What a lovely hat

The COOL protocol of Chen (DISC'21) is a major advance that enables perfect security for various tasks (in particular, Byzantine Agreement in Synchrony and Reliable Broadcast in Asynchrony). For an input of size $L$ bits, its communication complexity is $O(nL+n^2 \log n)$, which is optimal up to a $\log n$ factor. Unfortunately, Chen’s analysis is rather intricate and complex. Our main contribution is a simple analysis of a new variant of COOL based on elementary counting arguments....

A Byzantine consensus protocol is essential in decentralized systems as the protocol ensures system consistency despite node failures. Research on consensus in wireless networks receives relatively less attention, while significant advancements in wired networks. However, consensus in wireless networks has equal significance as in wired networks. In this paper, we propose a new reliable broadcast protocol that can achieve reliability with high fault tolerance over than the SOTA (PODC...

\textit{Federated Learning} (FL) is a distributed machine learning paradigm that allows multiple clients to train models collaboratively without sharing local data. Numerous works have explored security and privacy protection in FL, as well as its integration with blockchain technology. However, existing FL works still face critical issues. \romannumeral1) It is difficult to achieving \textit{poisoning robustness} and \textit{data privacy} while ensuring high \textit{model accuracy}....

Understanding the fault tolerance of Byzantine Agreement protocols is an important question in distributed computing. While the setting of Byzantine faults has been thoroughly explored in the literature, the (arguably more realistic) omission fault setting is far less studied. In this paper, we revisit the recent work of Loss and Stern who gave the first protocol in the mixed fault model tolerating $t$ Byzantine faults, $s$ send faults, and $r$ receive faults, when $2t+r+s<n$ and omission...

Byzantine fault-tolerant (BFT) protocols are known to suffer from the scalability issue. Indeed, their performance degrades drastically as the number of replicas $n$ grows. While a long line of work has attempted to achieve the scalability goal, these works can only scale to roughly a hundred replicas. In this paper, we develop BFT protocols from the so-called committee sampling approach that selects a small committee for consensus and conveys the results to all replicas. Such an...

State-of-the-art asynchronous Byzantine Fault Tolerance (BFT) protocols integrate a partially-synchronous optimistic path. The holy grail in this paradigm is to match the performance of a partially-synchronous protocol in favorable situations and match the performance of a purely asynchronous protocol in unfavorable situations. Several prior works have made progress toward this goal by matching the efficiency of a partially-synchronous protocol in favorable conditions. However, their...

In recent years, decentralized computing has gained popularity in various domains such as decentralized learning, financial services and the Industrial Internet of Things. As identity privacy becomes increasingly important in the era of big data, safeguarding user identity privacy while ensuring the security of decentralized computing systems has become a critical challenge. To address this issue, we propose ADC (Anonymous Decentralized Computing) to achieve anonymity in decentralized...

Sharding enhances blockchain scalability by dividing the network into shards, each managing specific unspent transaction outputs or accounts. As an introduced new transaction type, cross-shard transactions pose a critical challenge to the security and efficiency of sharding blockchains. Currently, there is a lack of a generic sharding blockchain consensus pattern that achieves both security and low overhead. In this paper, we present Kronos, a secure sharding blockchain consensus...

Shared randomness in blockchain can expand its support for randomized applications and can also help strengthen its security. Many existing blockchains rely on external randomness beacons for shared randomness, but this approach reduces fault tolerance, increases latency, and complicates application development. An alternate approach is to let the blockchain validators generate fresh shared randomness themselves once for every block. We refer to such a design as the \emph{on-chain}...

To improve the throughput of Byzantine Fault Tolerance (BFT) consensus protocols, the Directed Acyclic Graph (DAG) topology has been introduced to parallel data processing, leading to the development of DAG-based BFT consensus. However, existing DAG-based works heavily rely on Reliable Broadcast (RBC) protocols for block broadcasting, which introduces significant latency due to the three communication steps involved in each RBC. For instance, DAGRider, a representative DAG-based protocol,...

The conventional Byzantine fault tolerance (BFT) paradigm requires replicated state machines to execute deterministic operations only. In practice, numerous applications and scenarios, especially in the era of blockchains, contain various sources of non-determinism. Despite decades of research on BFT, we still lack an efficient and easy-to-deploy solution for BFT with non-determinism—BFT-ND, especially in the asynchronous setting. We revisit the problem of BFT-ND and provide a formal and...

