Whispering Secrets in a Crowd: Leveraging Non-Covert Users for Covert Communications

TL;DR

This paper explores the fundamental limits of multi-user covert communication systems, revealing how non-covert users can enhance covert communication capacity and emphasizing the importance of multiplexing strategies.

Contribution

It introduces a comprehensive analysis of covert and non-covert user interactions, establishing the capacity region and secret-key tradeoffs in multi-access covert communication.

Findings

Non-covert users can improve covert user capacities.

Multiplexing strategies are essential to achieve optimal rate regions.

The covert-capacity secret-key tradeoff is characterized for multi-access systems.

Abstract

This paper establishes the fundamental limits of a multi-access system where multiple users communicate to a legitimate receiver in presence of an external warden. Only a specific subset of the users, called covert users, needs their communication to remain undetected to the warden, while the remaining non-covert users have no such constraint. The fundamental limits show a tradeoff between the different rates that are simultaneously achievable at the various users in function of the secret-key rates that the different users share with the legitimate receiver. Interestingly, the presence of the non-covert users can enhance the capacities of the covert users, especially under stringent secret-key budgets. Our findings underscore the essential requirement of employing a multiplexing (coded time-sharing) strategy to exhaust the fundamental region of all rates that are simultaneously achievable at the covert and the non-covert users. As a side-product of our results, we also establish the covert-capacity secret-key tradeoff for standard single-user and multi-access covert communication systems (without non-covert users), i.e., the largest covert rates that are achievable under given secret-key rate budgets. Previous works had only established the minimum secret-key rates required at largest covert rates.