Trust is the basis of any distributed, fault-tolerant, or secure system. A trust assumption specifies the failures that a system, such as a blockchain network, can tolerate and determines the conditions under which it operates correctly. In systems subject to Byzantine faults, the trust assumption is usually specified through sets of processes that may fail together. Trust has traditionally been symmetric, such that all processes in the system adhere to the same, global assumption about potential faults. Recently, asymmetric trust models have also been considered, especially in the context of blockchains, where every participant is free to choose who to trust. In both cases, it is an open question how to compose trust assumptions. Consider two or more systems, run by different and possibly disjoint sets of participants, with different assumptions about faults: how can they work together? This work answers this question for the first time and offers composition rules for symmetric and for asymmetric quorum systems. These rules are static and do not require interaction or agreement on the new trust assumption among the participants. Moreover, they ensure that if the original systems allow for running a particular protocol (guaranteeing consistency and availability), then so will the joint system. At the same time, the composed system tolerates as many faults as possible, subject to the underlying consistency and availability properties. Reaching consensus with asymmetric trust in the model of personal Byzantine quorum systems (Losa et al., DISC 2019) was shown to be impossible, if the trust assumptions of the processes diverge from each other. With asymmetric quorum systems, and by applying our composition rule, we show how consensus is actually possible, even with the combination of disjoint sets of processes.
翻译：信任是任何分布、 错误容忍或安全系统的基础。 信任假设具体指明了系统, 如链链网络能够容忍的失败, 并确定其正确运行的条件。 在受拜占庭缺陷影响的系统中, 信任假设通常通过一系列可能共同失败的程序来具体化。 信任传统上是对称的, 以至于系统中的所有进程都遵守同样的全球潜在缺陷假设。 最近, 也考虑了不对称的信任模式, 特别是在块链中, 每个参与者都可以自由选择谁信任。 在这两种情况下, 如何构建信任假设是一个开放的问题。 考虑两种或两种以上系统, 由不同的参与者运行, 并且可能相互脱节, 有关错误的假设是: 它们如何一起工作? 这项工作第一次回答了这个问题, 提供了对潜在缺陷的匹配和不对称的法定人数系统的构成规则。 这些规则是静止的, 不需要在参与者之间就新的信任假设进行模式性互动或协议。 此外, 它们确保原始系统允许执行特定的协议( 保证一致性, 甚至是无法信任的可靠性, 也能够显示其他规则的稳定性, —— 也就是我们所显示的稳定性的稳定性的稳定性, 。 —— —— —— —— —— —— —— —— —— —— —— —— 和 —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— —— ——
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