We present DAG-Rider, the first asynchronous Byzantine Atomic Broadcast protocol that achieves optimal resilience, optimal amortized communication complexity, and optimal time complexity. DAG-Rider is post-quantum safe and ensures that all values proposed by correct processes eventually get delivered. We construct DAG-Rider in two layers: In the first layer, processes reliably broadcast their proposals and build a structured Directed Acyclic Graph (DAG) of the communication among them. In the second layer, processes locally observe their DAGs and totally order all proposals with no extra communication. CCS CONCEPTS• Theory of computation → Distributed algorithms; • Security and privacy → Distributed systems security.
We present DAG-Rider, the first asynchronous Byzantine Atomic Broadcast protocol that achieves optimal resilience, optimal amortized communication complexity, and optimal time complexity. DAG-Rider is post-quantum safe and ensures that all messages proposed by correct processes eventually get decided. We construct DAG-Rider in two layers: In the first layer, processes reliably broadcast their proposals and build a structured Directed Acyclic Graph (DAG) of the communication among them. In the second layer, processes locally observe their DAGs and totally order all proposals with no extra communication. INTRODUCTIONThe amplified need in scalable geo-replicated Byzantine fault-tolerant reliability systems has motivated an enormous amount of study on the Byzantine State Machine Replication (SMR) problem [17,31]. Many variants of the problem were defined in recent years [28,32,43] to capture the needs of blockchain systems. To address the fairness issues that naturally arise in interorganizational deployments, we focus on the classic long-lived Byzantine Atomic Broadcast (BAB) problem [13,19], which in addition to total order, progress, and validity, guarantees eventual fairness. That is, all proposals by correct processes are eventually included.Up until recently, asynchronous protocols for the Byzantine consensus problem [13,16,26] have been considered too costly or complicated to be used in practical SMR solutions. However, two recent single-shot Byzantine consensus papers, VABA [1] and later Dumbo [35], presented asynchronous solutions with (1) optimal resilience, (2) expected constant time complexity, and (3) optimal quadratic communication and optimal amortized linear communication complexity (for the latter). In this paper, we follow this recent line of work and present DAG-Rider: the first asynchronous BAB protocol with optimal resilience, optimal round complexity, and optimal amortized communication complexity. In addition, given a perfect shared coin abstraction, our protocol does not use signatures and does not rely on asymmetric cryptographic assumptions. Therefore, when using a deterministic threshold-based coin implementation with an information theoretical agreement guarantee [14,34], the safety properties of our BAB protocol are post-quantum secure.Overview. We construct DAG-Rider in two layers: a communication layer and a zero-overhead ordering layer. In the communication layer, processes reliably broadcast their proposals with some meta-data that help them form a Directed Acyclic Graph (DAG) of the messages they deliver. That is, the DAG consists of rounds s.t. every process broadcasts at most one message in every round and every message has 𝑂 (𝑛) references to messages in previous rounds, where 𝑛 is the total number of processes. The ordering layer does not require any extra communication. Instead, processes observe their local DAGs and with the help of a little randomization (one coin flip per 𝑂 (𝑛) decisions on values proposed by different processes) locally order all the delivered messages in th...
Blockchains such as Bitcoin and Ethereum execute payment transactions securely, but their performance is limited by the need for global consensus. Payment networks overcome this limitation through off-chain transactions. Instead of writing to the blockchain for each transaction, they only settle the final payment balances with the underlying blockchain. When executing off-chain transactions in current payment networks, parties must access the blockchain within bounded time to detect misbehaving parties that deviate from the protocol. This opens a window for attacks in which a malicious party can steal funds by deliberately delaying other parties' blockchain access and prevents parties from using payment networks when disconnected from the blockchain. We present Teechain, the first layer-two payment network that executes off-chain transactions asynchronously with respect to the underlying blockchain. To prevent parties from misbehaving, Teechain uses treasuries, protected by hardware trusted execution environments (TEEs), to establish off-chain payment channels between parties. Treasuries maintain collateral funds and can exchange transactions efficiently and securely, without interacting with the underlying blockchain. To mitigate against treasury failures and to avoid having to trust all TEEs, Teechain replicates the state of treasuries using committee chains, a new variant of chain replication with threshold secret sharing. Teechain achieves at least a 33× higher transaction throughput than the state-of-the-art Lightning payment network. A 30-machine Teechain deployment can handle over 1 million Bitcoin transactions per second. CCS Concepts • Security and privacy → Distributed systems security.
Many distributed protocols in the partial synchrony setting with Byzantine nodes divide the local state of the nodes into views, and the transition from one view to the next dictates a leader change. In order to provide liveness, all honest nodes need to stay in the same view for a sufficiently long time. This requires intricate mechanisms that are typically intertwined with the rest of the protocol, making it hard to understand and to reason about. Furthermore, state-machine replication, which is consisted of multiple instances of single-shot consensus can use the same view synchronization protocol.We define the Byzantine View Synchronization problem, which is responsible for eventually bringing all nodes to the same view for a sufficiently long time.Two approaches for implementing a protocol that achieves view synchronization exhibit the following tradeoffs: a view doubling solution has zero communication costs but unbounded latency, while a broadcastbased solution has quadratic communication costs but constant latency. We describe both protocols, prove their correctness, and also introduce a third protocol, named Lumière 1 , that has optimistically linear communication complexity and constant latency, and faced with benign failures, has expected linear communication and constant latency. Lumière is particularly useful for a family of consensus protocols that exhibit linear communication under various circumstances.
State Machine Replication (SMR) solutions often divide time into rounds, with a designated leader driving decisions in each round.Progress is guaranteed once all correct processes synchronize to the same round, and the leader of that round is correct. Recently suggested Byzantine SMR solutions such as HotStuff, Tendermint, and LibraBFT achieve progress with a linear message complexity and a constant time complexity once such round synchronization occurs. But round synchronization itself incurs an additional cost.By Dolev and Reischuk's lower bound, any deterministic solution must have Ω(n 2 ) communication complexity. Yet the question of randomized round synchronization with an expected linear message complexity remained open.We present an algorithm that, for the first time, achieves round synchronization with expected linear message complexity and expected constant latency. Existing protocols can use our round synchronization algorithm to solve Byzantine SMR with the same asymptotic performance.
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