Quantum key distribution (QKD) is a method for exchanging keys between two parties (Alice and Bob) with an absolute level of security in the ideal scenario. QKD systems are vulnerable to hacking through secondary emissions of photons by SPAD detectors, known as backflash. In this work, we derive an information leakage probability model for the scenario of QKD in a free space communication (FSO) system, and suggest a method for its minimization.
Quantum key distribution (QKD) is a method for exchanging keys between two parties (Alice and Bob) with an absolute level of security in the ideal scenario. QKD systems are vulnerable to hacking through secondary emissions of photons by SPAD detectors, known as backflash. In this work, we derive an information leakage probability model for the scenario of QKD in a free space communication (FSO) system, and suggest a method for its minimization.
Quantum key distribution (QKD) is a potential technology suitable for fulfiling the stringent requirements of upcoming 6G wireless networks in terms of privacy and security. Nevertheless, QKD systems have several practical limitations in terms of hardware design and implementation, making them vulnerable to malicious attacks. Specifically, we adopt a detection scheme using a single-photon avalanche diode and due to the avalanche effect, a secondary photon is emitted, called backflash, which can be captured by an eavesdropper. To mitigate this security loophole and enhance the secrecy performance, we consider a selection mechanism which schedules a single user for secret key exchange purposes. In particular, we propose an intelligent two-stage selection scheme in a multiuser free-space optical QKD deployment. We derive closed-form expressions for the secrecy success probability by considering a generalized pointing error model. Our results confirm that the proposed scheme yields significant secrecy gains over conventional methods.
Quantum key distribution (QKD)1 is a quantum technology already present in the market. This technology will become an essential point to secure our communication systems and infrastructure when today’s public key cryptography will be broken by either a mathematical algorithm or by, eventually, the development of quantum computers. One of the main task of quantum metrology and standardization in the next future is ensuring that QKD apparatus works as expected, and appropriate countermeasures against quantum hacking are taken. In this paper, we discuss the security of one of the QKD most critical (and quantum-hackered) components, i.e., single photon detectors based on fiber-pigtailed InGaAs SPADs. We analyze their secondary photon emission (backflash light) that can be exploited by an eavesdropper (Eve) to gain information without introducing errors in the key. We observed a significant light leakage from the detection event of fiber-pigtailed InGaAs SPADs. This may represent a significant security threat in all QKD apparatus. We provide a method to quantify the amount of potential information leakage, and we propose a solution to fix this potential security bug in practical QKD apparatus.
For examining QKD systems for sustainability to the “backflash” attack, it is necessary to measure the probability of photon re-emission and calculate the maximum possible probability of backflash for the secret key to remain secure. In this paper, we present research on backflash probabilities dependencies on parameters of fiber-based QKD systems. The SPAD gate width for each QKD system and quantum communication protocol is individual, therefore, it is necessary to consider the dependence of the probability of photon re-emission on the SPAD gate width. Moreover, as a result of Laser Damage Attack, active elements, such as a variable attenuator or a pulsed laser, may not work quite stable, eventually, as far as the mean photon number per pulse (μ) varies within a small range, the maximum backflash probability can fluctuate. We demonstrate an experimental setup of correlation “backflash” measurements by optical reflectometry with very low dark count rate and parasitic noise, furthermore, the contribution of reflected photons is negligible in comparison with the classical optical reflectometry scheme. Therefore, the signal-to-noise ratio was increased by at least two orders of magnitude. The obtained experimental data demonstrate the variation of the backflash probability of the single-photon detector depending on the SPAD gate width and the mean photon number per pulse sent by Alice to Bob. Analysis of the calculated backflash probabilities enables to estimate the maximum possible information leakage, depending on the parameters of equipment. In the future, the obtained experimental results can be used to adjust the optimal parameters of QKD systems to guarantee communication protocol security.
Nowadays, high security levels are required to transmit critical information for government, private and personal sectors. As a countermeasure, the Quantum Key Distribution systems are the best option in order to protect this information because it provides unconditional security. In addition, increasing the transmission distance is a highlight. Therefore, the Free-Space Optical Quantum Key Distribution systems (FSO-QKD) present an innovative way for sharing secure information between two parties located at ground stations, spacecraft or aircraft. However, these scenarios present several challenges regarding the hardware, protocols and techniques used that must be solved in order to enhance the performance parameters (security level, distance link, final secret key rate, among others) for any QKD system; although, in particular, a high transmission performance is required for both the classical and quantum channels. These issues impose the roadmap and trends in the research, academic and manufacturing sectors around the world.
This paper proposes a novel quantum key distribution (QKD) set-up designed to enhance the security of a free-space optical (FSO) quantum communication system. By incorporating channel ordering information, the proposed QPSK-QKD (P-QPSK-QKD) system leverages the inherent randomness of FSO channels, providing dual protection against eavesdroppers (Eve). The first layer of security is achieved through the QPSK-QKD mechanism, while the second arises from the randomness of the FSO channels. To ensure a comprehensive investigation, the performance of P-QPSK-QKD and the existing QPSK-QKD (E-QPSK-QKD) system is analyzed using a Málaga distributed atmospheric turbulence channel model incorporating pointing errors. Novel closed-form expressions are derived for different performance metrics, including probability of sifted keys, quantum bit error rate, probability of Eve (Peve), and ergodic secret key rate (ESKR), under both perfect and imperfect feedback conditions. Intensive examination indicates that, unlike E-QPSK-QKD, wherePeveshows a water-falling trend with increasing signal-to-noise ratios (SNR) under dual thresholds, P-QPSK-QKD achieves a saturatedPeve, enhancing security. Moreover, under the single threshold,Peveof P-QPSK-QKD is twice that of E-QPSK-QKD. Additionally, due to channel randomness, the mutual information between Alice and Eve remains zero (for both single and dual thresholds), even at infinite SNR for P-QPSK-QKD, providing consistently higher ESKR than E-QPSK-QKD system under both perfect and imperfect feedback conditions.
Single photon avalanche diodes (SPADs) are the most commercially diffused solution for single-photon counting in quantum key distribution (QKD) applications. However, the secondary photon emission, arising from the avalanche of charge carriers during a photon detection, may be exploited by an eavesdropper to gain information without forcing errors in the transmission key. In this paper, we characterise such backflash light in gated InGaAs/InP SPADs, and its spectral and temporal characterization for different detector models and different operating parameters. We qualitatively bound the maximum information leakage due to backflash light, and propose a solution.