Covert Optical Communication

TL;DR

This paper establishes the fundamental limits of covert optical communication, demonstrating that reliable and undetectable data transmission is possible in noisy channels with a square root scaling, supported by experimental validation.

Contribution

It characterizes the ultimate covert communication limit over noisy channels and provides a practical demonstration of achieving covert data transmission at this limit.

Findings

Covert communication is impossible over pure-loss channels.

In noisy channels, $ ext{O}( oot n)$ bits can be transmitted covertly.

Experimental results confirm theoretical predictions.

Abstract

Encryption prevents unauthorized decoding, but does not ensure stealth---a security demand that a mere presence of a message be undetectable. We characterize the ultimate limit of covert communication that is secure against the most powerful physically-permissible adversary. We show that, although it is impossible over a pure-loss channel, covert communication is attainable in the presence of any excess noise, such as a $300$K thermal blackbody. In this case, $\mathcal{O}(\sqrt{n})$ bits can be transmitted reliably and covertly in $n$ optical modes using standard optical communication equipment. The all-powerful adversary may intercept all transmitted photons not received by the intended receiver, and employ arbitrary quantum memory and measurements. Conversely, we show that this square root scaling cannot be outperformed. We corroborate our theory in a proof-of-concept experiment. We believe that our findings will enable practical realizations of covert communication and sensing, both for point-to-point and networked scenarios.