Quantum Key Distribution (QKD) provides information-theoretic security based on the laws of quantum mechanics, making it a promising candidate for secure communication in a post-quantum future. However, the gap between the theoretical security proofs and practical implementations introduces vulnerabilities that a capable adversary can exploit without disturbing the quantum channel. This thesis investigates such vulnerabilities within the framework of discrete-variable (DV) QKD, with a particular focus on the BB84 protocol with time-bin encoding. A simulation of a Trojan-Horse Attack (THA) targeting a time-bin BB84 transmitter is implemented in Lumerical Interconnect simulation software. In the attack, an eavesdropper injects bright light through the quantum channel using time-multiplexing at the protocol wavelength. The back-reflected light, carrying phase information from Alice’s phase modulator, is analyzed using homodyne detection. The simulation demonstrates that all four BB84 states are distinguishable at a base signal attenuation of 30 dB. Between 30 dB and 70 dB, the signatures gradually become more suppressed, providing full or partial state preparation information. At 70 dB total signal attenuation, the state preparation signatures are fully suppressed by the shot noise, establishing a practical security threshold for this attack scenario. At intermediate attenuation levels where individual state identification becomes ambiguous while basis information remains extractable, a combined basis-informed intercept-and-resend attack is possible without raising the quantum bit error rate. The 70 dB threshold is shown to be achievable in practice through the combined use of optical isolators, anti-reflective optical connections, and careful placement of Alice’s attenuator after the phase modulator. While the classical quadrature measurement is suppressed at this level, a rigorous security analysis cannot exclude the possibility of phase information carried by individual back-reflected photons being measured. The residual back-reflected photon number must, therefore, be treated as a leakage parameter in the security proof, with privacy amplification increased to account for an adversary’s potential information gain.
Quantum Key Distribution (QKD) provides information-theoretic security based on the laws of quantum mechanics, making it a promising candidate for secure communication in a post-quantum future. However, the gap between the theoretical security proofs and practical implementations introduces vulnerabilities that a capable adversary can exploit without disturbing the quantum channel. This thesis investigates such vulnerabilities within the framework of discrete-variable (DV) QKD, with a particular focus on the BB84 protocol with time-bin encoding. A simulation of a Trojan-Horse Attack (THA) targeting a time-bin BB84 transmitter is implemented in Lumerical Interconnect simulation software. In the attack, an eavesdropper injects bright light through the quantum channel using time-multiplexing at the protocol wavelength. The back-reflected light, carrying phase information from Alice’s phase modulator, is analyzed using homodyne detection. The simulation demonstrates that all four BB84 states are distinguishable at a base signal attenuation of 30 dB. Between 30 dB and 70 dB, the signatures gradually become more suppressed, providing full or partial state preparation information. At 70 dB total signal attenuation, the state preparation signatures are fully suppressed by the shot noise, establishing a practical security threshold for this attack scenario. At intermediate attenuation levels where individual state identification becomes ambiguous while basis information remains extractable, a combined basis-informed intercept-and-resend attack is possible without raising the quantum bit error rate. The 70 dB threshold is shown to be achievable in practice through the combined use of optical isolators, anti-reflective optical connections, and careful placement of Alice’s attenuator after the phase modulator. While the classical quadrature measurement is suppressed at this level, a rigorous security analysis cannot exclude the possibility of phase information carried by individual back-reflected photons being measured. The residual back-reflected photon number must, therefore, be treated as a leakage parameter in the security proof, with privacy amplification increased to account for an adversary’s potential information gain.
