Blockchain leader-based protocols elect leaders for proposing the next block of transactions. Proposed blocks need to pass a validation routine in order to be added to the blockchain. Proposers may prioritize certain transactions based on their fees or accounts. A fair block selection follows a random selection of transactions among pending transactions that a proposer is aware of. The validators may only have partial knowledge of the network transactions making it challenging to validate the random selection. We propose a protocol to encourage fair block selection in a leader-based blockchain network. Our protocol offers two main contributions. First, suggesting an algorithm that evaluates the proposed blocks based on both their transactions’ issuance times and zone structure. Second, providing incentives for acting honestly and diminishing malicious and dishonest nodes. To accomplish this, we use a reputation system, whereby each node is given a reputation score based on its actions (i.e. latest proposals and evaluations). We demonstrate the improved accuracy of our protocol by implementing experiments based on Ethereum topology, comparing it with Helix [1], an existing consensus algorithm for a fair block selection.
Abstract We proposed a decentralized cryptocurrency protocol named proof of activity and stake in which participants have no admittance threshold and communicate in peer-to-peer network. Our protocol adopts the idea of stakeholders in proof of stake and combined it with the propagation activity to replace the resource-costly proof of work. It is also proposed to solve the contradiction of security and throughput, which discouraged the promotion of blockchain applications. With analysis of security assurance and performance, proof of activity and stake is proved to be robust and economical against double spending attack.
Proof-of-Work~(PoW) based blockchains typically allocate only a tiny fraction (e.g., less than 1% for Ethereum) of the average interarrival time~$\mathbbI $ between blocks for validating smart contracts present in transactions. In such systems, block validation and PoW mining are typically performed sequentially, the former by CPUs and the latter by ASICs. A trivial increase in validation time~$(τ)$ introduces the popularly known Verifier's Dilemma, and as we demonstrate, causes more forking and hurts fairness. Large τ also reduces the tolerance for safety against a Byzantine adversary. Solutions that offload validation to a set of non-chain nodes (a.k.a. off-chain approaches) suffer from trust and performance issues that are non-trivial to resolve. In this paper, we present Tuxedo, the first on-chain protocol to theoretically scale τ/\mathbbI \approx 1$ in PoW blockchains. The key innovation in Tuxedo is to perform CPU-based block processing in \em parallel to ASIC mining. We achieve this by allowing miners to delay validation of transactions in a block by up to ζ blocks, where ζ is a system parameter. We perform security analysis of Tuxedo considering all possible adversarial strategies in a synchronous network with maximum end-to-end delay Δ and demonstrate that Tuxedo achieves security equivalent to known results for longest chain PoW Nakamoto consensus. Our prototype implementation of Tuxedo atop Ethereum demonstrates that it can scale τ without suffering the harmful effects of naïve scaling up of τ/\mathbbI $ in existing blockchains.
Trust lies at the center of the paradigm shift required to realize the ultra-dense networks needed by future radio communication systems. We propose a distributed, three-layer, trust-based hardware sharing scheme between operators that overcomes the limitations of a single-operator-owned monolithic network. Our system scales to tens of thousands of operators without requiring explicit contracts between them, or between operators and user equipment. User equipment in turn is free to requisition the services of any available hardware. This is achieved by abstracting the communication process as a transaction, and casting it within a distributed ledger technology framework paired with an efficient, fault-tolerant, distributed consensus protocol. A trust model associates a behavioral measure with each hardware device that signals its reliability, as well as its payoff. The proposed system offers multiple advantages for users, operators, and regulators.
Lehlogonolo P. I. Ledwaba, Gerhard P. Hancke, Sherrin J. Isaac
With limited processing, energy and storage along with a deadline sensitive operational environment, the combination of the Industrial IoT (IIoT) with distributed ledger technologies (DLTs) could serve to introduce intolerable latency into network processes; counteracting the potential advantages that come from combining the two technologies. In an effort to improve the compatibility of DLTs for the industrial informatics context, the authors developed a lightweight consensus for IIoT environments based off delegated proof of stake (DPoS), called Delegated Proof of Proximity (DPoP), to limit the processing and energy effort required by DLTs. DPoP will, however, require an existing, IIoT neighbour discovery process to facilitate a proof of proximity for the consensus process. Thus, this preliminary work aims to evaluate distance bounding as a possible mechanism for establishing a secure proof of proximity and neighbour discovery between nodes during the DPoP consensus process to improve the scalability and flexibility of DLT solutions, like Ethereum, for IIoT use cases.
