Whispering Agents: A Event-Driven Covert Communication Protocol for the Internet of Agents

The "Internet of Agents" (IoA) paradigm, where complex goals are met via large-scale, dynamic Agent-to-Agent (A2A) interactions, creates a vast attack surface for covert channels hidden within seemingly normal activities. Existing security models ignore this hidden communication, and traditional steganography, which generates static carriers, is ill-suited for the interactive, event-driven nature of A2A dialogues. To address this critical gap, we make three contributions. First, we establish a formal threat model for interactive agent ecosystems, defining two hierarchical security properties: statistical imperceptibility (IND-STAT) for entire communication traces and the stronger intention imperceptibility (IND-INT) for individual agent events. Second, we propose the Covert Communication Agent Protocol $\Pi{\text{CCAP}}$ as a practical instantiation of our model. Third, we comprehensively evaluate $\Pi{\text{CCAP}}$ across the critical dimensions of capacity, robustness, and imperceptibility, demonstrating its ability to facilitate reliable communication while satisfying our rigorous security requirements. Our work provides a foundational framework to analyze and defend against this new class of threats in the IoA.