Byzantine broadcast is one of the fundamental problems in distributed computing. Many of its practical applications, from multiparty computation to consensus mechanisms for blockchains, require increasingly weaker trust assumptions, as well as scalability for an ever-growing number of users n. This rules out existing solutions which run in a linear number of rounds in n or rely on trusted setup requirements. In this paper, we propose the first sublinear-round and trustless Byzantine broadcast protocol for the dishonest majority setting. Unlike previous sublinear-round protocols, our protocol assumes neither the existence of a trusted dealer who honestly issues keys and correlated random strings to the parties nor random oracles. Instead, we present a solution whose setup is limited to an unstructured uniform reference string and a plain public key infrastructure (a.k.a. bulletin-board PKI).

Recent advancements on DAG-based consensus protocols allow for blockchains with improved metrics and properties, such as throughput and censorship-resistance. Variants of the Bullshark [18] consensus protocol are adopted for practical use by the Sui blockchain, for improved latency. However, the protocol is leader-based, and is strongly affected by crashed leaders that can lead to various performance issues, for example, decreased transaction throughput. In this paper, we propose HammerHead, a DAG-based consensus protocol, that is inspired by Carousel [8] and provides Leader-Utilization. Our proposal differs from Carousel, which is built for a chained consensus protocol; in HammerHead chain quality is inherited by the DAG. HammerHead needs to preserve safety and liveness, despite validators committing leader vertices asynchronously. The key idea is to update leader schedules dynamically, based on the validators' scores during the previous schedule. We implement HammerHead and show a minor improvement in performance for cases without faults. The major improvements in comparison to Bullshark appear in faulty settings. Specifically, we show a drastic, 2x-latency improvement and up to 40% increased throughput when crash faults occur (100 validators, 33 faults).

We present a deterministic synchronous protocol for binary Byzantine Agreement against a corrupt minority with adaptive O(n · f) communication complexity, where f is the exact number of corruptions. Our protocol improves the previous best-known deterministic Byzantine Agreement protocol developed by Momose and Ren (DISC 2021), whose communication complexity is quadratic, independent of the exact number of corruptions. Our approach combines two distinct primitives that we introduce and implement with O(n · f) communication, Reliable Voting and Weak Byzantine Agreement. In Reliable Voting, all honest parties agree on the same value only if all honest parties start with that value, but there is no agreement guarantee in the general case. In Weak Byzantine Agreement we achieve agreement, but validity requires that the inputs to the protocol satisfy certain properties. Our Weak Byzantine Agreement protocol is an adaptation of the recent Cohen et al. protocol (OPODIS 2022), in which we identify and address various issues.