Permissionless blockchains achieve consensus while allowing unknown nodes to join and leave the system at any time. They typically come in two flavors: proof of work (PoW) and proof of stake (PoS), and both are vulnerable to attacks. PoS protocols suffer from long-range attacks, wherein attackers alter execution history at little cost, and PoW protocols are vulnerable to attackers with enough computational power to subvert execution history. PoS protocols respond by relying on external mechanisms like social consensus; PoW protocols either fall back to probabilistic guarantees, or are slow. We present Sieve-MMR, the first fully-permissionless protocol with deterministic security and constant expected latency that does not rely on external mechanisms. We obtain Sieve-MMR by porting a PoS protocol (MMR) to the PoW setting. From MMR we inherit constant expected latency and deterministic security, and proof-of-work gives us resilience against long-range attacks. The main challenge to porting MMR to the PoW setting is what we call time-travel attacks, where attackers use PoWs generated in the distant past to increase their perceived PoW power in the present. We respond by proposing Sieve, a novel algorithm that implements a new broadcast primitive we dub time-travel-resilient broadcast (TTRB). Sieve relies on a black-box, deterministic PoW primitive to implement TTRB, which we use as the messaging layer for MMR.

翻译：无许可区块链在允许未知节点随时加入和离开系统的同时实现共识。它们通常分为两类：工作量证明（PoW）和权益证明（PoS），两者均易受攻击。PoS协议遭受长程攻击，攻击者可以极低成本篡改执行历史；而PoW协议则易受拥有足够算力以颠覆执行历史的攻击者威胁。PoS协议通过依赖社会共识等外部机制应对；PoW协议要么退守概率性保证，要么运行缓慢。本文提出Sieve-MMR——首个具备确定性安全性和恒定预期延迟、且不依赖外部机制的完全无许可协议。我们通过将PoS协议（MMR）移植到PoW环境而构建Sieve-MMR。从MMR继承了恒定预期延迟和确定性安全性，工作量证明则赋予我们抵御长程攻击的能力。将MMR移植到PoW环境的主要挑战是我们所称的时空穿越攻击，即攻击者利用远古时期生成的工作量证明来增强当前时刻的感知算力。我们通过提出Sieve算法予以应对，该算法实现了一种新型广播原语——时空穿越弹性广播（TTRB）。Sieve依赖黑盒式确定性PoW原语实现TTRB，并将其作为MMR的消息传递层。