In the modern digital world, large-scale data and the analytic processing of the data make privacy-preserving computation even more critical. SMPC is a cryptographic protocol used to compute a function over the inputs of multiple parties such that the other party’s input is unknown. This then provides for computing in parallel with other participants, without requiring a coordinator, which, in today’s privacy-conscious world, is beneficial in avoiding using a central authority in data-entrusted activities. In a nutshell, a decentralized AI approach is based on distributed computing principles and the blockchain to create a solid architecture for SMPC implementation. In this manner, decentralized AI eliminates several drawbacks of data centralization, such as single points of failure and data breaches. SMPC and decentralized networks are the foundation of the privacy-preserving ML, where sensitive data train models without revealing the data points. Specifically, the growing necessity for protecting data with the help of laws like the GDPR and CCPA enhances SMPC’s application in decentralized AI. Blockchain technology extends this implementation by having additional qualities of having an unchangeable record and consensus mechanisms that guarantee computation reliability and openness. However, scalability, ITY, computational cost, and system compatibility are drawbacks to integrating decentralized AI and SMPC. Solving these needs more be a continuous effort in the search for cryptographic techniques in communication, network design, and protocol formation. The combination of decentralized AI and SMPC presents a new and revolutionary way of multi-party computation through data privacy and access to cooperation and innovation in sectors such as health, finance, and supply chain. With the development of technology, these intelligent computing applications of decentralized AI and SMPC will continue to develop and open up new areas for efficient and secure data usage.
Blockchain has a great potential to change the way we interact and transact on the Internet. It has features like decentralized systems, security, immutability and transparency. Given that it is a decentralized system and there is no third party involved to verify transactions, security is at risk. Such a system can be highly prone to attacks like double spending and might become vulnerable at times. How is it avoided without having a centralized system? For this system to function at a global scale, a secure algorithm is highly needed. That is where a consensus algorithm comes into play. This chapter discusses various Blockchain consensus algorithms like Proof of Work, Proof of Stake, etc. and gives a brief overview of each.
Riyad Almakki, Lulwah AlSuwaidan, Shakir Khan, Abdul Rauf Baig · 6 authors
A new algorithm for practical Byzantine fault tolerance (PBFT), called score-PBFT or S-PBFT, is proposed to solve the problems of high communication overhead and low algorithm efficiency. This algorithm is based on the characteristics of the consortium chain. The scoring mechanism for nodes is added. All the nodes are broken up into consensus nodes, candidate nodes, and early nodes. To make sure the consensus nodes are as reliable as possible, the nodes are changed dynamically based on how each node is behaving. Improved: the election method for the controller node has been changed. The node’s score and behaviour are used as the election basis to make the algorithm more stable. In this paper, we want to improve the consensus protocol’s execution process, cut down on how many nodes are involved in the consensus process, simplify it, and make it more efficient. Results show that, when compared with the PBFT algorithm, the S-PBFT algorithm has a shorter consensus delay, less communication overhead and throughput, and better consensus node reliability.
Blockchain-based IoT systems can manage IoT devices and achieve a high level of data integrity, security, and provenance. However, incorporating existing consensus protocols in many IoT systems limits scalability and leads to high computational cost and consensus latency. In addition, location-centric characteristics of many IoT applications paired with limited storage and computing power of IoT devices bring about more limitations, primarily due to the location-agnostic designs in blockchains. We propose a hierarchical and location-aware consensus protocol (LH-Raft) for IoT-blockchain applications inspired by the original Raft protocol to address these limitations. The proposed LH-Raft protocol forms local consensus candidate groups based on nodes' reputation and distance to elect the leaders in each sub-layer blockchain. It utilizes a threshold signature scheme to reach global consensus and the local and global log replication to maintain consistency for blockchain transactions. To evaluate the performance of LH-Raft, we first conduct an extensive numerical analysis based on the proposed reputation mechanism and the candidate group formation model. We then compare the performance of LH-Raft against the classical Raft protocol from both theoretical and experimental perspectives. We evaluate the proposed threshold signature scheme using Hyperledger Ursa cryptography library to measure various consensus nodes' signing and verification time. Experimental results show that the proposed LH-Raft protocol is scalable for large IoT applications and significantly reduces the communication cost, consensus latency, and agreement time for consensus processing.
