Most popular blockchain solutions, like Bitcoin, rely on proof-of-work, guaranteeing that the output of the consensus is agreed upon with high probability. However, this probability depends on the delivery of messages and that the computational power of the system is sufficiently scattered among pools of nodes in the network so that no pool can mine more blocks faster than the crowd. New approaches, like Ethereum, generalise the proof-of-work approach by letting individuals deploy their own private blockchain with high transaction throughput. As companies are starting to deploy private chains, it has become crucial to better understand the guarantees blockchains offer in such a small and controlled environment. In this paper, we present the , an execution that we experienced when building our private chain at NICTA Data61. Even though this anomaly has never been acknowledged before, it may translate into dramatic consequences for the user of blockchains. Named after the infamous Paxos anomaly, this anomaly makes dependent transactions, like "Bob sends money to Carole after he received money from Alice" impossible. This anomaly relies on the fact that existing blockchains do not ensure consensus safety deterministically: there is no way for Bob to make sure that Alice actually sent him coins without Bob using an external mechanism, like converting these coins into a fiat currency that allows him to withdraw. We also explore smart contracts as a potential alternative to transactions in order to freeze coins, and show implementations of smart contract that can suffer from the Blockchain anomaly and others that may cope with it.
Another part of this research @cite_20 discusses the probabilistic guarantees of proof-of-work systems and the deterministic guarantees of Byzantine fault tolerance. The proof-of-work consensus is compared to Byzantine agreement protocols along two axes, scalability and performance, where proof-of-work consensus protocols are considered as scalable but inefficient while Byzantine agreement protocols are considered as efficient but not scalable. For example, Bitcoin scales beyond 1000 nodes while achieving a performance lower than 100 transactions per second with a high latency, whereas standard Byzantine fault tolerant protocols achieve more than 10,000 transactions per second but scale only to tens of nodes.
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While showing great promise, Bitcoin requires users to wait tens of minutes for transactions to commit, and even then, offering only probabilistic guarantees. This paper introduces ByzCoin, a novel Byzantine consensus protocol that leverages scalable collective signing to commit Bitcoin transactions irreversibly within seconds. ByzCoin achieves Byzantine consensus while preserving Bitcoin's open membership by dynamically forming hash power-proportionate consensus groups that represent recently-successful block miners. ByzCoin employs communication trees to optimize transaction commitment and verification under normal operation while guaranteeing safety and liveness under Byzantine faults, up to a near-optimal tolerance of f faulty group members among 3f + 2 total. ByzCoin mitigates double spending and selfish mining attacks by producing collectively signed transaction blocks within one minute of transaction submission. Tree-structured communication further reduces this latency to less than 30 seconds. Due to these optimizations, ByzCoin achieves a throughput higher than PayPal currently handles, with a confirmation latency of 15-20 seconds. The surging interest in blockchain technology has revitalized the search for effective Byzantine consensus schemes. In particular, the blockchain community has been looking for ways to effectively integrate traditional Byzantine fault-tolerant (BFT) protocols into a blockchain consensus layer allowing various financial institutions to securely agree on the order of transactions. However, existing BFT protocols can only scale to tens of nodes due to their @math message complexity. In this paper, we propose FastBFT, a fast and scalable BFT protocol. At the heart of FastBFT is a novel message aggregation technique that combines hardware-based trusted execution environments (TEEs) with lightweight secret sharing. Combining this technique with several other optimizations (i.e., optimistic execution, tree topology and failure detection), FastBFT achieves low latency and high throughput even for large scale networks. Via systematic analysis and experiments, we demonstrate that FastBFT has better scalability and performance than previous BFT protocols. Byzantine fault-tolerance is costly, both in terms of increased latency and limited scalability. Most recent contributions in the area have addressed the latency aspect, leaving the throughput scalability problem largely untouched. We propose in this short paper to build scalable Byzantine fault-tolerant storage systems based on the concept of mini-transactions. To achieve this goal we propose a novel atomic commit protocol that tolerates malicious clients and servers. A