Optimistic Reliable Broadcast in Three Steps

Abstract

We study the latency tradeoffs of signature-free asynchronous reliable broadcast with optimistic two-step delivery. We consider $n$ parties, of which at most $f$ are Byzantine. An optimistic fault budget $b\le f$ guarantees two-step delivery with an honest broadcaster and at most $b$ faults, while correctness tolerates $f$ faults. We write $(d_o,d_s,d_t)$ for optimistic latency, honest-broadcaster latency with up to $f$ faults, and totality delay-the elapsed time from first to last honest delivery. For $3f+1\le n<4f$, our first construction improves Opt-RBC's $(2,4,4)$ guarantees to $(2,3,4)$, preserving its maximal optimistic budget $b_*=\lfloor n/2\rfloor-f$. Thus maximal optimism is compatible with the optimal three-step good case. We prove a complementary lower bound: for $3f+1\le n<4f$, every deterministic reliable broadcast protocol in our asynchronous model with two-step optimistic budget $b\ge n-3f$ has totality delay at least three, even when the broadcaster is honest. Our second construction achieves $(2,3,3)$, matching this lower bound at maximal optimism when $f\ge2$ and $\lceil(10f-3)/3\rceil\le n<4f$. Both constructions use quadratic messages and tolerate adaptive corruption. In the matching range, maximal optimism therefore costs exactly one totality step relative to Bracha's $(3,3,2)$, without increasing good-case latency.