Although having been popular for a long time, Byzantine Fault Tolerance (BFT) consensus under the partially-synchronous network is denounced to be inefficient or even infeasible in recent years, which calls for a more robust asynchronous consensus. On the other hand, almost all the existing asynchronous consensus are too complicated to understand and even suffer from the termination problem. Motivated by the above problems, we propose SimpleFT in this paper, which is a simple asynchronous...

This paper examines the vulnerabilities inherent in prevailing Public Key Infrastructure (PKI) systems reliant on centralized Certificate Authorities (CAs), wherein a compromise of the CA introduces risks to the integrity of public key management. We present PKChain, a decentralized and compromise-tolerant public key management system built on blockchain technology, offering transparent, tamper-resistant, and verifiable services for key operations such as registration, update, query,...

Regular access to unpredictable and bias-resistant randomness is important for applications such as blockchains, voting, and secure distributed computing. Distributed random beacon protocols address this need by distributing trust across multiple nodes, with the majority of them assumed to be honest. Numerous applications across the blockchain space have led to the proposal of several distributed random beacon protocols, with some already implemented. However, many current random beacon...

Central Bank Digital Currencies refer to the digitization of lifecycle's of central bank money in a way that meets first of a kind requirements for transparency in transaction processing, interoperability with legacy or new world, and resilience that goes beyond the traditional crash fault tolerant model. This comes in addition to legacy system requirements for privacy and regulation compliance, that may differ from central bank to central bank. This paper introduces a novel framework for...

The Byzantine Fault Tolerance (BFT) protocol is a long-standing topic. Recently, a lot of efforts have been made in the research of asynchronous BFT. However, the existing solutions cannot adapt well to the flexible network environment, and suffer from problems such as high communication complexity or long latency. To improve the efficiency of BFT consensus in flexible networks, we propose FaBFT. FaBFT's clients can make their own assumptions about the network conditions, and make the most...

Byzantine atomic broadcast (ABC) is at the heart of permissioned blockchains and various multi-party computation protocols. We resolve a long-standing open problem in ABC, presenting the first information-theoretic (IT) and signature-free asynchronous ABC protocol that achieves optimal $O(n^2)$ messages and $O(1)$ expected time. Our ABC protocol adopts a new design, relying on a reduction from---perhaps surprisingly---a somewhat neglected primitive called multivalued Byzantine agreement (MBA).

Byzantine fault-tolerant state machine replication (BFT-SMR) replicates a state machine across a set of replicas, and processes requests as a single machine even in the presence of Byzantine faults. Recently, synchronous BFT-SMRs have received tremendous attention due to their simple design and high fault-tolerance threshold. In this paper, we propose Arena, the first multi-leader synchronous BFT-SMR. Thanks to the synchrony assumption, Arena gains the performance benefit from...

To reduce latency and communication overhead of asynchronous Byzantine Fault Tolerance (BFT) consensus, an optimistic path is often added, with Ditto and BDT as state-of-the-art representatives. These protocols first attempt to run an optimistic path that is typically adapted from partially-synchronous BFT and promises good performance in good situations. If the optimistic path fails to make progress, these protocols switch to a pessimistic path after a timeout, to guarantee liveness in an...

Due to the significant development of the intelligence industry worldwide, various initiatives have increasingly recognized the value of the Internet of Things (IoT). IoT systems, however, are often hin- dered by fundamental challenges, such as the need for a central server to manage them. Decentralizing these systems can be achieved through the use of blockchains. Recently, there has been an increase in the popularity of blockchain in various fields, such as banking, IoT, and the...

Asynchronous common subset (ACS) is a powerful paradigm enabling applications such as Byzantine fault-tolerance (BFT) and multi-party computation (MPC). The most efficient ACS framework in the information-theoretic setting is due to Ben-Or, Kelmer, and Rabin (BKR, 1994). The BKR ACS protocol has been both theoretically and practically impactful. However, the BKR protocol has an $O(\log n)$ running time (where $n$ is the number of replicas) due to the usage of $n$ parallel asynchronous...

