analyze side channels in a free-space QKD sender and receiver implementation and focus especially on the dependencies and side channels for the spatial degree of freedom.
While quantum key distribution is possible with today's technology, the security of systems built with today's components is a difficult question. Real systems might have subtle weaknesses, so called "side-channels", which allow an eavesdropper to gain information about the key without direct polarization measurements. Other degrees of freedom of the photons might carry information which is correlated to their polarization. In this paper the author investigated the possible side-channels and show, by evaluating the amount of information leakage, that this weakness can be overcome by suitable countermeasures.
Practical implementations of quantum key distribution (QKD) protocols can introduce additional degrees of freedom in the quantum states that may render them distinguishable to an eavesdropper. This is the case of QKD systems using a different laser source to generate each quantum states, which can lead to temporal, spectral and/or spatial differences among them that can be exploited by a malicious party to extract information of the key. In this work we characterize, and experimentally verify, a side-channel attack on spatially distinguishable states against free-space QKD systems with misaligned laser sources. Specifically, for those emitting Gaussian beams, which is the most common case in free-space QKD. The attack makes theoretically unsafe any QKD system with any angular misalignment between the laser sources. Finally, we propose two countermeasures to eliminate the spatial distinguishability and secure the key exchange.
For a compact quantum key distribution (QKD) sender for the polarization encoding BB84 protocol, an eavesdropper could take a side-channel attack by measuring the spatial information of photons to infer their polarizations. The possibility of this attack can be reduced by introducing an aperture in the QKD sender, however, the effect of the aperture on the QKD security lacks of quantitative analysis. In this paper, we analyze the mutual information between the actual keys encoded at this QKD sender and the inferred keys at the eavesdropper (Eve), demonstrating the effect of the aperture to eliminate the spatial side-channel information quantitatively. It shows that Eve’s potential on eavesdropping spatial side-channel information is totally dependent on the optical design of the QKD sender, including the source arrangement and the aperture. The height of compact QKD senders with integrated light-emitting diode (LED) arrays could be controlled under several millimeters, showing great potential on applications in portable equipment.
Practical implementations of QKD protocols involve devices which are not perfect, and an eavesdropper may exploit this to gain information leading to attacks. Here we have considered the effects of coupling mismatch between the detectors. We find possible information leakage to Eve due to coupling mismatch at the receiver’s detectors in terms of mutual information between the eavesdropper and receiver. The experiment has been performed for the Gaussian and Laguerre-Gaussian modes of the signal. This aspect becomes essential while implementing free space QKD using a satellite. The results suggest that accounting for detection coupling mismatch is crucial to avoid side-channel attacks.
Side-channel attacks on practical quantum key distribution systems compromise its security. Although some of these attacks can be taken into account, the general recipe of how to eliminate all side-channel flaws is still missing. In this work, we propose a method for estimation of the total passive side-channel information leakage from the Alice's light source. The method relies on Hong-Ou-Mandel interference between different signals emitted by Alice, which reveals their overall mode mismatch without the necessity to measure all individual degrees of freedom independently. We include experimental values of interference visibility in the security proof for the decoy-state BB84 protocol, and lower-bound the secure key rate for realistic light sources. The obtained results provide a tool that can be used for certification of the current QKD systems and pave the way towards the loophole-free design of the future ones.
Quantum Key Distribution (QKD) leverages the principles of quantum mechanics to provide theoretically unconditional security for cryptographic key sharing. However, practical implementations remain vulnerable due to non-ideal devices and potential security loopholes at both the source and detection sides of QKD systems. The side-channel-secure (SCS) protocol addresses these challenges by encoding bits in vacuum and non-vacuum states and introducing a third-party measurement node, thereby repelling attacks targeting the detection side as well as external lab attacks on the source side. In this work, we consider the state-dependent correlated errors and Trojan-horse attack while preserving the SCS protocol's key advantage-specifically, requiring only upper bounds on intensities characterization without needing a full description of quantum states in infinite dimensions. Numerical results demonstrate that when the reflected light intensity from Trojan-horse attacks falls below 10−610^{-6}10−6, Eve can scarcely extract additional key information from the reflections. This work makes the SCS protocol more practical.