Quantum physical layer security protocols offer significant advantages, particularly in space communications, but their achievable secrecy rates are often limited to the asymptotic regime. Realistic communication systems, however, demand non-asymptotic solutions. Recently, the α-order information-theoretic metrics based on Rényi entropy has been proposed. We study their practical applicability to engineering secure quantum communication systems. By deriving a composable security bound using sandwiched Rényi entropy, we apply our framework to a practical scenario involving Binary Phase Shift Keying modulation over a lossy bosonic channel, consistent with Digital Video Broadcasting—Satellite—Second Generation Extension standards. We highlight a critical tradeoff between reliability and security, by showing that current coding rates may fall short of supporting positive secure coding rates under physical layer security considerations. This emphasizes the need for higher coding rates or additional frame lengths to balance reliability-security tradeoffs effectively. Through this application, we demonstrate the utility of α-order measures for advancing secure communication in quantum physical layer designs.
Quantum physical layer security protocols offer significant advantages, particularly in space communications, but their achievable secrecy rates are often limited to the asymptotic regime. Realistic communication systems, however, demand non-asymptotic solutions. Recently, the α-order information-theoretic metrics based on Rényi entropy has been proposed. We study their practical applicability to engineering secure quantum communication systems. By deriving a composable security bound using sandwiched Rényi entropy, we apply our framework to a practical scenario involving Binary Phase Shift Keying modulation over a lossy bosonic channel, consistent with Digital Video Broadcasting—Satellite—Second Generation Extension standards. We highlight a critical tradeoff between reliability and security, by showing that current coding rates may fall short of supporting positive secure coding rates under physical layer security considerations. This emphasizes the need for higher coding rates or additional frame lengths to balance reliability-security tradeoffs effectively. Through this application, we demonstrate the utility of α-order measures for advancing secure communication in quantum physical layer designs.
With the increasing demand for secure communication in optical space networks, it is essential to develop physical-layer scalable security solutions. In this context, we present the asymptotic security analysis of a keyless quantum private communication protocol that transmits classical information over quantum states. Different from the previous literature, our protocol sends dummy (decoy) states optimally obtained from the true information to deceive the eavesdropper. We analyze optical on-off keying (OOK) and binary phase shift keying (BPSK) for several detection scenarios. Our protocol significantly improves the protocol without decoy states whenever Bob is at a technological disadvantage with respect to Eve. Our protocol guarantees positive secrecy capacity when the eavesdropper gathers up to 90-99.9% (depending on the detection scenario) of the photon energy that Bob detects, even when Eve is only limited by the laws of quantum mechanics. We apply our results to the design of an optical inter-satellite link (ISL) study case with pointing losses, and introduce a new design methodology whereby the link margin is guaranteed to be secure by our protocol. Hence, our design does not require knowing the eavesdropper’s location and/or channel state: the protocol aborts whenever the channel drops below the secured margin. Our protocol can be implemented with state-of-the-art space-proof technology. Finally, we also show the potential secrecy advantage when using (not yet available) squeezed quantum states technology.
This dissertation investigates relative entropies, also called generalized divergences, and how they can be used to characterize information-theoretic tasks in quantum information theory. The main goal is to further refine characterizations of the optimal rates for quantum source coding, state redistribution, and measurement compression with quantum side information via second order asymptotic expansions and strong converse theorems. The dissertation consists of a mathematical and an information-theoretic part. In the mathematical part, we focus on the α-sandwiched Rényi divergence (α-SRD). We first investigate the limit α→0 to determine whether this recovers the well-known 0-Rényi relative divergence. We then prove various new results for entropic quantities derived from the α-SRD, including dimension bounds and useful bounds in terms of the fidelity between two quantum states. Furthermore, we derive a necessary and sufficient algebraic condition for equality in the data processing inequality (viz. monotonicity under quantum operations) for the α-SRD, and give applications to entropic bounds. In the information-theoretic part, we first derive the second order asymptotics of visible quantum source coding using a mixed source. For the achievability part, we develop universal quantum source codes achieving a given second order rate for a memoryless source. As a corollary of the main result, we obtain the second order asymptotics of quantum source coding using a single memoryless source. We then prove strong converse theorems for state redistribution (with or without feedback) and measurement compression with quantum side information. The key ingredients in proving these theorems are the aforementioned fidelity bounds on Rényi entropic quantities derived from the α-SRD.