Asynchronous Byzantine fault-tolerance (BFT) protocols (e.g., HoneyBadger and Dumbo family protocols) have received increasing attention as the consensus mechanism of permissioned blockchains, given their particular robustness against timing and performance attacks. However, there is a substantial performance gap before they can be applied in real systems. In this paper, we identify and address two critical issues, and design Dory, an asynchronous BFT consensus protocol with improved...

Bitcoin's longest-chain protocol pioneered consensus under dynamic participation, also known as sleepy consensus, where nodes do not need to be permanently active. However, existing solutions for sleepy consensus still face two major issues, which we address in this work. First, existing sleepy consensus protocols have high latency (either asymptotically or concretely). We tackle this problem and achieve $4\Delta$ latency ($\Delta$ is the bound on network delay) in the best case, which is...

We provide an expressive framework that allows analyzing and generating provably secure, state-of-the-art Byzantine fault-tolerant (BFT) protocols. Our framework is hierarchical, including three layers. The top layer is used to model the message pattern and abstract key functions on which BFT algorithms can be built. The intermediate layer provides the core functions with high-level properties sufficient to prove the security of the top-layer algorithms. The bottom layer carefully defines...

State-of-the-art Byzantine fault-tolerant (BFT) protocols assuming partial synchrony such as SBFT and HotStuff use \textit{regular certificates} obtained from $2f+1$ (partial) signatures. We show that one can use \textit{weak certificates} obtained from only $f+1$ signatures to \textit{assist} in designing more robust and more efficient BFT protocols. We design and implement two BFT systems: Dashing (a family of two HotStuff-style BFT protocols) and Star (a parallel BFT framework). We...

This paper studies dynamic BFT, where replicas can join and leave the system dynamically, a primitive that is nowadays increasingly needed. We provide a formal treatment for dynamic BFT protocols, endowing them with a flexible syntax and various security definitions. We demonstrate the challenges of extending static BFT to dynamic BFT. Then we design and implement Dyno, a highly efficient dynamic BFT protocol under the partial synchrony model. We show that Dyno can seamlessly handle...

As the first Byzantine fault-tolerant (BFT) protocol with linear communication complexity, HotStuff (PODC 2019) has received significant attention. HotStuff has three round-trips for both normal case operations and view change protocols. Follow-up studies attempt to reduce the number of phases for HotStuff. These protocols, however, all give up of one thing in return for another. This paper presents Marlin, a BFT protocol with linearity, having two phases for normal case operations and two...

Asynchronous BFT consensus can implement robust mission-critical decentralized services in the unstable or even adversarial wide-area network without relying on any form of timing assumption. Starting from the work of HoneyBadgerBFT (CCS 2016), several studies tried to push asynchronous BFT towards practice. In particular, in a recent work of Dumbo (CCS 2020), they redesigned the protocol backbone and used one multi-valued validated Byzantine agreement (MVBA) to replace $n$ concurrent...

The classic asynchronous Byzantine fault tolerance (BFT) framework of Ben-Or, Kemler, and Rabin (BKR) and its descendants rely on reliable broadcast (RBC) and asynchronous binary agreement (ABA). However, BKR does not allow all ABA instances to run in parallel, a well-known performance bottleneck. We propose PACE, a generic framework that removes the bottleneck, allowing fully parallelizable ABA instances. PACE is built on RBC and reproposable ABA (RABA). Different from the conventional...

Blockchain as a potentially disruptive technology can advance many different fields, e.g., cryptocurrencies, supply chains, and the industrial Internet of Things. The next-generation blockchain ecosystem is expected to consist of various homogeneous and heterogeneous distributed ledgers. These ledger systems will inevitably require a certain level of proper cooperation of multiple blockchains to enrich advanced functionalities and enhance interoperable capabilities for future applications....

The problem of Byzantine Fault Tolerance (BFT) has received a lot of attention in the last 30 years. The seminal work by Fisher, Lynch, and Paterson (FLP) shows that there does not exist a deterministic BFT protocol in complete asynchronous networks against a single failure. In order to address this challenge, researchers have designed randomized BFT protocols in asynchronous networks and deterministic BFT protocols in partial synchronous networks. For both kinds of protocols, a basic...