Quantum cryptography, or, more precisely, quantum key distribution (QKD) allows two distant parties, Alice and Bob, to expand a previously shared secret key by sending quantum states through a quantum channel. The goal here is for Alice and Bob to communicate securely in the presence of an eavesdropper, Eve. The most well-known QKD protocol is the Bennett-Brassard 1984 (BB84) protocol, which has been proved unconditionally secure against any attacks allowed by quantum mechanics. The main advantage of QKD over conventional cryptography is its absolute security. Thus, it is imperative to evaluate the security and performance of QKD protocols and realistic QKD systems. In this thesis, we study the security and performance aspects of QKD in four parts. In the first part, we study the Scarani-Acin-Ribordy-Gisin 2004 (SARG04) protocol. This protocol differs from the BB84 protocol only in the classical post-processing step and uses the same four quantum states as the BB84 protocol. We compared these two protocols and showed that in typical situations, the classical post-processing software for BB84 should be used in order to have a higher performance. In the second part, we prove the security of a three-state protocol. This protocol is similar to the BB84 protocol in that they share the same three qubit states. In the third part, we consider a practical Trojan horse attack on practical QKD systems implementing the BB84 protocol. The attack works by taking advantage of the finite response time of practical phase modulators to remap the states sent out by the legitimate user, Alice. We showed that if Alice and Bob are unaware of our attack, the final key shared between them can be compromised in some situations. In the fourth part, we consider another type of Trojan horse attack, which is based on mismatch between detection efficiencies of detectors. Practical detectors in QKD systems usually do not have identical efficiencies and an eavesdropper Eve may be able to exploit this efficiency mismatch to steal information about the final secret key. We analyze the security of the attack and show that it is possible for Eve to acquire some information on the final secret key if Alice and Bob are unaware of this attack.
This paper presents a theoretical and experimental demonstration of a security analysis of a Trojan-horse attack (THA) on a real-world quantum key distribution (QKD) system. We show that the upper bound on the information leakage depends solely on the fidelity between the states of the adversary. We find the lower bound for fidelity between THA states in both the polarization- and phase-coding BB84 protocols, considering both pure and mixed states. Our bounds depend only on the mean photon number per pulse available to an adversary. We also present an experimental analysis of a QKD system, including optical time-domain reflectometry measurements with centimeter resolution and spectral transmittance measurements for optical defense elements ranging from 1100 to 1800 nm with a noise floor lower than − 100 dB. Finally, by considering the optimal attack, we obtain the value of the mean photon number per pulse available to an adversary and calculate the key leakage that needs to be eliminated during the privacy amplification procedure. Published by the American Physical Society 2024
Quantum Key Distribution (QKD) is increasingly recognized as a robust solution to the vulnerabilities posed by quantum computing to classical encryption systems. This paper presents a comprehensive investigation of the BB84 protocol, the foundational QKD scheme, through simulated quantum communication using IBM’s Qiskit framework. The study models quantum key exchange under both ideal and adversarial conditions, including channel noise and intercept-resend attacks. The Quantum Bit Error Rate (QBER) was measured as a function of noise intensity and eavesdropping, confirming that QBER increases predictably in compromised channels. A threshold of 11% QBER was used to determine the viability of secure key generation. Final key retention rates were analyzed across multiple channel scenarios, revealing BB84’s sensitivity to both photon loss and adversarial interference.\n\nSecurity vulnerabilities such as photon-number-splitting and detector blinding were assessed, and countermeasures including decoy-state protocols and authenticated classical channels were proposed. Four key performance graphs were generated to visualize system behavior under realistic operational conditions. The results support BB84’s theoretical advantage in ensuring information-theoretic security while acknowledging real-world limitations in transmission range and hardware reliability. The paper concludes by addressing future improvements in QKD systems, including satellite-based distribution, quantum repeaters, and device-independent implementations. Through simulation-driven analysis, this study validates BB84 as a practical defense against quantum-enabled attacks and underscores the need for continued development and standardization in post-quantum cryptographic infrastructure.
Quantum Key Distribution (QKD) provides an innovative approach for secure communication by utilizing the principles of quantum mechanics. Among various QKD protocols, the BB84 protocol remains the most widely studied and implemented due to its simplicity and theoretical security guarantees. However, practical implementations of BB84 are vulnerable to several eavesdropping attacks and channel imperfections. This paper presents a comprehensive study on the detection and analysis of eavesdropping attacks in the BB84 protocol. The work focuses on common attacks such as intercept-resend attack, photon-number splitting attack, Trojan horse attack, and phase-covariant cloning attack. The effect of these attacks on Quantum Bit Error Rate (QBER) is analyzed in detail. Simulation-based observations are discussed to demonstrate how QBER increases under malicious interference. The study further explores statistical methods for detecting eavesdropping and examines practical countermeasures including decoy states, privacy amplification, and error correction techniques. The paper concludes that BB84 remains robust against eavesdropping when proper detection and mitigation mechanisms are employed.
