Igor Machado Coelho, Vitor N. Coelho, Rodolfo Pereira Araújo, Wang Yong Qiang · 5 authors
Consensus mechanisms are a core feature for handling negotiation and agreements. Blockchain technology has seen the introduction of different sorts of consensus mechanism, ranging from tasks of heavy computation to the subtle mathematical proofs of Byzantine agreements. This paper presents the pioneer Delegated Byzantine Fault Tolerance (dBFT) protocol of Neo Blockchain, which was inspired by the Practical Byzantine Fault Tolerance (PBFT). Besides introducing its history, this study describes proofs and didactic examples, as well as novel design and extensions for Neo dBFT with multiple block proposals. Finally, we discuss challenges when dealing with strong Byzantine adversaries, and propose solutions inspired on PBFT for current weak-synchrony problems and increasing system robustness against attacks. Key Contribution: Presents an overview of the history of PBFT-inspired consensus for blockchain, highlighting its current importance on the literature, challenges and assumptions. Contributes to the field of Distributed Consensus, proposing novel extensions for the Neo dBFT (dBFT 2.0+, dBFT 3.0 and dBFT 3.0+), with new insights on innovative consensus mechanisms.
Tiantian Gong, Mohsen Minaei, Wenhai Sun, Aniket Kate
A fixed block reward and voluntary transaction fees are two sources of economic incentives for mining in Bitcoin and other cryptocurrencies. For Bitcoin, the block reward halves every 210,000 blocks and it is supposed to vanish gradually. The remaining incentive of transaction fees is optional and arbitrary, and an undercutting attack becomes a potential threat, where the attacker deliberately forks an existing chain by leaving wealthy transactions unclaimed to attract other miners. We look into the profitability of the undercutting attack in this work.
Our numerical simulations and experiments demonstrate that (i) only miners with mining power > 40% have a reasonable probability of successfully undercutting. (ii) As honest miners do not shift to the fork immediately in the first round, an undercutter's profit drops with the number of honest miners. Given the current transaction fee rate distribution in Bitcoin, with half of the miners being honest, undercutting cannot be profitable at all; With 25% honest mining power, an undercutter with > 45% mining power can expect income more than its fair share; With no honest miners present, the threshold mining power for a profitable undercutting is 42%. (iii) For the current largest Bitcoin mining pool with 17.2% mining power, the probability of successfully launching an undercutting attack is tiny and the expected returns are far below honest mining gains. (iv) While the larger the prize the undercutter left unclaimed, the higher is the probability of the attack succeeding but the attack's profits also go down. Finally, we analyze the best responses to undercutting for other rational miners. (v) For two rational miners and one of them being the potential undercutter with 45% mining power, we find the dominant strategy for the responding rational miner is to typical rational.
Duncan Thomson, Steven Bodie, David E. Bryson, Timothy S. Luc · 5 authors
This paper describes a concept and a prototype for sharing flight information using blockchain technology. Providing all stakeholders access to complete, consistent, and up-to-date information about each flight facilitates efficient aviation operations. Existing flight information exchange methods are limited; the concept of a “flight object” that provides a complete solution for all stakeholders has yet to be realized. A complete solution requires either a centrally administered data store, which is unsuitable for an international context, or a distributed ledger. Distributed ledgers are at the heart of blockchain technology, which makes this technology a good match for implementing the flight object. The authors have proven it is possible to provide a complete flight object solution using blockchain technology by demonstrating a prototype based on Tendermint: an open-source blockchain implementation. The demonstrated solution offers additional benefits like strong integrity guarantees and role-based update permissions enforcement. Prototype performance indicates a production implementation is likely to provide sufficient speed and capacity to support global aviation operations for flight planning negotiation, as well as possible future concepts such as trajectory-based operations and international flow management. The paper’s conclusion discusses steps necessary for the flight object sharing capability to be adopted as the basis for a real-world international aviation solution.
Blockchain protocols differ in fundamental ways, including the mechanics of selecting users to produce blocks (e.g., proof-of-work vs. proof-of-stake) and the method to establish consensus (e.g., longest chain rules vs. BFT-inspired protocols). These fundamental differences have hindered "apples-to-apples" comparisons between different categories of blockchain protocols and, in turn, the development of theory to formally discuss their relative merits. This paper presents a parsimonious abstraction sufficient for capturing and comparing properties of many well-known permissionless blockchain protocols, simultaneously capturing essential properties of both proof-of-work and proof-of-stake protocols, and of both longest-chain-type and BFT-type protocols. Our framework blackboxes the precise mechanics of the user selection process, allowing us to isolate the properties of the selection process which are significant for protocol design. We illustrate our framework's utility with two results. First, we prove an analog of the CAP theorem from distributed computing for our framework in a partially synchronous setting. This theorem shows that a fundamental dichotomy holds between protocols (such as Bitcoin) that are adaptive, in the sense that they can function given unpredictable levels of participation, and protocols (such as Algorand) that have certain finality properties. Second, we formalize the idea that proof-of-work (PoW) protocols and non-PoW protocols can be distinguished by the forms of permission that users are given to carry out updates to the state.
