Paper 2025/2133 Original: eprint.iacr.org/2025/2133 ↗

Byzantine Broadcast with Unknown Participants

Abstract

A sender wishes to broadcast a message across an open, permissionless network (e.g., on the dark web) such that whoever is around and active reaches agreement, even if the sender is malicious. Crucially, no a priori knowledge of the number of parties or their identities is given, and no assumptions are made regarding the number of honest parties or existence of resource-allocation mechanisms, like proof-of-work or stake. The above is an instance of Byzantine broadcast (BB) with unknown-participants (UP Broadcast). Despite extensive research on dishonest-majority BB, existing approaches (such as Dolev-Strong broadcast) fail in this setting, as they rely on knowing the number of parties or corruption bounds. A key challenge, which might appear as an inherent limitation, is that malicious parties can join dynamically, making it difficult for honest parties to terminate without violating consistency. In this work, we provide the first definitions of UP Broadcast, accommodating both static and dynamic participation under arbitrarily many corruptions. We show that the problem is solvable by introducing a novel primitive called active-parties set agreement (APSA), which we use to construct a polynomial-time deterministic UP Broadcast protocol (as well as UP Interactive Consistency). Our protocols guarantee consistency and validity for all honest parties active throughout the execution. Furthermore, we settle the round complexity of UP Broadcast by proving that our deterministic protocols are round-optimal, matching a lower bound that holds even for randomized protocols. Finally, while we establish negative results for unrestricted dynamic participation where parties may join late, we formulate best-possible guarantees for late-joining parties alongside matching round-optimal constructions.