blockchain is a distributed ledger for recording transactions, maintained by many nodes without central authority through a distributed cryptographic protocol. All nodes validate the information to be appended to the blockchain, and a consensus protocol ensures that the nodes agree on a unique order in which entries are appended. Consensus protocols for tolerating Byzantine faults have received renewed attention because they also address blockchain systems. This work discusses the process of assessing and gaining confidence in the resilience of a consensus protocols exposed to faults and adversarial nodes. We advocate to follow the established practice in cryptography and computer security, relying on public reviews, detailed models, and formal proofs; the designers of several practical systems appear to be unaware of this. Moreover, we review the consensus protocols in some prominent permissioned blockchain platforms with respect to their fault models and resilience against attacks. The protocol comparison covers Hyperledger Fabric, Tendermint, Symbiont, R3 Corda, Iroha, Kadena, Chain, Quorum, MultiChain, Sawtooth Lake, Ripple, Stellar, and IOTA. A major approach to overcoming the performance and scalability limitations of current blockchain protocols is to use sharding which is to split the overheads of processing transactions among multiple, smaller groups of nodes. These groups work in parallel to maximize performance while requiring significantly smaller communication, computation, and storage per node, allowing the system to scale to large networks. However, existing sharding-based blockchain protocols still require a linear amount of communication (in the number of participants) per transaction, and hence, attain only partially the potential benefits of sharding. We show that this introduces a major bottleneck to the throughput and latency of these protocols. Aside from the limited scalability, these protocols achieve weak security guarantees due to either a small fault resiliency (e.g., 1 8 and 1 4) or high failure probability, or they rely on strong assumptions (e.g., trusted setup) that limit their applicability to mainstream payment systems. We propose RapidChain, the first sharding-based public blockchain protocol that is resilient to Byzantine faults from up to a 1 3 fraction of its participants, and achieves complete sharding of the communication, computation, and storage overhead of processing transactions without assuming any trusted setup. RapidChain employs an optimal intra-committee consensus algorithm that can achieve very high throughputs via block pipelining, a novel gossiping protocol for large blocks, and a provably-secure reconfiguration mechanism to ensure robustness. Using an efficient cross-shard transaction verification technique, our protocol avoids gossiping transactions to the entire network. Our empirical evaluations suggest that RapidChain can process (and confirm) more than 7,300 tx sec with an expected confirmation latency of roughly 8.7 seconds in a network of 4,000 nodes with an overwhelming time-to-failure of more than 4,500 years. A fault-scalable service can be configured to tolerate increasing numbers of faults without significant decreases in performance. The Query Update (Q U) protocol is a new tool that enables construction of fault-scalable Byzantine fault-tolerant services. The optimistic quorum-based nature of the Q U protocol allows it to provide better throughput and fault-scalability than replicated state machines using agreement-based protocols. A prototype service built using the Q U protocol outperforms the same service built using a popular replicated state machine implementation at all system sizes in experiments that permit an optimistic execution. Moreover, the performance of the Q U protocol decreases by only 36 as the number of Byzantine faults tolerated increases from one to five, whereas the performance of the replicated state machine decreases by 83 . The surprising success of cryptocurrencies has led to a surge of interest in deploying large scale, highly robust, Byzantine fault tolerant (BFT) protocols for mission-critical applications, such as financial transactions. Although the conventional wisdom is to build atop a (weakly) synchronous protocol such as PBFT (or a variation thereof), such protocols rely critically on network timing assumptions, and only guarantee liveness when the network behaves as expected. We argue these protocols are ill-suited for this deployment scenario. We present an alternative, HoneyBadgerBFT, the first practical asynchronous BFT protocol, which guarantees liveness without making any timing assumptions. We base our solution on a novel atomic broadcast protocol that achieves optimal asymptotic efficiency. We present an implementation and experimental results to show our system can achieve throughput of tens of thousands of transactions per second, and scales to over a hundred nodes on a wide area network. We even conduct BFT experiments over Tor, without needing to tune any parameters. Unlike the alternatives, HoneyBadgerBFT