Blockchain Technology is exclusively used to make online transactions secure by maintaining a distributed and decentralized ledger of records across multiple computers. Tendermint is a general-purpose blockchain engine that is composed of two parts; Tendermint Core and the blockchain application interface. The application interface makes Tendermint suitable for a wide range of applications. In this paper, we analyze and improve Practical Byzantine Fault Tolerant (PBFT), a consensus-based Tendermint blockchain algorithm. In order to avoid negative issues of locks, we first propose a lock-free algorithm for blockchain in which the proposal and voting phases are concurrent whereas the commit phase is sequential. This consideration in the algorithm allows parallelism. Secondly, a new methodology is used to decide the size of the voter set which is a subset of blockchain nodes, further investigating the block sensitivity and trustworthiness of nodes. Thirdly, to fairly select the voter set nodes, we employ the random walk algorithm. Fourthly, we imply the wait-freedom property by using a timeout due to which all blocks are eventually committed or aborted. In addition, we have discussed voting conflicts and consensuses issues that are used as a correctness property, and provide some supportive techniques.
SHA256 algorithm was one of the reasons people thought that led to professional mining machines and mining pools, since this algorithm could be easily computed. The success of Litecoin induced a variety of algorithmic innovations. Between 2012 and 2014, algorithmic innovation has always been a hot topic in community discussions, and every currency that uses innovative algorithms can make a new wave. Ethereum’s Proof-of-Work (PoW) algorithm is called Ethash, which is slightly different from Bitcoin’s PoW algorithm, which makes it possible to mine with ordinary hardware. Tendermint consists of two main technologies: the blockchain consensus engine and a common application interface. One of the most important parts of Casper’s proof-of-stake (PoS) consensus is checkpoints. The chapter discusses the three main PoS protocols that use PoS in cryptocurrencies: Casper the Friendly Ghost (CTFG), Casper the Friendly Finality Gadget and Tendermint. CTFG is a correct construction consensus agreement designed to combat the real environment of oligopoly.
Lăcrămioara Aştefănoaei, Pierre Chambart, Antonella Del Pozzo, Thibault Rieutord · 6 authors
First-generation blockchains provide probabilistic finality: a block can be revoked, albeit the probability decreases as the block "sinks" deeper into the chain. Recent proposals revisited committee-based BFT consensus to provide deterministic finality: as soon as a block is validated, it is never revoked. A distinguishing characteristic of these second-generation blockchains over classical BFT protocols is that committees change over time as the participation and the blockchain state evolve. In this paper, we push forward in this direction by proposing a formalization of the Dynamic Repeated Consensus problem and by providing generic procedures to solve it in the context of blockchains. Our approach is modular in that one can plug in different synchronizers and single-shot consensus. To offer a complete solution, we provide a concrete instantiation, called {{Tenderbake}}, and present a blockchain synchronizer and a single-shot consensus algorithm, working in a Byzantine and partially synchronous system model with eventually synchronous clocks. In contrast to recent proposals, our methodology is driven by the need to bound the message buffers. This is essential in preventing spamming and run-time memory errors. Moreover, {{Tenderbake}} processes can synchronize with each other without exchanging messages, leveraging instead the information stored in the blockchain.
Amani Altarawneh, Tom Herschberg, Sai Medury, Farah Kandah · 5 authors
Consensus algorithms in distributed systems have attracted much attention in recent studies. However, there is a need for a classification that leads to better understanding and helps direct the deployment of such algorithms. In this paper, we classify common consensus algorithms based on how they decide the order of system state changes. We then determine the extent to which each category prioritizes scalability, decentralization, and security. As with other engineering design scenarios, this is a choose-two tradeoff. Our key contribution is that, based on this taxonomy of tradeoffs, we are able to discern the types of consensus algorithms that work well within the application area(s) for a given distributed system. We find that a dichotomy of algorithms between leader-based and voting-based consensus algorithms emerges from this taxonomy. Applications of this classification scheme include several different categories of distributed ledgers such as blockchains and directed acyclic graphs (DAGs).
In 2008, bitcoin disrupted the transactional ecosystem with its value propositions. However, sustaining autonomous, deregulated systems in markets filled with laws and regulations has had its challenges. There are also open questions of scalability, resilience, availability, speed and finality of transactions. Sound technical components have emerged but we need to take this one step further and integrate the units to provide an accepted packaged solution. Using a game application as an entry point, we propose a simple, inclusive asset transfer system which will stimulate adoption and create traction in the distributed ledger universe.
