Wait a second: Playing Hanabi without giving hints

Hanabi is a cooperative card game in which communication plays a key role. The game provides an interesting challenge for AI agents, because the game state is only partially observable, and the game limits what players can tell each other. This limit on communication channels is similar to a common scenario in system security research, and has been researched extensively in that context, for example by bypassing a system's isolation by establishing a covert communication channel. Such channels can be established through anything that the sending party can influence and the receiving party can observe, such as photonic emission, resource contention, or latency. In this paper, we present Hanabi agents that utilize timing as a covert channel so effectively that they can eschew the communicative actions provided by the game entirely. In addition to a thorough evaluation of the effectiveness of our approach, and a comparison to other Hanabi agents, we provide its context in the area of security, and an outlook on how it could be related to human behavior in future work.