simply does not care about the underlying network. Inspired by the characteristics of biologically-motivated systems consisting of autonomous agents, we define the notion of stabilizing consensus in fully decentralized and highly dynamic ad hoc systems. Stabilizing consensus requires non-faulty nodes to eventually agree on one of their inputs, but individual nodes do not necessarily know when agreement is reached. First we show that, similar to the original consensus problem in the synchronous model, there exist deterministic solutions to the stabilizing consensus problem tolerating crash faults. Similarly, stabilizing consensus can also be solved deterministically in presence of Byzantine faults with the assumption that n > 3f where n is the number of nodes and f is the number of faulty nodes. Our main result is a Byzantine consensus protocol in a model in which the input to each node can change finitely many times during execution and eventually stabilizes. Finally we present an impossibility result for stabilizing consensus in systems of identical nodes. Cryptocurrencies, based on and led by Bitcoin, have shown promise as infrastructure for pseudonymous online payments, cheap remittance, trustless digital asset exchange, and smart contracts. However, Bitcoinderived blockchain protocols have inherent scalability limits that trade off between throughput and latency, which withhold the realization of this potential. This paper presents Bitcoin-NG (Next Generation), a new blockchain protocol designed to scale. Bitcoin-NG is a Byzantine fault tolerant blockchain protocol that is robust to extreme churn and shares the same trust model as Bitcoin. In addition to Bitcoin-NG, we introduce several novel metrics of interest in quantifying the security and efficiency of Bitcoin-like blockchain protocols. We implement Bitcoin-NG and perform large-scale experiments at 15 the size of the operational Bitcoin system, using unchanged clients of both protocols. These experiments demonstrate that Bitcoin-NG scales optimally, with bandwidth limited only by the capacity of the individual nodes and latency limited only by the propagation time of the network. Cryptocurrencies, based on and led by Bitcoin, have shown promise as infrastructure for pseudonymous online payments, cheap remittance, trustless digital asset exchange, and smart contracts. However, Bitcoin-derived blockchain protocols have inherent scalability limits that trade-off between throughput and latency and withhold the realization of this potential. This paper presents Bitcoin-NG, a new blockchain protocol designed to scale. Based on Bitcoin's blockchain protocol, Bitcoin-NG is Byzantine fault tolerant, is robust to extreme churn, and shares the same trust model obviating qualitative changes to the ecosystem. In addition to Bitcoin-NG, we introduce several novel metrics of interest in quantifying the security and efficiency of Bitcoin-like blockchain protocols. We implement Bitcoin-NG and perform large-scale experiments at 15 the size of the operational Bitcoin system, using unchanged clients of both protocols. These experiments demonstrate that Bitcoin-NG scales optimally, with bandwidth limited only by the capacity of the individual nodes and latency limited only by the propagation time of the network. This paper describes an architecture for secure and fault-tolerant service replication in an asynchronous network such as the Internet, where a malicious adversary may corrupt some servers and control the network. It relies on recent protocols for randomized Byzantine agreement and for atomic broadcast, which exploit concepts from threshold cryptography. The model and its assumptions are discussed in detail and compared to related work from the last decade in the first part of this work, and an overview of the broadcast protocols in the architecture is provided. The standard approach in fault-tolerant distributed systems is to assume that at most a certain fraction of servers fails. In the second part, novel general failure patterns and corresponding protocols are introduced. The allow for realistic modeling of real-world trust assumptions, beyond (weighted) threshold models. Finally, the application of our architecture to trusted services is discussed. In this paper, we show how to build synchronized clocks of arbitrary size atop of existing small-sized clocks, despite arbitrary faults. Our solution is both self-stabilizing and Byzantine fault-tolerant, and needs merely single-bit channels. It involves a reduction to Byzantine fault-tolerant consensus, which allows different consensus algorithms to be plugged in for matching the actual clock sizes and resilience requirements best. We demonstrate the practicability of our approach by means of an FPGA implementation and its experimental evaluation. To also address the cases where deterministic algorithms hit fundamental limits, we provide a novel randomized self-stabilizing Byzantine consensus algorithm that works very well also in these settings, along with its correctness proof and stabilization time analysis. Awareness of