Dynamic participation has recently become a crucial requirement for devising permissionless consensus protocols. It enhances robustness against events involving portions of participants going offline, thus preserving the safety and liveness of such dynamically available protocols. This concept, formalized by Pass and Shi (ASIACRYPT 2017) through the sleepy model, has been implicitly adopted to model various blockchain protocols, such as Ethereum's consensus protocol, Gasper. Neu, Tas, and Tse (S&P 2021) demonstrated that LMD-GHOST, Gasper's dynamic availability component, is not secure even in a full-participation context, i.e., with all validators online. Subsequent mitigations were developed to address its shortcomings. However, the resulting protocol still fails to achieve dynamic availability, motivating further research into more secure, dynamically available protocols. In this work, we introduce RLMD-GHOST, a synchronous, dynamically available protocol that maintains safety during bounded periods of asynchrony. This protocol is particularly appealing for practical systems where strict synchrony assumptions may not always hold, contrary to general assumptions in standard synchronous protocols. Additionally, we present the generalized sleepy model, within which our results are proven. Building upon the original sleepy model proposed by Pass and Shi, this model extends it with more generalized and stronger constraints on the corruption and sleepiness power of the adversary. This approach allows us to explore a wide range of dynamic participation regimes, spanning from complete dynamic participation to no dynamic participation, i.e., with every participant online. Consequently, this model provides a foundation for analyzing dynamically available protocols.

翻译：摘要：动态参与最近已成为设计无需许可共识协议的关键要求。它增强了协议在部分参与者离线场景下的鲁棒性，从而保障这类动态可用协议的安全性和活性。这一概念由Pass和Shi（ASIACRYPT 2017）通过睡眠模型形式化，并被隐式应用于建模多种区块链协议，例如以太坊的共识协议Gasper。Neu、Tas和Tse（S&P 2021）证明，即使在全参与场景（即所有验证者在线）下，Gasper的动态可用性组件LMD-GHOST也不安全。后续开发了多种缓解方案以弥补其缺陷。然而，最终形成的协议仍未能实现动态可用性，这促使研究者进一步探索更安全的动态可用协议。本文提出RLMD-GHOST，一种同步动态可用协议，能在有界异步期间维持安全性。该协议特别适用于严格同步假设未必成立的现实系统，这与标准同步协议的一般假设形成对比。此外，我们提出了广义睡眠模型，并在该模型框架内证明了研究结果。该模型在Pass和Shi提出的原始睡眠模型基础上扩展，对对手的腐败能力和睡眠能力施加了更泛化且更强的约束。通过这种方法，我们能够探索从完全动态参与到无动态参与（即所有参与者在线）的广泛动态参与模式。因此，该模型为分析动态可用协议提供了基础。