Abstract

Quorum systems are a key abstraction in distributed fault-tolerant computing for capturing trust assumptions. They can be found at the core of many algorithms for implementing reliable broadcasts, shared memory, consensus and other problems. This talk introduces asymmetric Byzantine quorum systems that model subjective trust. Every process is free to choose which combinations of other processes it trusts and which ones it considers faulty. Asymmetric quorum systems strictly generalize standard Byzantine quorum systems, which have only one global trust assumption for all processes. The talk presents also several protocols that tolerate Byzantine faults with asymmetric trust, such as shared-register implementations and reliable Byzantine broadcasts. Consensus is arguably one of the most important notions in distributed computing and also relevant for practical systems. We also show how to realize consensus protocols with asymmetric trust, illustrating our approach for protocols in partially synchronous systems and for asynchronous protocols that use randomization with asymmetric trust. Asymmetric quorum systems offer a way to understand some ideas behind the Ripple and Stellar blockchain protocols, which aim at relaxing symmetric trust assumptions and permit flexible trust. The presentation is based on joint work with Bjorn Tackmann and Luca Zanolini [1, 2].

Highlights

  • Byzantine quorum systems [21] are a fundamental primitive for building resilient distributed systems from untrusted components

  • A middle ground between permissionless blockchains and Byzantine-fault tolerant (BFT)-based ones has been introduced by the blockchain networks of Ripple and Stellar

  • Standard Byzantine quorum systems and federated Byzantine quorum systems (FBQS) are not comparable because (1) an FBQS when instantiated with the same trust assumption for all processes does not reduce to a symmetric quorum system and (2) existing protocols do not generalize to FBQS

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Summary

Introduction

Byzantine quorum systems [21] are a fundamental primitive for building resilient distributed systems from untrusted components. A middle ground between permissionless blockchains and BFT-based ones has been introduced by the blockchain networks of Ripple (https://ripple.com) and Stellar (https: //stellar.org) Their stated model for achieving network-level consensus uses subjective trust in the sense that each process declares a local list of processes that it “trusts” in the protocol. Standard Byzantine quorum systems and FBQS are not comparable because (1) an FBQS when instantiated with the same trust assumption for all processes does not reduce to a symmetric quorum system and (2) existing protocols do not generalize to FBQS Understanding how such ideas of subjective trust, as manifested in the Ripple and Stellar blockchains, relate to traditional quorum systems is the main motivation for this work. The long version of the paper contains more details and all proofs [5]

Related work
System model
Symmetric trust
Asymmetric trust
Shared memory
Definitions
Protocol with authenticated data
Findings
Broadcast

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