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.
Quantum computing hardware is improving in robustness, but individual computers still have small number of qubits (for storing quantum information). Computations needing a large number of qubits can only be performed by distributing them over a network of smaller quantum computers. In this paper, we consider the problem of distributing a quantum computation, represented as a quantum circuit, over a homogeneous network of quantum computers, minimizing the number of communication operations needed to complete every step of the computation. We propose a two-step solution: dividing the given circuit’s qubits among the computers in the network, and scheduling communication operations, called migrations, to share quantum information among the computers to ensure that every operation can be performed locally. While the first step is an intractable problem, we present a polynomial-time solution for the second step in a special setting, and a O (log n )-approximate solution in the general setting. We provide empirical results which show that our two-step solution outperforms existing heuristic for this problem by a significant margin (up to 90%, in some cases).
The problem of transaction reordering in blockchains, also known as the blockchain anomaly [Christopher Natoli and Vincent Gramoli, 2016], can lead to fairness limitations [Kelkar et al., 2020] and front-running activities [Philip Daian et al., 2020] in cryptocurrency. To cope with this problem despite f < n/3 byzantine processes, Zhang et al. [Zhang et al., 2020] have introduced the ordering linearizability property ensuring that if two transactions or commands are perceived by all correct processes in the same order, then they are executed in this order. They proposed a generic distributed protocol that first orders commands and then runs a leader-based consensus protocol to agree on these orders, hence requiring at least 11 message delays. In this paper, we parallelize the ordering with the execution of the consensus to require only 6 message delays. For the ordering, we introduce the ordered reliable broadcast primitive suitable for broadcast-based cryptocurrencies (e.g., [Daniel Collins et al., 2020]). For the agreement, we build upon the DBFT leaderless consensus protocol [Tyler Crain et al., 2018] that was recently formally verified [Bertrand et al., 2021]. The combination is thus suitable to ensure ordering linearizability in consensus-based cryptocurrencies (e.g., [Tyler Crain et al., 2021]).
Small trusted hardware primitives can improve fault tolerance of Byzantine Fault Tolerant (BFT) protocols to one-half faults. However, existing works achieve this at the cost of increased communication complexity. In this work, we explore the design of communication-efficient BFT protocols that can boost fault tolerance to one-half without worsening communication complexity. Our results include a version of HotStuff that retains linear communication complexity in each view and a version of the VABA protocol with quadratic communication, both leveraging trusted hardware to tolerate a minority of corruptions. As a building block, we present communication-efficient provable broadcast, a core broadcast primitive with increased fault tolerance. Our results use expander graphs to achieve efficient communication in a manner that may be of independent interest.
We give a protocol for information dissemination in asynchronous networks of rational players, where each player may have its own desires and preferences as to the outcome of the protocol, and players may deviate from the protocol if doing so achieves their goals. We show that under minimalistic assumptions, it is possible to solve the information dissemination problem in a truthful manner, such that no participant has an incentive to deviate from the protocol we design. Our protocol works in any asynchronous network, provided the network graph is at least 2-connected. We complement the protocol with two impossibility results, showing that 2-connectivity is necessary, and also that our protocol achieves optimal bit complexity. As an application, we show that truthful information dissemination can be used to implement a certain class of communication equilibria , which are equilibria that are typically reached by interacting with a trusted third party. Recent work has shown that communication equilibria can be implemented in synchronous networks, or in asynchronous, complete networks; we show that in some useful cases, our protocol yields a lightweight mechanism for implementing communication equilibria in any 2-connected asynchronous network.