Though recent breakthroughs greatly improved the efficiency of asynchronous Byzantine agreement (BA) protocols, they mainly focused on the setting with private setups, e.g., assuming established non-interactive threshold cryptosystems. Challenges remain to reduce the large communication complexities in the absence of such setups. For example, Abraham et al. (PODC'21) recently gave the first private-setup free construction for asynchronous validated BA (VBA) with expected $\mathcal{O}(n^3)$...

Classic Byzantine fault tolerant (BFT) protocols are designed for a specific timing model, most often one of the following: synchronous, asynchronous or partially synchronous. It is well known that the timing model and fault tolerance threshold present inherent trade-offs. Synchronous protocols tolerate up to $n/2$ Byzantine faults, while asynchronous or partially synchronous protocols tolerate only up to $n/3$ Byzantine faults. In this work, we generalize the fault thresholds of BFT and...

BFT protocols in the synchronous setting rely on a strong assumption: every message sent by a party will arrive at its destination within a known bounded time. To allow some degree of asynchrony while still tolerating a minority corruption, recently, in Crypto'19, a weaker synchrony assumption called mobile sluggish faults was introduced. In this work, we investigate the support for mobile sluggish faults in existing synchronous protocols such as Dfinity, Streamlet, Sync HotStuff, OptSync...

A great challenge for distributed payment systems is their compliance with regulations, such as anti-money laundering, insolvency legislation, countering the financing of terrorism and sanctions laws. After Bitcoin's MtGox scandal, one of the most needed auditing functionalities for financial solvency and tax reporting purposes is to prove ownership of blockchain reserves, a process known as Proof of Assets (PoA). This work formalizes the PoA requirements in account-based blockchains,...

Verifiable Secret-Sharing (VSS) is a fundamental primitive in secure distributed computing. It is used as an important building block in several distributed computing tasks, such as Byzantine agreement and secure multi-party computation. VSS has been widely studied in various dimensions over the last three decades and several important results have been achieved related to the fault-tolerance, round-complexity and communication efficiency of VSS schemes. In this article, we consider VSS...

Blockchain has been practiced in crypto-currencies and crossborder banking settlement. However, no clear evidence that a distributed ledger network (or Blockchain) is built within domestic payment systems, although many experts believe that Blockchain has wide applicability in various industries and disciplines. As the author’s best knowledge, no one has published a clear architecture and a feasible framework for a Blockchain-based banking network. Thus, \how Blockchain can be implemented in...

Agreement protocols for partially synchronous or asynchronous networks tolerate fewer than one-third Byzantine faults. If parties are equipped with trusted hardware that prevents equivocation, then fault tolerance can be improved to fewer than one-half Byzantine faults, but typically at the cost of increased communication complexity. In this work, we present results that use small trusted hardware without worsening communication complexity assuming the adversary controls a fraction of the...

Random beacon protocols provide a continuous public source of randomness and their applications range from public lotteries to zero-knowledge proofs. Existing random beacon protocols in the bounded synchronous model sacrifice either the fault tolerance or the communication complexity for security, or ease of reconfigurability. This work overcomes the challenges with the existing works through a novel communication efficient combination of state machine replication and (publicly) verifiable...

We consider leader-based Byzantine state machine replication, a.k.a. "BFT", under partial synchrony. We provide a generic solution enabling to match simultaneously, for the first time, three arguably gold standards of BFT: in two phases, with a responsive view change and a linear complexity per view. It is based on a new threshold primitive, which we call Proofs of non-Supermajority (or PnS for short). A PnS system enables players, each with an input number, to report their input to a...

Most state machine replication protocols are either based on the 40-years-old Byzantine Fault Tolerance (BFT) theory or the more recent Nakamoto’s longest chain design. Longest chain protocols, designed originally in the Proof-of-Work (PoW) setting, are available under dynamic participation, but has probabilistic confirmation with long latency dependent on the security parameter. BFT protocols, designed for the permissioned setting, has fast deterministic confirmation, but assume a fixed...

