Most state machine replication protocols are either based on the 40-years-old Byzantine Fault Tolerance (BFT) theory or the more recent Nakamoto's longest chain design. Longest chain protocols, designed originally in the Proof-of-Work (PoW) setting, are available under dynamic participation, but has probabilistic confirmation with long latency dependent on the security parameter. BFT protocols, designed for the permissioned setting, has fast deterministic confirmation, but assume a fixed number of nodes always online. We present a new construction which combines a longest chain protocol and a BFT protocol to get the best of both worlds. Using this construction, we design TaiJi, the first dynamically available PoW protocol which has almost deterministic confirmation with latency independent of the security parameter. In contrast to previous hybrid approaches which use a single longest chain to sample participants to run a BFT protocol, our native PoW construction uses many independent longest chains to sample propose actions and vote actions for the BFT protocol. This design enables TaiJi to inherit the full dynamic availability of Bitcoin, as well as its full unpredictability, making it secure against fully-adaptive adversaries with up to 50% of online hash power.

中文翻译：

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太极：拥有BFT快速确认的最长链条可用性

大多数状态机复制协议要么基于已有40年历史的拜占庭式容错（BFT）理论，要么是基于中本聪最长的链设计。最长链协议最初是在工作量证明（PoW）设置中设计的，可在动态参与下使用，但具有概率确认，且取决于安全参数的延迟较长。为允许的设置而设计的BFT协议具有快速的确定性确认，但是假定始终在线的节点数固定。我们提出了一种结合了最长的链协议和BFT协议的新结构，以实现两全其美。使用这种构造，我们设计了TaiJi，这是第一个动态可用的PoW协议，它具有几乎确定性的确认，并且其延迟与安全参数无关。与以前的使用单个最长链来采样参与者以运行BFT协议的混合方法相比，我们的本地PoW结构使用许多独立的最长链来对BFT协议的提议动作和投票动作进行采样。这种设计使TaiJi能够继承比特币的全部动态可用性以及其完全的不可预测性，从而使其具有高达50％的在线哈希能力，从而可以抵御完全自适应的对手。