The Sum of Its Parts: Analysis of Federated Byzantine Agreement Systems
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Federated Byzantine Agreement Systems (FBASs) are a fascinating new paradigm in the context of consensus protocols. Originally proposed for powering the Stellar payment network, FBASs can be thought of as a middle way between typical permissionless systems (like Bitcoin) and permissioned approaches for solving consensus (like classical BFT protocols). Unlike Bitcoin and the like, validators must be explicitly chosen by peers. Unlike permissioned protocols, there is no need for the whole system to agree on the same set of validators. Instead, every node is free to decide for itself with whom it requires agreement. In this paper, we propose an intuitive yet precise methodology for determining whether the quorum systems resulting from such individual configurations can enable liveness and safety, respectively how many (byzantine) node failures they are away from losing these qualities. We apply our analysis approach and software to evaluate the effects of different node configuration policies, i.e., logics through which node configurations result from strategic considerations or an existing inter-node relationship graph. Lastly, we also investigate the reported "open-membership" property of FBASs. We observe that an often small group of nodes is exclusively relevant for determining safety and liveness "buffers", and prove that these top tiers are effectively "closed-membership" if maintaining safety is a core requirement.
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