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IET conference proceedings.

Radiofrequency emanations of a single-photon detector

John J. Pantoja, Alfonso Tello Castillo, Dimitris E. Anagnostou, J. Kirrane, Mark Stonehouse, A. Koehler-Sidki, Michele Natrella, Ross J. Donaldson

Published Dec 18, 2023• 2 citations
Publisher

Abstract

Quantum key distribution (QKD) offers the possibility of information theoretic secure key establishment. However, non-ideal behaviour of some components used in practical implementations causes security risks and loopholes that need to be addressed. Previously identified loopholes have resulted in changes to hardware and the invention of new QKD protocols. In this document, we report an investigation into the eavesdropping risk due to out-of-band radiofrequency emanations produced by a QKD receiver. We present experimental results that show a QKD receiver may produce significant pulsed-like emissions that can be detected by a passive eavesdropper antenna to identify clicks in single-photon detectors.

DOI
10.1049/icp.2023.3271

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Quantum key distribution (QKD) offers the possibility of information theoretic secure key establishment. However, non-ideal behaviour of some components used in practical implementations causes security risks and loopholes that need to be addressed. Previously identified loopholes have resulted in changes to hardware and the invention of new QKD protocols. In this document, we report an investigation into the eavesdropping risk due to out-of-band radiofrequency emanations produced by a QKD receiver. We present experimental results that show a QKD receiver may produce significant pulsed-like emissions that can be detected by a passive eavesdropper antenna to identify clicks in single-photon detectors.
Quantum key distribution (QKD) promises information theoretic secure key as long as the device performs as assumed in the theoretical model. One of the assumptions is an absence of information leakage about individual photon detection outcomes of the receiver unit. Here we investigate the information leakage from a QKD receiver due to photon emission caused by detection events in single-photon detectors (backflash). We test commercial silicon avalanche photodiodes and a photomultiplier tube, and find that the former emit backflashes. We study the spectral, timing and polarization characteristics of these backflash photons. We experimentally demonstrate on a free-space QKD receiver that an eavesdropper can distinguish which detector has clicked inside it, and thus acquire secret information. A set of countermeasures both in theory and on the physical devices are discussed.
The security provided by Quantum Key Distribution (QKD) can be strongly compromised by interception of the raw and sifted key through side channels, such as the practical characteristics of electronic components used in the transmitter and receiver modules. Some out-of-band electromagnetic attacks have already been identified and tested in components used in QKD, such as quantum random number generators. In this presentation, we explore out-of-band electromagnetic attacks of other components used in a quantum receiver, such as Single Photon Avalanche Diodes (SPADs), and the time-correlated single-photon counting module. We measured the electromagnetic (EM) radiated emissions of the components to quantify the emanation levels and evaluate the vulnerability that this QKD side channel may present. The test was conducted in an anechoic chamber up to 1 GHz, at 3 m distance, and rotating the SPAD to provide radiation from four azimuth angles. Results show that measurable radiated pulses are generated by the SPAD in this frequency range due to dark count pulses and due to incoming Single-Photon level pulses. Dark counts of few kHz and Single-Photon level counts of hundreds of kHz were considered in the tests. EM radiation frequency bands with main emissions and electric field strengths are identified for both operation conditions.
Abstract The single-photon detectionefficiency of the detector unit is crucial for the security of common quantum key distribution protocols like Bennett-Brassard 1984 (BB84). A low value for the efficiency indicates a possible eavesdropping attack that exploits the photon receiver’s imperfections. We present a method for estimating the detection efficiency, and calculate the corresponding secure key generation rate. The estimation is done by testing gated detectors using a randomly activated photon source inside the receiver unit. This estimate gives a secure rate for any detector with non-unity single-photon detection efficiency, both inherit or due to blinding. By adding extra optical components to the receiver, we make sure that the key is extracted from photon states for which our estimate is valid. The result is a quantum key distribution scheme that is secure against any attack that exploits detector imperfections.
Quantum key distribution (QKD) theoretically offers unconditional security. Unfortunately, the gap between theory and practice threatens side-channel attacks on practical QKD systems. Many well-known QKD protocols use weak coherent laser pulses to encode the quantum information. These sources differ from ideal single photon sources and follow Poisson statistics. Many protocols, such as decoy state and coincidence detection protocols, rely on monitoring the photon statistics to detect any information leakage. The accurate measurement and characterization of photon statistics enable the detection of adversarial attacks and the estimation of secure key rates, strengthening the overall security of the QKD system. We have rigorously characterized our source to estimate the mean photon number employing multiple detectors for comparison against measurements made with a single detector. Furthermore, we have also studied intensity fluctuations to help identify and mitigate any potential information leakage due to state preparation flaws. We aim to bridge the gap between theory and practice to achieve information-theoretic security.
Reference-frame-independent quantum key distribution (RFI-QKD) enables the generation of secure keys when the reference frame suffers from unknown and slow drifting. To combat the potential photon-number-splitting attacks in RFI-QKD, the decoy-state method should be adopted. Generally, the decoy states are prepared by actively modulating different photon numbers of weak coherent pulses, which, however, may leak the side-channel information due to the imperfect modulation. To address this security loophole, we propose to implement the improved RFI-QKD protocol [Opt. Lett.47, 4219 (2022)10.1364/OL.470558], which bounds the eavesdropper's information analytically and is less sensitive to statistical fluctuations, with passive decoy states. By interfering two weak coherent pulses on a beam splitter and then directing one arm of the outputs into the single-photon avalanche detector (SPAD), different photon-number distributions of pulses can be prepared according to the response status of SPAD. To make this scheme more practical, we consider the inevitable afterpulses of the SPADs for both the transmitter and receiver, and construct an afterpulse-compatible analysis model. Simulation results show that our scheme can achieve good performance in practical scenarios.
The security of quantum key distribution (QKD) can easily be obscured if the eavesdropper can utilize technical imperfections in the actual implementation. Here, we describe and experimentally demonstrate a very simple but highly effective attack that does not need to intercept the quantum channel at all. Only by exploiting the dead time effect of single-photon detectors is the eavesdropper able to gain (asymptotically) full information about the generated keys without being detected by state-of-the-art QKD protocols. In our experiment, the eavesdropper inferred up to 98.8% of the key correctly, without increasing the bit error rate between Alice and Bob significantly. However, we find an even simpler and more effective countermeasure to inhibit this and similar attacks.