Optimization of intensity-modulation/direct-detection optical key distribution under passive eavesdropping

TL;DR

This paper provides a theoretical analysis of optimizing optical key distribution using intensity modulation and direct detection, focusing on security against passive eavesdropping and practical system design.

Contribution

It derives closed-form expressions for optimal modulation depth and detection thresholds, simplifying the design of secure optical key distribution systems under varying conditions.

Findings

Key amount depends on the ratio of detection capabilities of legitimate users and eavesdroppers.

Derived proportionality relation in strong eavesdropping regime.

Presented formulas for optimal modulation depth and detection thresholds.

Abstract

We analyze theoretically optimal operation of an optical key distribution (OKD) link based on fine intensity modulation of an optical signal transmitted over an attenuating channel to a direct detection receiver. With suitable digital postprocessing, the users may generate a secret key that will be unknown to an unauthorized party collecting passively a fraction of the signal that escapes detection by the legitimate recipient. The security is ensured by the presence of the shot noise that inevitably accompanies eavesdropper's attempt to detect the collected signal. It is shown that the key amount depends on a ratio that compares legitimate recipient's and eavesdropper's capabilities to detect the signal, including noise contributed by their respective detectors. A simple proportionality relation is derived in the strong eavesdropping regime and closed expressions for the optimal depth of binary intensity modulation as well as the discrimination thresholds for hard-decoded direct detection are given. The presented results substantially simplify design of practical OKD systems operating under changing external conditions, e.g. variable atmospheric absorption in the case of free-space optical links.