Asymmetric Distributed Trust

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 Björn Tackmann and Luca Zanolini [1, 2].