The specific order of commands agreed upon when running state machine replication (SMR) is immaterial to fault-tolerance: all that is required is for all correct deterministic replicas to follow it. In the permissioned blockchains that rely on Byzantine fault tolerant (BFT) SMR, however, nodes have a stake in the specific sequence that ledger records, as well as in preventing other parties from manipulating the sequencing to their advantage. The traditional specification of SMR correctness,...

The advent of decentralized trading markets introduces a number of new challenges for consensus protocols. In addition to the 'usual' attacks - a subset of the validators trying to prevent disagreement -- there is now the possibility of financial fraud, which can abuse properties not normally considered critical in consensus protocols. We investigate the issues of attackers manipulating or exploiting the order in which transactions are scheduled in the blockchain. More concretely, we look...

Security and Scalability are two major challenges that IoT is currently facing. Access control to critical IoT infrastructure is considered as top security challenge that IoT faces. Data generated by IoT devices may be driving many hard real time systems, thus it is of utmost importance to guarantee integrity and authenticity of the data and resources at the first place itself. Due to heterogeneous and constrained nature of IoT devices, traditional IoT security frameworks are not able to...

Ethereum Research team has proposed a family of Casper blockchain consensus protocols. It has been shown in the literature that the Casper Friendly Finality Gadget (Casper FFG) cannot achieve liveness property in partially synchronous networks such as the Internet environment. The ``Correct-by-Construction'' family of Casper blockchain consensus protocols (CBC Casper) has been proposed as a finality gadget for the future Proof-of-Stake (PoS) based Ethereum blockchain. Unfortunately, no...

We present a novel framework for asynchronous permissioned blockchain with high performance and post-quantum security for the first time. Specifically, our framework contains two asynchronous Byzantine fault tolerance (aBFT) protocols SodsBC and SodsBC++. We leverage concurrently preprocessing to accelerate the preparation of three cryptographic objects for the repeated consensus procedure, including common random coins as the needed randomness, secret shares of symmetric encryption keys for...

The problem of Byzantine Fault Tolerance (BFT) in partial synchronous networks has received a lot of attention in the last 30 years. There are two types of widely accepted definitions for partial synchronous networks. This paper shows that several widely deployed BFT protocols would reach deadlocks in the widely accepted Type II partial synchronous networks (that is, they will not achieve liveness property). Based on the analysis of BFT security requirements for partial synchronous networks,...

Metering is a critical process in large-scale distributed industrial plants, which enables multiple plants to collaborate to offer mutual services without outside interference. When distributed plants measure the data from a shared common source, e.g., flow metering in an oil pipeline, trustworthiness and immutability must be guaranteed among them. In this paper, we propose a hierarchical and scalable blockchain-based secure metering system, \textit{SMChain}, to provide strong security,...

We review several solutions for the Byzantine Fault Tolerance (BFT) problem and discuss some aspects that are frequently overlooked by existing literatures. For example, PBFT and HotStuff BFT protocols (HotStuff has been adopted by Facebook Libra) require a reliable broadcast primitive. We show that if the broadcast primitive is not reliable then the PBFT and HotStuff BFT protocols could not achieve the liveness property (that is, the system will never reach an agreement on a proposal)....

Synchronous solutions for Byzantine Fault Tolerance (BFT) can tolerate up to minority faults. In this work, we present Sync HotStuff, a surprisingly simple and intuitive synchronous BFT solution that achieves consensus with a latency of $2\Delta$ in the steady state (where $\Delta$ is a synchronous message delay upper bound). In addition, Sync HotStuff ensures safety in a weaker synchronous model in which the synchrony assumption does not have to hold for all replicas all the time. Moreover,...

The importance of efficient MPC in today's world needs no retelling. An obvious barebones requirement to execute protocols for MPC is the ability of parties to communicate with each other. Traditionally, we solve this problem by assuming that every pair of parties in the network share a dedicated secure link that enables reliable message transmission. This assumption is clearly impractical as the number of nodes in the network grows, as it has today. In their seminal work, Dwork, Peleg,...

A blockchain system is a replicated state machine that must be fault tolerant. When designing a blockchain system, there is usually a trade-off between decentralization, scalability, and security. In this paper, we propose a novel blockchain system, DEXON, which achieves high scalability while remaining decentralized and robust in the real-world environment. We have two main contributions. First, we present a highly scalable sharding framework for blockchain. This framework takes an...