the need for robustness in distributed systems increases as distributed systems become integral parts of day-to-day systems. Self-stabilizing while tolerating ongoing Byzantine faults are wishful properties of a distributed system. Many distributed tasks (e.g. clock synchronization) possess efficient non-stabilizing solutions tolerating Byzantine faults or conversely non-Byzantine but self-stabilizing solutions. In contrast, designing algorithms that self-stabilize while at the same time tolerating an eventual fraction of permanent Byzantine failures present a special challenge due to the “ambition” of malicious nodes to hamper stabilization if the systems tries to recover from a corrupted state. This difficulty might be indicated by the remarkably few algorithms that are resilient to both fault models. We present the first scheme that takes a Byzantine distributed algorithm and produces its self-stabilizing Byzantine counterpart, while having a relatively low overhead of O(f′) communication rounds, where f′ is the number of actual faults. Our protocol is based on a tight Byzantine self-stabilizing pulse synchronization procedure. The synchronized pulses are used as events for initializing Byzantine agreement on every node’s local state. The set of local states is used for global predicate detection. Should the global state represent an illegal system state then the target algorithm is reset. Three decades ago, introduced the problem of Byzantine Agreement [PSL80] where nodes need to maintain a consistent view of the world in spite of the challenge posed by Byzantine faults. Subsequently, it is well known that Byzantine agreement over a completely connected synchronous network of n nodes tolerating up to t faults is (efficiently) possible if and only if t < n 3. further empowered the nodes with the ability to authenticate themselves and their messages and proved that agreement in this new model (popularly known as authenticated Byzantine agreement (ABA)) is possible if and only if t < n. (which is a huge improvement over the bound of t < n 3 in the absence of authentication for the same functionality). To understand the utility, potential and limitations of using authentication in distributed protocols for agreement, [GGBS10] studied ABA in new light. They generalize the existing models and thus, attempt to give a unified theory of agreements over the authenticated and non-authenticated domains. In this paper we extend their results to synchronous (undirected) networks and give a complete characterization of agreement protocols. As a corollary, we show that agreement can be strictly easier than all-pair point-to-point communication. It is well known that in a synchronous network over n nodes of which up to any t are corrupted by a Byzantine adversary, BA is possible only if all pair point-to-point reliable communication is possible [Dol82, DDWY93]. Thus, a folklore in the area is that maintaining global consistency (agreement) is at least as hard as the problem of all pair point-to-point communication. Equivalently, it is widely believed that protocols for BA over incomplete networks exist only if it is possible to simulate an overlay-ed complete network. Surprisingly, we show that the folklore is not always true. Thus, it seems that agreement protocols may be more fundamental to distributed computing than reliable communication. Byzantine agreement requires a set of parties in a distributed system to agree on a value even if some parties are maliciously misbehaving. A new protocol for Byzantine agreement in a completely asynchronous network is presented that makes use of new cryptographic protocols, specifically protocols for threshold signatures and coin-tossing. These cryptographic protocols have practical and provably secure implementations in the random oracle model. In particular, a coin-tossing protocol based on the Diffie-Hellman problem is presented and analyzed. The resulting asynchronous Byzantine agreement protocol is both practical and theoretically optimal because it tolerates the maximum number of corrupted parties, runs in constant expected rounds, has message and communication complexity close to the optimum, and uses a trusted dealer only once in a setup phase, after which it can process a virtually unlimited number of transactions. The protocol is formulated as a transaction processing service in a cryptographic security model, which differs from the standard information-theoretic formalization and may be of independent interest. This paper introduces a new leaderless Byzantine consensus called the Democratic Byzantine Fault Tolerance (DBFT) for blockchains. While most blockchain consensus protocols rely on a correct leader or coordinator to terminate, our algorithm can terminate even when its coordinator is faulty. The key idea is to allow processes to complete asynchronous rounds as soon as they receive a threshold of messages, instead of having to wait for a message from a coordinator that may be slow. The resulting decentralization is particularly appealing for blockchains for two reasons: (i) each