Christian Cachin (University of Bern)
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, from shared-register implementations to reliable broadcasts and consensus. Asymmetric quorum systems also offer a way to understand some ideas behind the Ripple and Stellar blockchain protocols, which also aim at relaxing symmetric trust assumptions and permit flexible trust.