Everlasting Secrecy by Exploiting Eavesdropper\u27s Receiver Non-Idealities

This dissertation focuses on secrecy, which is a primary concern in modern communication. Secrecy has traditionally been obtained by cryptography, which is based on assumptions on current and future computational capabilities of the eavesdropper. However, there are numerous examples of cryptographic schemes being broken that were supposedly secure, often when the signal was recorded by the adversary for later processing. This motivates seeking types of secrecy that are provably everlasting for sensitive applications. The desire for such everlasting security suggests considering information-theoretic approaches, where the eavesdropper cannot extract any information about the secret message from the received signal. However, since the location and channel state information of a passive eavesdropper is generally unknown, it is challenging to know whether the advantage required to achieve information-theoretic security for a given scenario is provided, and thus attempting to obtain information-theoretic security via commonly-envisioned approaches leads to a significant risk in wireless communication. In this dissertation, we present a new perspective on how to generate the necessary information-theoretic advantage required for secret communication in the wireless environment. The proposed technique does not rely on the channel between the transmitter and the eavesdropper\u27s receiver because we exploit receiver\u27s processing effects for security. In particular, we attack the eavesdropper\u27s analog-to-digital (A/D) converter to generate the advantage required to obtain information-theoretic secrecy, as follows. Based on a key pre-shared between the legitimate nodes that only needs to be kept secret during transmission (and we pessimistically assume it will be handed to the adversary immediately afterward) we insert intentional distortion on the transmitted signal. Since the intended recipient of the signal knows the key and hence the distortion, it can undo the distortion before his/her A/D, whereas the eavesdropper must store the signal in memory and try to compensate for the distortion after the A/D conversion. Since the A/D is necessarily a non-linear component of the receiver, the operations are not necessarily commutative and there is the potential for information-theoretic security. This dissertation studies two practical instantiations of this approach to obtain everlasting secrecy against eavesdroppers with different hardware capabilities. As a first step, the transmitted signal is modulated by two vastly different power levels at the transmitter based on the key. Since the intended recipient knows the key, he/she can undo the power modulation before the A/D, putting the signal in the appropriate range for analog-to-digital conversion. The eavesdropper, on the other hand, must compromise between larger quantization noise and more A/D overflows, and thus will lose information required to recover the message. Hence, information-theoretic security is obtained. We show that this method can provide information-theoretic secrecy even when the eavesdropper has perfect access to the output of the transmitter, and even when the eavesdropper has an A/D that has better quality than the legitimate receiver\u27s A/D. A risk of the power modulation approach is a sophisticated eavesdropper with multiple A/Ds. In our second approach, in order to attack such an eavesdropper, we introduce the idea of adding random jamming (based on the ephemeral key) to the signal. In this case the intended recipient can simply subtract off the jamming signal and its signal will be well-matched to the span of its A/D converter, while the eavesdropper has difficulty because it does not know the key during transmission: if it does not change the span of the A/D, it will lose information due to A/D overflows, and, if it enlarges the span of the A/D to cover all possible received signal values, the width of each quantization level will be increased, and thus the eavesdropper will lose information due to high quantization noise. Hence, the desired advantage for information-theoretic secrecy is obtained. Finally, we study the combination of random jamming and frequency hopping in wideband channels, and show that considering the current fundamental limits of analog-to-digital conversion, this method can provide everlasting secrecy in wireless environments against any eavesdropper

Covert Communication over Classical-Quantum Channels

The square root law (SRL) is the fundamental limit of covert communication over classical memoryless channels (with a classical adversary) and quantum lossy-noisy bosonic channels (with a quantum-powerful adversary). The SRL states that $\mathcal{O}(\sqrt{n})$ covert bits, but no more, can be reliably transmitted in $n$ channel uses with $\mathcal{O}(\sqrt{n})$ bits of secret pre-shared between the communicating parties. Here we investigate covert communication over general memoryless classical-quantum (cq) channels with fixed finite-size input alphabets, and show that the SRL governs covert communications in typical scenarios. %This demonstrates that the SRL is achievable over any quantum communications channel using a product-state transmission strategy, where the transmitted symbols in every channel use are drawn from a fixed finite-size alphabet. We characterize the optimal constants in front of $\sqrt{n}$ for the reliably communicated covert bits, as well as for the number of the pre-shared secret bits consumed. We assume a quantum-powerful adversary that can perform an arbitrary joint (entangling) measurement on all $n$ channel uses. However, we analyze the legitimate receiver that is able to employ a joint measurement as well as one that is restricted to performing a sequence of measurements on each of $n$ channel uses (product measurement). We also evaluate the scenarios where covert communication is not governed by the SRL

Artificial Intersymbol Interference (ISI) to Exploit Receiver Imperfections for Secrecy

Abstract—Secure communication over a wireless channel in the presence of a passive eavesdropper is considered. We present a method to exploit the eavesdropper’s inherent receiver vulnerabilities to obtain everlasting secrecy. An ephemeral cryptographic key is pre-shared between the transmitter and the legitimate receiver and is utilized to induce intentional intersymbol interference (ISI). The legitimate receiver uses the key to cancel the ISI while the eavesdropper, since it does not have the key, cannot do such. It is shown that although ISI reduces the capacity of the main channel, it can lead to a net gain in secrecy rate. The achievable secrecy rates for different ISI filter settings are evaluated and the proposed method is compared with other information-theoretic security schemes. I