A Secure Key Sharing Algorithm Exploiting Phase Reciprocity in Wireless Channels

TL;DR

This paper introduces a secure wireless key sharing method that leverages channel reciprocity and phase masking, creating multiple independent realizations to generate secure keys resistant to eavesdropping.

Contribution

It proposes a novel key exchange algorithm exploiting phase reciprocity and RF perturbation to enhance security over static or slow fading channels.

Findings

Secure key sharing using shared phase values and PSK constellation rotation.

Multiple independent channel realizations increase key size.

Eavesdropper faces under-determined systems, preventing key recovery.

Abstract

This article presents a secure key exchange algorithm that exploits reciprocity in wireless channels to share a secret key between two nodes $A$ and $B$. Reciprocity implies that the channel phases in the links $A\rightarrow B$ and $B\rightarrow A$ are the same. A number of such reciprocal phase values are measured at nodes $A$ and $B$, called shared phase values hereafter. Each shared phase value is used to mask points of a Phase Shift Keying (PSK) constellation. Masking is achieved by rotating each PSK constellation with a shared phase value. Rotation of constellation is equivalent to adding phases modulo-$2\pi$, and as the channel phase is uniformly distributed in $[0,2\pi)$, the result of summation conveys zero information about summands. To enlarge the key size over a static or slow fading channel, the Radio Frequency (RF) propagation path is perturbed to create several independent realizations of multi-path fading, each used to share a new phase value. To eavesdrop a phase value shared in this manner, the Eavesdropper (Eve) will always face an under-determined system of linear equations which will not reveal any useful information about its actual solution value. This property is used to establish a secure key between two legitimate users.