In this work we integrate Proof of Stake (PoS) consensus technique with multiple entry Practical Byzantine Fault Tolerance voting in a permissioned blockchain network. We introduce several PoS classes based on stake and truthfulness of voting. Each class is modeled using Semi Markov Process (SMP). We derive probability of reaching the two-thirds majority of total number of votes, and highlight the impact of the populations of individual stake/priority classes on achieving consensus. We have also connected stake classes with Enhanced Distribution Coordination Function EDCA for leader selection, which enables nodes from high stake classes to have higher frequency of leader role and gain more revenue for block handling.
In this paper we explore a context of application of Cob, a recently introduced Byzantine Fault Tolerant consensus protocol. Cob proves to be a leaderless consensus protocol which carries out the consensus process in parallel on each component of a list of events to be observed and recorded. We show how Cob can be used to define a consensus layer for scalable and sustainable blockchains. This layer is used to design consensus protocols based on sharding as a mean to achieve scalability, and on the fragmentation of time in time-slots (which get assigned to nodes that are instructed to create new blocks) as a mean to reduce the amount of computation and communication necessary for the maintenance of the distributed ledger. We explain why Cob is a viable candidate to implement such consensus layer through the introduction of an auxiliary blockchain that we name Synchronization Chain.
Sebastian Müller, Andreas Penzkofer, Nikita Polyanskii, Jonas Theis · 6 authors
The Unspent Transaction Output (UTXO) model is commonly used in the field of Distributed Ledger Technology (DLT) to transfer value between participants. One of its advantages is that it allows parallel processing of transactions, as independent transactions can be added in any order. This property of order invariance and parallelisability has potential benefits in terms of scalability. However, since the UTXO Ledger is an append-only data structure, this advantage is compromised through the presence of conflicting transactions. We propose an extended UTXO Ledger model that optimistically updates the ledger and keeps track of the dependencies of the possible conflicts. In the presence of a conflict resolution mechanism, we propose a method to reduce the extended ledger back to a consistent UTXO Ledger.
While the Bitcoin and Ethereum are still leading the world of permissionless blockchains, we are increasingly seeing a multipolar ecosystem where new blockchains keep emerging instead of migrating to the two big players. As a result, it is highly desired to enable multiple blockchains to interoperate, e.g., move assets from one blockchain to the other. The crosschain bridge service, as a solution to the blockchain interoperability problem, has been offered by a wide range of service providers. However, the existing bridges either rely on centralised notaries or require complicated preparations, therefore are far from sufficient in practice. In this demo proposal, we highlight the Poly Bridge for heterogeneous blockchains to interoperate with cryptocurrencies. In specific, Poly Bridge is based on an un-derlying Poly Chain and a pair of relays to confirm crosschain transactions and form consensus among relevant parties. More importantly, Poly Bridge delivers extensibility to flexibly inter-face to blockchains with different consensus models and atomicity as well in a way that a sequence of crosschain operations are either all confirmed or all rejected. Poly Bridge is now available as a web application to support crosschain requests with over 200 types of cryptocurrencies on 18 blockchains.
Sharding technique, which divides the whole network into multiple disjoint groups or committees, has been recognized as a revolutionary solution to enhance the scalability of blockchains. For account-based model, state data are partitioned over all committees and organized as Merkle trees to ensure data consistency and immutability. However, existing techniques on Merkle tree-based state storage fail to scale out due to a large amount of network and compute overheads incurred by data migration and Merkle tree reconstruction, respectively. In this paper, we propose $\mathcal{S}$-Store, a scalable data storage technique towards permissioned blockchain sharding based on Aggregate Merkle B+ tree (AMB-tree). $\mathcal{S}$-Store utilizes consistent hashing to reduce data migration among committees and uses split and merge on AMB-tree to decrease Merkle tree reconstruction overheads. $\mathcal{S}$-Store also employs a novel committee addition protocol that guarantees the system service availability during data migration. Extensive experiments show that $\mathcal{S}$-Sotre outperforms existing techniques by one order of magnitude in terms of transaction execution, data transmission, and committee addition.