Mahbub Majumdar, Mobashir Monim, Mohammad Muhtasim Shahriyer
Land Registry documents are legally binding documents provided by the government to owners of land as proof of their ownership. In developing countries such as Bangladesh, it is essential to combat the challenges which a traditional Land Registry system faces. To keep a digital ledger of information about land assets in a transparent and secured manner, Blockchain technology can be used to overcome the hurdle. However, any Blockchain based system needs to be integrated in a well thought out manner. We propose a novel approach on enhancing the Delegated Proof of Stake consensus to provide a private ledger based system for transacting land assets which can be easily integrated into the existing traditional Land Registry system for smooth operation.
The great increase of the Bitcoin crypto-currency value these last months has brought blockchain and distributed ledger technologies back on the front stage. However, it also gives incentive to malicious users who would like to break the system or take advantage of it (steal money, hide some information stored in the ledger, isolate a particular node from the rest of the network, etc.). Thus, our research focuses on overcoming potential attacks to distributed ledgers. In this paper, we focus on attacks that damage underlying networks of distributed ledgers. Underlying networks offer useful communication primitives such as an atomic broadcast, however such attacks may degrade the property of the primitives and make distributed ledgers relying on the primitives no longer work. Hence we should design algorithms to make the distributed ledgers still work even when some attacks degrade the primitives. As the first study of this problem, we consider distributed ledgers that rely on an atomic broadcast to maintain consistency. We define different cases of attacks affecting the uniform agreement property of the atomic broadcast: We first assume that a single server can fail to deliver a particular message and then we assume that several servers can fail to deliver the message. Afterwards, we present an algorithm to recover the states of the servers that fail to deliver some messages, and consider the situation where the maximum of servers which keep a wrong sequence is known and bounded.
In recent years, with hugh success of cryptocurrencies, blockchain has drawn attentions excessively. The consensus problem, as the key mechanism of blockchain, leads to a wide research. However, existing consensuses have difficulty being efficient and understandable at the same time. Therefore, we propose an understandable consensus protocol with high performance. It provides understandability by dividing consensus into log replication, commit confirmation and leader election, and it introduces digital signature and nested hash to survive byzantine failures. Our experimental results demonstrate that the proposed method shares similar performance and scalability with Raft.
Blockchain is a distributed system with decentralized, tamper-proof, traceability and other characteristics. The primary function of a distributed system is to enable consensus among nodes without trust. It is especially important to choose the appropriate consensus mechanism to improve the performance of the blockchain. Based on the original Proof of Stake (PoS), this paper introduces behavioral credits, establishes credit ratings, and interacts with the currency age in the PoS mechanism. By this way, it makes the revenue distribution of the nodes participating in the generation of the PoS mechanism more fair and reasonable, improves social stratification and resists the trend of system centralization.
Distributed Ledger Technology promises to solve multiple problems emergent of existing IT solutions and business processes on the intersection of trust, privacy, security, automation, authentication and authorization. However, multiple obstacles restrict the potential of the technology, specifically production-related challenges in the enterprise-centric ecosystem. The paper provides a holistic view of the top production readiness challenges of the decentralized application (dApp) for enterprise along with alternative solutions. We propose a novel approach to identify challenges and rank them. The identified challenges and possible solutions provide a baseline assessment. We present a novel, reusable, and extensible framework to help the system engineering teams to assess the challenges towards production readiness. We expect this study to benefit the DLT community to enhance the understanding of challenges and potential research areas for contribution.
This paper presents a new consensus protocol based on verifiable delay function. First, we introduce the concept of verifiable delay puzzle (VDP), which resembles the hashing puzzle used in the PoW mechanism but can only be solved sequentially. We then present a VDP implementation based on the continuous verifiable delay function. Further, we show that VDP can be combined with the Nakamoto consensus in a proof-of-stake/proof-of-delay hybrid protocol. We analyze the persistence and liveness of the protocol, and show that compared to PoW, our proposal consumes much less energy; compared to BFT leader-election based consensus algorithms, our proposal achieves better resistance to long-range attacks and DoS attacks targeting the block proposers.
Blockchain is the core technology of many cryptocurrencies. Blockchain as a distributed ledger technology has received extensive research attention. In addition to cryptography and P2P (peer-to-peer) technology, consensus protocols are also a fundamental part of the blockchain technology. A good consensus protocol can guarantee the fault tolerance and security of the blockchain systems. The consensus protocols currently used in most blockchain systems can be broadly divided into two categories: the probabilistic-finality consensus protocols and the absolute-finality consensus protocols. This paper introduces some of the main consensus protocols of these two categories, and analyzes their strengths and weaknesses as well as the applicable blockchain types.
