DAG-based Consensus with Asymmetric Trust [Extended Version]

TL;DR

This paper introduces the first DAG-based asynchronous consensus protocol that operates under asymmetric trust assumptions, extending existing models to accommodate individual trust differences among participants.

Contribution

It develops the first asymmetric protocol for computing a common core and a randomized DAG-based consensus protocol with asymmetric quorums, advancing trust model flexibility.

Findings

Replacing threshold quorums with asymmetric ones in existing protocols is not sound.

A new asymmetric common core protocol is introduced.

The proposed consensus protocol decides within an expected constant number of rounds.

Abstract

In protocols with asymmetric trust, each participant is free to make its own individual trust assumptions about others, captured by an asymmetric quorum system. This contrasts with ordinary, symmetric quorum systems and with threshold models, where all participants share the same trust assumption. It is already known how to realize reliable broadcasts, shared-memory emulations, and binary consensus with asymmetric quorums. In this work, we introduce Directed Acyclic Graph (DAG)-based consensus protocols with asymmetric trust. To achieve this, we extend the key building-blocks of the well-known DAG-Rider protocol to the asymmetric model. Counter to expectation, we find that replacing threshold quorums with their asymmetric counterparts in the existing constant-round gather protocol does not result in a sound asymmetric gather primitive. This implies that asymmetric DAG-based consensus protocols, specifically those based on the existence of common-core primitives, need new ideas in an asymmetric-trust model. Consequently, we introduce the first asymmetric protocol for computing a common core, equivalent to that in the threshold model. This leads to the first randomized asynchronous DAG-based consensus protocol with asymmetric quorums. It decides within an expected constant number of rounds after an input has been submitted, where the constant depends on the quorum system.