Jemin Andrew Choi, Sidi Mohamed Beillahi, Peilun Li, Andreas Veneris · 5 authors
We present the Layered Merkle Patricia Trie (LMPT), a performant storage data structure for processing transactions in high-throughput systems when com-pared to traditional Merkle Patricia Tries used in Ethereum clients. LMPTs keep smaller intermediary tries in memory to alleviate read and write amplification from high-latency disk storage. As an additional feat, they also allow for the I/O and transaction verifier threads to be scheduled in parallel and independently. LMPTs can ultimately reduce significant I/O traffic that happens on the critical path of transaction processing. Empirical results presented here confirm that LMPTs can process up to × 6 more transactions per second on real-life workloads when compared to existing Ethereum clients.
Classical and contemporary distributed consensus protocols, may they be for binary agreement, state machine replication, or blockchain consensus, require all protocol participants in a peer-to-peer system to agree on exactly the same information as part of the consensus payload. Although this model of consensus is extensively studied, and is useful for most consensus based decentralized applications, it falls short of defining correct distributed systems which mandate participant credential based privileged visibility into the consensus payload, through the consensus protocol itself. We introduce a new paradigm for distributed consensus, called functional blockchain consensus. Functional blockchain consensus allows each blockchain protocol participant to agree on some distinct sub-information of the list of transactions, as a function of the credentials of the participant in the blockchain system, instead of agreeing on the entire list of transactions. We motivate two adversary models, one with a standard crash-fault adversary and another with a novel rational-fault adversary, to compromise functional blockchain consensus. We then present two versions of a blockchain protocol called SightSteeple, that achieves functional blockchain consensus in the said fault models. SightSteeple relies on a novel combination of standard blockchain consensus and functional encryption, among other primitives, to achieve its goals of correctness. Finally, we discuss practical uses of functional blockchain consensus based asymmetric distributed ledgers, and motivate off-shoot constructions that can result from this new consensus paradigm.
In recent years, Blockchain arose as a key technology in building autonomous decentralised financial systems. Its ability to digitize trust enables building trustless systems such as cryptocurrencies where users do not need to rely on any third party to exchange value. The success of cryptocurrencies to operate without any intermediaries draw the interest of business operators who seek to bypass intermediation and thus to reduce cost and gain competitive advantages. As a result, Blockchain was used outside the crypto-sphere to build decentralized systems. However, this portage led to the inception of new types of Blockchains adapted to different specifications and with different designs. Consequently, the technology has diverged from its baseline (Bitcoin) to the point where some systems marketed as “blockchain” share only a few design concepts with the original Blockchain design proposed by Satoshi Nakamoto. This conceptual divergence alongside the lack of comprehensive models and standards made it difficult for both system designers and decision-makers to clearly understand what is a blockchain or to choose a suitable solution.This survey has a double goal; on the one hand, it attempts to contribute to the discussion on the ontological status of DLTs by providing a taxonomy oriented-framework (DCEA) for conceptualizing and examining DLT. On the other hand, it also attempts to present an up-to-date review and evaluation of current blockchains and their variants as constructed of four layers: the data, consensus, execution and application layers.
It has been recognized that one of the bottlenecks in the UTXO-based blockchain systems is the slow block validation - the process of validating a newly-received block by a node before locally storing it and further broadcasting it. As a block contains multiple inputs, the block validation mainly involves checking the inputs against the status data, which is also known as the Unspent Transaction Outputs (UTXO) set. As time goes by, the UTXO set becomes more and more expansive, most of which can only be stored on disks. This considerably slows down the input checking and thus block validation, which can potentially compromise system security. To deal with the above problem, we disassemble the function of input checking into three parts: existence validation (EV), unspent validation (UV), and script validation (SV). Based on the disassembly, we propose EBV, an efficient block validation mechanism to speed up EV, UV, and SV individually. First, EBV changes the representation of status data, from UTXO set to a bit-vector set, which drastically reduces its size. The smaller status data can be entirely maintained in memory, thereby accelerating UV and also block validation. Second, EBV requires each transaction to carry the proof data, which enables EV and SV without accessing the disks. Furthermore, we also cope with two challenges in the design of EBV, namely transaction inflation and fake positions. To evaluate the EBV mechanism, we implement a prototype on top of Bitcoin, the most widely known UTXO-based blockchain, and conduct extensive experiments to compare EBV and Bitcoin. The experimental results demonstrate that EBV successfully reduces the memory requirement by 93.1 % and the block validation time by up to 93.5%.