node plays a similar role in the execution of the consensus, hence making the decision inherently “democratic” (ii) decentralization avoids bottlenecks by balancing the load, making the solution scalable. DBFT is deterministic, assumes partial synchrony, is resilience optimal, time optimal and does not need signatures. We first present a simple safe binary Byzantine consensus algorithm, modify it to ensure termination, and finally present an optimized reduction from multivalue consensus to binary consensus whose fast path terminates in 4 message delays. One of the most recent members of the Paxos family of protocols is Generalized Paxos. This variant of Paxos has the characteristic that it departs from the original specification of consensus, allowing for a weaker safety condition where different processes can have different views on a sequence being agreed upon. However, much like the original Paxos counterpart, Generalized Paxos does not have a simple implementation. Furthermore, with the recent practical adoption of Byzantine fault tolerant protocols, it is timely and important to understand how Generalized Paxos can be implemented in the Byzantine model. In this paper, we make two main contributions. First, we provide a description of Generalized Paxos that is easier to understand, based on a simpler specification and the pseudocode for a solution that can be readily implemented. Second, we extend the protocol to the Byzantine fault model. Self-stabilizing protocols can tolerate any type and any number of transient faults. However, in general, self-stabilizing protocols provide no guarantee about their behavior against permanent faults. This paper considers self-stabilizing link-coloring resilient to (permanent) Byzantine faults in arbitrary anonymous networks. First, we show that stabilizing link-coloring is impossible in anonymous cycles when there is no constraint on the spatial scheduling of processes. Then, given the assumption that no correct neighbors execute their actions simultaneously, we present a self-stabilizing link-coloring protocol that is also resilient to Byzantine faults in arbitrary anonymous networks. The protocol uses 2d-1 colors where d is the maximum degree in the network. This protocol guarantees that any link (u, v) between non faulty processes u and v is assigned a color within 2d+2 rounds and its color remains unchanged thereafter. Our protocol is Byzantine insensitive in the sense that the subsystem of correct processes remains operating properly in spite of unbounded Byzantine faults. Consider a network of @math n processes, where each process inputs a @math d-dimensional vector of reals. All processes can communicate directly with others via reliable FIFO channels. We discuss two problems. The multidimensional Byzantine consensus problem, for synchronous systems, requires processes to decide on a single @math d-dimensional vector @math v?Rd, inside the convex hull of @math d-dimensional vectors that were input by the non-faulty processes. Also, the multidimensional Byzantine approximate agreement (MBAA) problem, for asynchronous systems, requires processes to decide on multiple @math d-dimensional vectors in @math Rd, all within a fixed Euclidean distance @math ∈ of each other, and inside the convex hull of @math d-dimensional vectors that were input by the non-faulty processes. We obtain the following results for the problems above, while tolerating up to @math f Byzantine failures in systems with complete communication graphs: (1) In synchronous systems, @math n>max 3f,(d+1)f is necessary and sufficient to solve the multidimensional consensus problem. (2) In asynchronous systems, @math n>(d+2)f is necessary and sufficient to solve the multidimensional approximate agreement problem. Our sufficiency proofs are constructive, giving explicit protocols for the problems. In particular, for the MBAA problem, we give two protocols with strictly different properties and applications. By analyzing the states of knowledge that the processors attain in an unreliable system of a simple type, we capture some of the basic underlying structure of such systems. In particular, we study what facts become common knowledge at various points in the execution of protocols in an unreliable system. This characterizes the simultaneous actions that can be carried out in such systems. For example, we obtain a complete characterization of the number of rounds required to reach simultaneous Byzantine agreement, given the pattern in which failures occur. From this we derive a new protocol for this problem that is optimal in all runs, rather than just always matching the worst-case lower bound. In some cases this protocol attains simultaneous Byzantine agreement in as few as two rounds. We also present a nontrivial simultaneous agreement problem called bivalent agreement for which there is a protocol that always halts in two rounds. Our analysis applies to simultaneous actions in general, and not just to Byzantine agreement. The lower bound proofs presented here generalize and simplify the previously known proofs.