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：无需许可区块链在允许未知节点随时加入和退出系统的情况下达成共识。这类系统通常分为两种类型：工作量证明（PoW）和权益证明（PoS），两者均易受攻击。PoS协议面临长程攻击问题，攻击者可以低成本篡改执行历史；而PoW协议容易遭受具备足够计算能力的攻击者颠覆执行历史的威胁。PoS协议通过依赖社会共识等外部机制应对攻击；PoW协议要么退而求其次采用概率性保证，要么运行缓慢。我们提出Sieve-MMR，这是首个无需任何外部机制即可实现确定性安全性与恒定预期延迟的完全无需许可协议。通过将PoS协议（MMR）移植到PoW环境，我们获得了Sieve-MMR。继承自MMR的恒定预期延迟与确定性安全性，以及工作量证明机制对长程攻击的抵抗能力，共同构成了该协议的核心优势。将MMR移植到PoW环境的主要挑战在于我们称之为时间穿越攻击的新型威胁——攻击者利用久远历史中生成的工作量证明来夸大当前感知到的计算能力。为此我们提出Sieve算法，该算法实现了一种名为时间穿越韧性广播（TTRB）的新型广播原语。Sieve通过黑盒式确定性PoW原语来实例化TTRB，并将其作为MMR的消息传输层。