Consensus in the Unknown-Participation Message-Adversary Model

TL;DR

This paper introduces a new distributed computing model inspired by permissionless systems like Bitcoin, allowing the study of consensus with an adversary impersonating processors, and presents two algorithms with safety guarantees under different assumptions.

Contribution

It defines a novel permissionless consensus model with impersonation and authentication, and provides two algorithms with safety and liveness guarantees under specific adversarial conditions.

Findings

Two consensus algorithms with safety guarantees

Deterministic safety and constant expected latency

Applicability to static, finitely many processes

Abstract

We propose a new distributed-computing model, inspired by permissionless distributed systems such as Bitcoin and Ethereum, that allows studying permissionless consensus in a mathematically regular setting. Like in the sleepy model of Pass and Shi, we consider a synchronous, round-by-round message-passing system in which the set of online processors changes each round. Unlike the sleepy model, the set of processors may be infinite. Moreover, processors never fail; instead, an adversary can temporarily or permanently impersonate some processors. Finally, processors have access to a strong form of message-authentication that authenticates not only the sender of a message but also the round in which the message was sent. Assuming that, each round, the adversary impersonates less than 1/2 of the online processors, we present two consensus algorithms. The first ensures deterministic safety and constant latency in expectation, assuming a probabilistic leader-election oracle. The second ensures deterministic safety and deterministic liveness assuming irrevocable impersonation and eventually-stabilizing participation. The model is unrealistic in full generality. However, if we assume finitely many processes and that the set of faulty processes remains constant, the model coincides with a practically-motivated model: the static version of the sleepy model.