Amos Fiat, Anna R. Karlin, Ηλίας Κουτσουπιάς, Christos H. Papadimitriou
The Bitcoin protocol induces miners, through monetary rewards, to expend energy in order to add blocks to the chain. We show that, when energy costs are substantial and taken into account, counterintuitive and unintended strategic behavior results: In a simple bounded-horizon setting with two identical miners there is a unique pure symmetric equilibrium in which both miners first "slow down" in order to decrease the crypto complexity and then take advantage of this decrease. If miners have different energy efficiencies and are restricted to choose the same hash rate for many epochs, there is a unique pure equilibrium in which miners either participate at low levels that depend in intricate ways on all the other miners' efficiencies, or choose to abstain from mining if their efficiency is too low. In the general setting in which miners can adapt their hash rates over time, we show that, unless the number of miners is very small, the only possible pure equilibria are rather chaotic, with miners quitting and starting again periodically --- or there is no pure equilibrium at all. We discuss the implications of these results for the stability of proof-of-work protocols.
We describe MaGPoS, a novel consensus mechanism which is well suited to decentralized blockchain based protocols. MaGPoS is based on a combination of the well known physics of nano-scale magnetism, and previous implementations of proof of stake. This system has been studied by hundreds of thousands of scientists worldwide for over a hundred years, giving it an extreme level of reliability that is needed for a consensus mechanism. We start by explaining the physics, and study the properties that make it particularly beneficial for use in a consensus mechanism. We then show how to apply the physical model to a decentralized network of nodes, each with their own copy of a blockchain. After this, we describe some example calculations that a node in the decentralized network would make, and provide pseudo code for implementation. Finally, we discuss the how the model achieves all of the important properties that one expects of a consensus mechanism.
Lachlan J. Gunn, Jian Liu, Bruno Vavala, N. Asokan
Consensus mechanisms used by popular distributed ledgers are highly scalable but notoriously inefficient. Byzantine fault tolerance (BFT) protocols are efficient but far less scalable. Speculative BFT protocols such as Zyzzyva and Zyzzyva5 are efficient and scalable but require a trade-off: Zyzzyva requires only $3f + 1$ replicas to tolerate $f$ faults, but even a single slow replica will make Zyzzyva fall back to more expensive non-speculative operation. Zyzzyva5 does not require a non-speculative fallback, but requires $5f + 1$ replicas in order to tolerate $f$ faults. BFT variants using hardware-assisted trusted components can tolerate a greater proportion of faults, but require that every replica have this hardware. We present SACZyzzyva, addressing these concerns: resilience to slow replicas and requiring only $3f + 1$ replicas, with only one replica needing an active monotonic counter at any given time. We experimentally evaluate our protocols, demonstrating low latency and high scalability. We prove that SACZyzzyva is optimally robust and that trusted components cannot increase fault tolerance unless they are present in greater than two-thirds of replicas.
Blockchain technologies and smart contracts were considered for their potential to provide tremendous benefits as decision support tools to the next generation of warfighters. An investigative team focused on novel security-enhanced information gathering and decision support Artificial-Intelligence (AI) based software agents that will serve as decision support co-pilots when commanders are developing real-time multi-domain orders of battle. As a core construct for this architecture, smart contracts allow for the governance of these systems to be time-bound and/or condition-bound. This disruptive technology propels progress made with distributed multi-agent systems with the numerous security benefits of blockchain technologies. Methods for implementing intelligent computing agents that follow and execute the logic embedded in a contract model will provide a transparent record of agents, chain of trust and chain of custody on the blockchain. This paper will explore mechanisms for maintaining a hierarchy of smart contracts allowing reasoning and decision support over different aspects of the overall command structure. Finally, the potential for maintaining multiple levels of classification across communication channels for platforms tasked with collection and reconnaissance missions will explored.
Current swarm robotics systems are not utilized as frequently in surveillance missions due to the limitations of the existing distributed systems' designs. The main limitation of swarm robotics is the absence of a framework for robots to be self-governing, secure, and scalable. As of today, a swarm of robots is not able to communicate and perform tasks in transparent and autonomous ways. Many believe blockchain is the imminent future of distributed autonomous systems. A blockchain is a system of computers that stores and distributes data among all participants. Every single participant is a validator and protector of the data in the blockchain system. The data cannot be modified since all participants are storing and watching the same records. In this thesis, we will focus on blockchain applications in swarm robotics using Ethereum smart contracts because blockchain can make a swarm globally connected and secure. A decentralized application (DApp) is used to deploy surveillance missions. After mission deployment, the swarm uses blockchain to communicate and make decisions on appropriate tasks within Ethereum private networks. We set a test swarm robotics system and evaluate the blockchain for its performance, scalability, recoverability, and responsiveness. We conclude that, although blockchain enables a swarm to be globally connected and secure, there are performance limitations that can become a critical issue.