In this paper, we provide secrecy metrics applicable to physical-layer coding techniques with finite blocklengths over Gaussian and fading wiretap channel models and analyze their secrecy performance over several cases of concatenated code designs. Our metrics go beyond some of the known practical secrecy measures, such as bit error rate and security gap, so as to make lower bound probabilistic guarantees on error rates over short blocklengths both preceding and following a secrecy decoder. Our techniques are especially useful in cases where application of traditional information-theoretic security measures is either impractical or simply not yet understood. The metrics can aid both practical system analysis, including cryptanalysis, and practical system design when concatenated codes are used for physical-layer security. Furthermore, these new measures fill a void in the current landscape of practical security measures for physical-layer security coding and may assist in the wide-scale adoption of physical-layer techniques for security in real-world systems. We also show how the new metrics provide techniques for reducing realistic channel models to simpler discrete memoryless wiretap channel equivalents over which existing secrecy code designs may achieve information-theoretic security.
Secure communications are becoming increasingly relevant in the development of space technology. Well-established cryptographic technology is already in place and is expected to continue to be so. On the other hand, information theoretical security emerges as a post-quantum versatile candidate to complement overall security strength. In order to prove such potential, performance analysis methods are needed that consider realistic legitimate and eavesdropper system assumptions and non-asymptotic coding lengths. In this paper, we propose the design of secure radio frequency (RF) satellite links with realistic system assumptions. Our contribution is three-fold. First, we propose a wiretap channel model for the finite-length regime. The model includes a stochastic wiretap encoding method using existing practical linear error correcting codes and hash codes. Secrecy is provided with privacy amplification, for which the finite-length secrecy metric is given that upper bounds semantic secrecy. Second, we derive a novel RF (broadcast) satellite wiretap channel model that parameterizes the stochastic degraded channel around the legitimate channel, a necessary condition to enable secure communication. Finally, we show the design of a secure satellite physical layer and finite-length performance evaluation. In doing so, we define as sacrifice rate the fixed fraction of the overall coding rate budget for reliability that needs to be allocated to secrecy. Our methodology does not make use of channel side information of the eavesdropper, it only makes assumptions on Eve's noise power. We illustrate our proposed design method with numerical results using practical error correcting codes in current standards of satellite communication.
Quantum key distribution (QKD) is a form of secure communication which transmits secret keys between two trusted parties. Evaluation of the achievable secret key rates are essential to ensure robust security of any QKD protocols. As such, the optimization of the secret key rate to obtain tight and reliable bounds are required especially for instances when the number of transmitted signals are finite. In this work, we make use of the generalized Rényi entropic quantities to obtain a tighter bound on the key rate compared to previous works which uses the von Neumann entropy. However, the optimization of Rényi entropic quantities is often non-trivial. Hence, we derive a generalized bound on the Rényi entropy in terms of the Rényi divergence which allows us to apply the Frank-Wolfe minimization algorithm for the optimization of secret key rates, thus generalizing existing numerical frameworks. With this generalized framework, we show improvements in regimes where finite signals experience high loss, thus ensuring relevance for real-world scenarios, particularly for satellite-based QKD implementations.
Physical Layer Security (PLS) exploits characteristics and properties of the physical layer for data encryption and supplements conventional cryptography for enhanced overall security. Most of the PLS methodologies rely on the (statistical) characteristics of the transmission channel to either generate secure encryption keys, or to exploit them together with other physical layer characteristics (i.e. advanced modulation schemes) for secure transmission. However, these approaches often lead to increased complexity and therefore become impractical for actual system implementation. Recent advancements in Quantum Key Distribution (QKD) systems allow for the utilization of ultra-secure and robust high-rate key exchange. In this work, we propose and describe practical techniques for exploiting and seamlessly integrating highrate QKD keys to encrypt modulation parameters and quantities of conventional modulation schemes like M-QAM, DMT and OFDM of communication links. Moreover, we present transmission scenarios, integrating QKD-PLS in free space optics links, together with their numerical evaluation. The main advantage of QKD exploitation to the proposed solutions comes from the seamless and transparent integration and application of high-rate keys which can either be used in their original form or feed a pseudo-random number generator, to modify the modulation properties/symbols in very high rates, such that eavesdropping and decoding of the encrypted information becomes almost impossible. Additionally, we present the architecture of a real time practical system utilizing and seamlessly integrating the QKD keys into transceiver links to form a robust and ultra-secure PLS ecosystem.