# Shared Secret Key Generation via Carrier Frequency Offsets

## TL;DR

This paper introduces a new secret key generation method leveraging reciprocal carrier frequency offsets between legitimate nodes, enhancing security by exploiting physical layer characteristics and analyzing different frequency stability regimes.

## Contribution

The work proposes a novel key generation algorithm based on CFO estimation, quantization, and reconciliation, with analysis of frequency stability regimes and key rate computation.

## Key findings

- Key disagreement rate decreases exponentially with SNR.
- Eve's decipher probability reduces when her link quality degrades.
- Simulation confirms effectiveness of CFO-based key generation.

## Abstract

This work presents a novel method to generate secret keys shared between a legitimate node pair (Alice and Bob) to safeguard the communication between them from an unauthorized node (Eve). To this end, we exploit the {\it reciprocal carrier frequency offset} (CFO) between the legitimate node pair to extract common randomness out of it to generate shared secret keys. The proposed key generation algorithm involves standard steps: the legitimate nodes exchange binary phase-shift keying (BPSK) signals to perform blind CFO estimation on the received signals, and do equi-probable quantization of the noisy CFO estimates followed by information reconciliation--to distil a shared secret key. Furthermore, guided by the Allan deviation curve, we distinguish between the two frequency-stability regimes---when the randomly time-varying CFO process i) has memory, ii) is memoryless; thereafter, we compute the key generation rate for both regimes. Simulation results show that the key disagreement rate decreases exponentially with increase in the signal to noise ratio of the link between Alice and Bob. Additionally, the decipher probability of Eve decreases as soon as either of the two links observed by the Eve becomes more degraded compared to the link between Alice and Bob.