Key Exchange in the Quantum Era: Evaluating a Hybrid System of Public-Key Cryptography and Physical-Layer Security

TL;DR

This work proposes a hybrid key-exchange framework that leverages classical public-key cryptography to bootstrap a short-term secret and then employs a physical-layer jamming key exchange to derive a long-term secret, aiming for everlasting security in the quantum era. The approach introduces a temporal race between quantum-capable attackers and the JKE, analyzes storage-based threats to the jamming signal, and derives conditions under which positive secrecy rates can be achieved irrespective of Eve's channel quality. Key contributions include the notions of temporal advantage and non-storage channels, a detailed two-phase protocol, and quantitative discussion of ADC-related information loss and practical technology constraints. If validated, the framework could offer a pragmatic path to long-term security by tying cryptographic security to physical-layer imperfections and controlled jamming, though it requires formal security proofs, storage-model analyses, and consideration of advancing quantum and storage technologies.

Abstract

Today's information society relies on cryptography to achieve security goals such as confidentiality, integrity, authentication, and non-repudiation for digital communications. Here, public-key cryptosystems play a pivotal role to share encryption keys and create digital signatures. However, quantum computers threaten the security of traditional public-key cryptosystems as they can tame computational problems underlying the schemes, i.e., discrete logarithm and integer factorization. The prospective arrival of capable-enough quantum computers already threatens today's secret communication in terms of their long-term secrecy when stored to be later decrypted. Therefore, researchers strive to develop and deploy alternative schemes. In this work, evaluate a key exchange protocol based on combining public-key schemes with physical-layer security, anticipating the prospect of quantum attacks. If powerful quantum attackers cannot immediately obtain private keys, legitimate parties have a window of short-term secrecy to perform a physical-layer jamming key exchange (JKE) to establish a long-term shared secret. Thereby, the protocol constraints the computation time available to the attacker to break the employed public-key cryptography. In this paper, we outline the protocol, discuss its security, and point out challenges to be resolved.

Figures (3)

• Figure 1: Alice and Bob use classical public-key cryptography, e.g., RSA or DH, to establish a key $k_{AB}$ which enables their JKE to establish $k_L$. Concurrently, Eve seeks to compute $k_{AB}$ and obtain $k_L$. Everlasting security of $k_L$ is given if $t_{j} < t_{QC}$.
• Figure 2: JKE in a wiretap setting.
• Figure 3: JKE secrecy rate evaluation for a 40 MHz signal bandwidth and 500 fs rms aperture jitter of Bob's ADC. (a) Secrecy rate versus SNR of Eve's channel and SNR of Bob's channel for 5 fs aperture jitter of Eve's ADC and 14 bits per jamming symbol. (b) Minimum SNR of Bob to achieve a positive secrecy rate versus the number of jamming bits per symbol and the aperture jitter of Eve's ADC when Eve's channel is noiseless.