Flexible Byzantine Fault Tolerance (1904.10067v2)

Abstract: This paper introduces Flexible BFT, a new approach for BFT consensus solution design revolving around two pillars, stronger resilience and diversity. The first pillar, stronger resilience, involves a new fault model called alive-but-corrupt faults. Alive-but-corrupt replicas may arbitrarily deviate from the protocol in an attempt to break safety of the protocol. However, if they cannot break safety, they will not try to prevent liveness of the protocol. Combining alive-but-corrupt faults into the model, Flexible BFT is resilient to higher corruption levels than possible in a pure Byzantine fault model. The second pillar, diversity, designs consensus solutions whose protocol transcript is used to draw different commit decisions under diverse beliefs. With this separation, the same Flexible BFT solution supports synchronous and asynchronous beliefs, as well as varying resilience threshold combinations of Byzantine and alive-but-corrupt faults. At a technical level, Flexible BFT achieves the above results using two new ideas. First, it introduces a synchronous BFT protocol in which only the commit step requires to know the network delay bound and thus replicas execute the protocol without any synchrony assumption. Second, it introduces a notion called Flexible Byzantine Quorums by dissecting the roles of different quorums in existing consensus protocols.

• The paper introduces Flexible Byzantine Fault Tolerance (BFT), a novel approach achieving enhanced resilience through a new "alive-but-corrupt" fault model.
• Flexible BFT enhances diversity by allowing the protocol transcript to support diverse commit decisions based on varying beliefs and fault threshold combinations.
• The approach utilizes a synchronous BFT protocol where only the commit step requires a network delay bound and introduces Flexible Byzantine Quorums.

The paper "Flexible Byzantine Fault Tolerance" (1904.10067) introduces a novel approach to BFT consensus, emphasizing enhanced resilience and diversity through two key pillars. The first pillar is stronger resilience, achieved via a new fault model that considers "alive-but-corrupt" faults. In this model, replicas that are alive but corrupted may deviate arbitrarily to break the protocol's safety. However, if they cannot compromise safety, they will not attempt to prevent liveness. By incorporating alive-but-corrupt faults, Flexible BFT achieves resilience to higher corruption levels compared to a standard Byzantine fault model. The second pillar, diversity, involves designing consensus solutions where the protocol transcript is used to make different commit decisions based on diverse beliefs. This separation allows the same Flexible BFT solution to support both synchronous and asynchronous beliefs, as well as varying resilience threshold combinations of Byzantine and alive-but-corrupt faults.

Flexible BFT uses two main technical ideas to achieve its results. First, it presents a synchronous BFT protocol in which only the commit step requires knowledge of the network delay bound. This allows replicas to execute the protocol without needing synchrony assumptions. Second, the paper introduces Flexible Byzantine Quorums by dissecting the roles of different quorums in existing consensus protocols.

In summary, "Flexible Byzantine Fault Tolerance" (1904.10067) enhances BFT consensus by introducing the concepts of "alive-but-corrupt" faults and Flexible Byzantine Quorums. This leads to greater resilience and the ability to support diverse beliefs within the same BFT solution.