Most classical consensus protocols rely on a leader to coordinate nodes’ voting efforts. One novel idea that stems from blockchain-style consensus is to rely, instead, on a “longest-chain” idea for such coordination. Such a longest-chain idea was initially considered in randomized protocols, where in each round, a node has some probability of being elected a leader who can propose the next block. Recently, well-known systems have started implementing the deterministic counterpart of such...

In state-of-the-art e-voting systems, a bulletin board (BB) is a critical component for preserving election integrity and availability. Although it is common in the literature to assume that a BB is a centralized entity that is trusted, in the recent works of Culnane and Schneider [CSF 2014] and Chondros et al. [ICDCS 2016], the importance of removing BB as a single point of failure has been extensively discussed. Motivated by these works, we introduce a framework for the formal security...

Bitcoin and blockchain technologies have proven to be a phenomenal success. The underlying techniques hold huge promise to change the future of financial transactions, and eventually the way people and companies compute, collaborate, and interact. At the same time, the current Bitcoin-like proof-of-work based blockchain systems are facing many challenges. For example, a huge amount of energy/electricity is needed for maintaining the Bitcoin blockchain. We propose a new approach to...

Bitcoin and its underlying blockchain mechanism have been attracting much attention. One of their core innovations, Proof-of-Work (PoW), is notoriously inefficient which potentially motivates a centralization of computing power, defeating the original goal of decentralization. Proof-of-Stake (PoS) is later proposed to replace PoW. However, both PoW and PoS have different inherent advantages and disadvantages, so does Proof-of-Activity (PoA) of Bentov et al. (SIGMETRICS 2014) which only...

We present new protocols for Byzantine state machine replication and Byzantine agreement in the synchronous and authenticated setting. The celebrated PBFT state machine replication protocol tolerates f Byzantine faults in an asynchronous setting using 3f +1 replicas, and has since been studied or deployed by numerous works. In this work, we improve the Byzantine fault tolerance threshold to n = 2f + 1 by utilizing a relaxed synchrony assumption. We present a synchronous state machine...

Peer-to-peer (P2P) systems such as BitTorrent and Bitcoin are susceptible to serious attacks from byzantine nodes that join as peers. Research has explored many adversarial models with additional assumptions, ranging from mild (such as pre-established PKI) to strong (such as the existence of common random coins). One such widely-studied model is the general-omission model, which yields simple protocols with good efficiency, but has been considered impractical or unrealizable since it...

Secure multi-party computation (MPC) allows multiple parties to compute a known function over inputs held by each party, without any party having to reveal its private input. Unfortunately, traditional MPC algorithms do not scale well to large numbers of parties. In this paper, we describe several recent MPC algorithms that are designed to handle large networks. All of these algorithms rely on recent techniques from the Byzantine agreement literature on forming and using quorums. Informally,...

In this paper we present first ever error-free, asynchronous broadcast (called as A-cast) and Byzantine Agreement (called as ABA) protocols with optimal communication complexity and fault tolerance. Our protocols are multi-valued, meaning that they deal with $\ell$ bit input and achieve communication complexity of $O(n\ell)$ bits for large enough $\ell$ for a set of $n \geq 3t+1$ parties in which at most t can be Byzantine corrupted. In synchronous settings, Fitzi and Hirt (PODC'06) ...

Pease {\em et al.}\/ introduced the problem of Byzantine Generals (BGP) to study the effects of Byzantine faults in distributed protocols for reliable broadcast. It is well known that BGP among $n$ players tolerating up to $t$ faults is (efficiently) possible if and only if $n > 3t$. To overcome this severe limitation, Pease {\em et al.} introduced a variant of BGP, \emph{Authenticated Byzantine General} (ABG). Here players are supplemented with digital signatures (or similar tools) to...

In this work we focus on two basic secure distributed computation tasks- Probabilistic Weak Secret Sharing (PWSS) and Probabilistic Verifiable Secret Sharing (PVSS). PVSS allows a dealer to share a secret among several players in a way that would later allow a unique reconstruction of the secret with negligible error probability. PWSS is slightly weaker version of PVSS where the dealer can choose not to disclose his secret later. Both of them are well-studied problems. While PVSS is used as...