In this thesis we develop practical tools for quantum key distribution (QKD) security proofs. We apply the tools to provide security proofs for several protocols, ranging from discrete variable protocols in high dimensions, protocols with realistic implementations, measurement device independent QKD and continuous-variable QKD. The security proofs are based on the Devetak-Winter security framework [I. Devetak and A. Winter, Proc. of the Roy. Soc. of London Series A, 461, 207 (2005); B. Kraus, N. Gisin, and R. Renner, Phys. Rev. Lett., 95 080501 (2005)]. \n \nIn the key rate calculation, it is often convenient to assume that the optimal attack is symmetric. \nUnder the assumption that the parameter estimation is based on coarse-grained observations, we show that the optimal attack is symmetric, if the protocol and the postselection have sufficient symmetries. \nAs an example we calculate the key rates of protocols using 2, d and d+1 mutually unbiased bases in d-dimensional Hilbert spaces. \n \nWe investigate the connection between the optimal collective eavesdropping attack and the optimal cloning attack, in which the eavesdropper employs an optimal cloner to attack the protocol. \nWe find that, in general, it does not hold that the optimal attack is an optimal cloner. However, there are classes of protocols, for which we can identify the optimal attack by an optimal cloner. \nWe analyze protocols with mutually unbiased bases in d dimensions, and show that for the protocols with 2 and d+1 mutually unbiased bases the optimal attack is an optimal cloner, but for the protocols with d mutually unbiased bases, it is not. \n \nIn optical implementations of the phase-encoded BB84 protocol, the bit information is usually encoded in the phase of two consecutive photon pulses generated in a Mach-Zehnder interferometer. In the actual experimental realization, the loss in the arms of the Mach-Zehnder interferometer is not balanced, for example because only one arm contains a lossy phase modulator. \nSince the imbalance changes the structure of the signals states and measurements, the BB84 security analysis no longer applies in this scenario. \nWe provide a security proof for the unbalanced phase-encoded BB84. The loss does lower the key rate compared to a protocol without loss. However, for a realistic parameter regime, the same key rate is found by applying the original BB84 security analysis. \n \nRecently, the security of a measurement device-independent QKD setup with BB84 signal states was proven in Refs. [H.-K. Lo, M. Curty, and B. Qi, Phys. Rev. Lett., 108 (2012); S. L. Braunstein and S. Pirandola, Phys. Rev. Lett., 108 (2012)]. In this setup Alice and Bob send quantum states to an intermediate node, which performs the measurement, and is assumed to be controlled by Eve. \nWe analyze the security of a measurement device-independent QKD protocol with B92 signal states, and calculate the key rates numerically for a realistic implementation. \nBased on our security proof we were able to prove the security of the strong reference pulse B92 protocol. \n \nWe analyze the security of continuous-variable protocols using the entropic uncertainty relations established in Ref. [M. Berta, M. Christandl, R. Colbeck, J. M. Renes, and R. Renner, Nature Physics 6, 659 (2010)] to provide an estimate of the key rate based on the observed first and second moments. We analyze a protocol with squeezed coherent states and the 2-state protocol with two coherent states with opposite phases.
Quantum key distribution (QKD) leverages the principles of quantum mechanics to exchange a secret key between two parties. Unlike classical cryptographic systems, the security of QKD is not reliant on computational assumptions but is instead rooted in the fundamental laws of physics. In a QKD protocol, any attempt by an eavesdropper to intercept the key is detectable: this provides an unprecedented level of security, making QKD an attractive solution for secure communication in an era increasingly threatened by the advent of quantum computers and their potential to break classical cryptographic systems. However, QKD also faces several practical challenges such as transmission loss and noise in quantum channels, finite key size effects, and implementation flaws in QKD devices. Addressing these issues is crucial for the large-scale deployment of QKD and the realization of a global quantum Internet. A whole body of research is dedicated to the hacking of the quantum state source, for example using (THAs), where the eavesdropper injects light into the system and analyzes the back-reflected signal. In this paper, we study the vulnerabilities against THAs of the iPOGNAC encoder, first introduced in [M. Avesani , ], to propose adapted countermeasures that can mitigate such attacks.