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1. Hamraz: Resilient Partitioning and Replication

   https://doi.org/10.1109/SP46214  

   Li, Xiao; Houshmand, Farzin (January 2022, S&P'22 (IEEE Symposium on Security and Privacy))

   Inter-organizational systems where subsystems with partial trust need to cooperate are common in healthcare, finance and military. In the face of malicious Byzantine attacks, the ultimate goal is to assure end-to-end policies for the three aspects of trustworthiness: confidentiality, integrity and availability. In contrast to confidentiality and integrity, provision and validation of availability has been often sidestepped. This paper guarantees end-to-end policies simultaneously for all the three aspects of trustworthiness. It presents a security-typed object-based language, a partitioning transformation, an operational semantics, and an information flow type inference system for partitioned and replicated classes. The type system provably guarantees that well-typed methods enjoy noninterference for the three properties, and that their types quantify their resilience to Byzantine attacks. Given a class and the specification of its end-to-end policies, the Hamraz tool applies type inference to automatically place and replicate the fields and methods of the class on Byzantine quorum systems, and synthesize trustworthy-by-construction distributed systems. The experiments show the resiliency of the resulting systems; they can gracefully tolerate attacks that are as strong as the specified policies. 

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2. Quorum subsumption for heterogeneous quorum systems

   https://doi.org/10.4230/LIPIcs.DISC.2023.28  

   Li, Xiao; Chan, Eric; Lesani, Mohsen (January 2023, Leibniz International Proceedings in Informatics (LIPIcs):37th International Symposium on Distributed Computing (DISC 2023))

   Oshman, Rotem (Ed.)

   Byzantine quorum systems provide higher throughput than proof-of-work and incur modest energy consumption. Further, their modern incarnations incorporate personalized and heterogeneous trust. Thus, they are emerging as an appealing candidate for global financial infrastructure. However, since their quorums are not uniform across processes anymore, the properties that they should maintain to support abstractions such as reliable broadcast and consensus are not well-understood. It has been shown that the two properties quorum intersection and availability are necessary. In this paper, we prove that they are not sufficient. We then define the notion of quorum subsumption, and show that the three conditions together are sufficient: we present reliable broadcast and consensus protocols, and prove their correctness for quorum systems that provide the three properties. 

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3. Brief Announcement: Reconfigurable Heterogeneous Quorum Systems

   https://doi.org/10.4230/LIPIcs.DISC.2024.52  

   Li, Xiao; Lesani, Mohsen (January 2024, Schloss Dagstuhl – Leibniz-Zentrum für Informatik)

   Alistarh, Dan (Ed.)

   In contrast to proof-of-work replication, Byzantine quorum systems maintain consistency across replicas with higher throughput, modest energy consumption, and deterministic liveness guarantees. If complemented with heterogeneous trust and open membership, they have the potential to serve as blockchains backbone. This paper presents a general model of heterogeneous quorum systems where each participant can declare its own quorums, and captures the consistency, availability and inclusion properties of these systems. In order to support open membership, it then presents reconfiguration protocols for heterogeneous quorum systems including joining and leaving of a process, and adding and removing of a quorum, and further, proves their correctness in the face of Byzantine attacks. The design of the protocols is informed by the trade-offs that the paper proves for the properties that reconfigurations can preserve. The paper further presents a graph characterization of heterogeneous quorum systems, and its application for reconfiguration optimization. 

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4. Hamband: RDMA Replicated Data Types

   https://doi.org/10.1145/3519939.3523426  

   Houshmand, Farzin; Saberlatibari, Javad; Lesani, Mohsen (January 2022, PLDI'22 (ACM SIGPLAN Conference on Programming Language Design and Implementation))

   Data centers are increasingly equipped with RDMAs. These network interfaces mark the advent of a new distributed system model where a node can directly access the remote memory of another. They have enabled microsecond-scale replicated services. The underlying replication protocols of these systems execute all operations under strong consistency. However, strong consistency can hinder response time and availability, and recent replication models have turned to a hybrid of strong and relaxed consistency. This paper presents RDMA replicated data types, the first hybrid replicated data types for the RDMA network model. It presents a novel operational semantics for these types that considers three distinct categories of methods and captures their re- quired coordination, and formally proves that they preserve convergence and integrity. It implements these semantics in a system called Hamband that leverages direct remote accesses to efficiently implement the required coordination protocols. The empirical evaluation shows that Hamband outperforms the throughput of existing message-based and SMR-based implementations by more than 4x. 

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5. Constant Latency in Sleepy Consensus

   https://doi.org/10.1145/3548606.3559347  

   Momose, Atsuki; Ren, Ling (November 2022, Proceedings of the 2022 ACM SIGSAC Conference on Computer and Communications Security)

   Dynamic participation support is an important feature of Bitcoin's longest-chain protocol and its variants. But these protocols suffer from long latency as a fundamental trade-off. Specifically, the latency depends at least on the following two factors: 1) the desired security level of the protocol, and 2) the actual participation level of the network. Classic BFT protocols, on the other hand, can achieve constant latency but cannot make progress under dynamic participation. In this work, we present a protocol that simultaneously supports dynamic participation and achieves constant latency. Our core technique is to extend the classic BFT approach from static quorum size to dynamic quorum size, i.e., according to the current participation level, while preserving important properties of static quorum. We also present a recovery mechanism for rejoining nodes that is efficient in terms of both communication and storage. Our experimental evaluation shows our protocol has much lower latency than a longest-chain protocol, especially when there is a sudden decrease of participation. 

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