Blockchain protocols typically aspire to run in the permissionless setting, in which nodes are owned and operated by a large number of diverse and unknown entities, with each node free to start or stop running the protocol at any time. This setting is more difficult than the traditional permissioned setting, in which the set of nodes that will be running the protocol is fixed and known at the time of protocol deployment. The goal of this paper is to provide a model for reasoning about the rich design space of blockchain protocols and their capabilities and limitations in the permissionless setting. Our results demonstrate senses in which the permissionless setting is provably more challenging than its permissioned counterpart, and identify unavoidable protocol ingredients and additional assumptions that are necessary to achieve various forms of consensus in the permissionless setting.

翻译：区块链协议通常旨在运行于无许可（permissionless）环境中，在该环境下节点由大量多样且未知的实体拥有和运营，每个节点可随时自由启动或停止运行协议。相较于传统许可（permissioned）环境，该设置更具挑战性——后者中运行协议的节点集合在协议部署时是固定且已知的。本文旨在构建一个模型，用以分析区块链协议在无许可环境中的丰富设计空间及其能力与局限。我们的研究结果表明，无许可环境在多个方面被证明比其许可对应环境更具挑战性，并识别出在无许可环境中实现各类共识所必需的协议核心要素与附加假设条件。