Hybrid transaction in blockchain enlarges the application domain of smart contracts by enabling off-chain data access. In support of the hybrid transaction, blockchain oracle is an indispensable middleware responsible for receiving data-request tasks and pushing the result back to chain. Though solutions supporting hybrid transactions do exist, they are less featured or applicable for both contract and application developers. On one hand, the racing-condition among several concurrent hybrid transactions should be manually resolved by contract developers. On the other hand, application developers have to associate each individual task with its parent hybrid transaction in order to obtain the result. In this paper, a novel block-chain oracle platform with a set of simple but unified API for both hybrid and traditional transactions is proposed. Firstly, we design and implement a transactional storage layer and a transaction scheduler for transaction preservation and ordering. Secondly, with the help of an automatic result tracing mechanism, application developers are able to acquire transaction results transparently without awareness of oracles and underlying task execution. Experiments are conducted on one of current prevalent permissioned blockchain platforms, RepChain, and the result shows that all of the system design goals are fulfilled with merely an acceptable loss of performance.
Blockchain has recently gained massive attention thanks to the success of cryptocurrencies and decentralized applications. With immutability and tamper-resistance features, it can be seen as a promising secure database solution. To address the need of searches over blockchain databases, prior work vChain proposed a novel verifiable processing framework that ensures query integrity without maintaining a full copy of the blockchain database. It however suffers from several limitations, including linear-scan search performance in the worst case and impractical public key management. In this paper, we propose a new searchable blockchain system, vChain+, that supports efficient verifiable boolean range queries with additional features. Specifically, we propose a sliding window accumulator index to achieve efficient query processing even for the worst case. We also design an object registration index to enable practical public key management without compromising the security guarantee. To support richer queries, we employ optimal tree-based indexes to index both keywords and numerical attributes of the data objects. Several optimizations are also proposed to further improve the query performance. Security analysis and empirical study validate the robustness and performance improvement of the proposed system. Compared with vChain, vChain+ improves the query performance by up to 913x.
Min Fang, Xinna Zhou, Zhao Zhang, Cheqing Jin · 5 authors
The current blockchain system suffers from serious scalability bottleneck, which greatly limits the application in large-scale. Furthermore, with the emergence of high-throughput consensus algorithms in permissioned blockchain, how to efficiently execute smart contracts becomes a critical challenge. To solve this issue, a two-phase concurrent execution mechanism has been adopted recently, where the primary executes a batch of transactions concurrently in the first phase, then the rest replays them in the second phase to achieve consistency. However, these works only enable intra-node concurrency, not inter-node concurrency. This demonstration showcases SEFrame, a novel execution framework for smart contracts of permissioned blockchain to achieve intra- and inter-node concurrency with the confidentiality guarantee of Intel Software Guard Extensions (SGX). We use real-time dashboards containing the output of SEFrame, which allows attendees to interactively explore how SEFrame achieves efficient inter- and intra-node concurrency.
Piotr Macek, Daria Dziubałtowska, Bartosz Kuśmierz, Radosław Michalski
Every distributed system in which there is no central coordinator requires some form of agreement on its state and other aspects. Depending on the setting, there is also some possibility of additional risks, such as malicious or failing nodes, and this also contributes to the challenge of reaching consensus. In this work we investigate the problem of metastability in peer-to-peer networks that are used for distributed ledgers, especially focusing on cellular automata protocol with majority dynamics. Our results demonstrate that networks using this protocol do not reach consensus easily, even in the absence of an adversary. As a remedy for that, we propose a set of fixtures significantly increasing the probability of reaching network agreement that in best cases allow for achieving it two orders of magnitude more often compared to the basic cellular automata approach.
Rodrigo Dutra Garcia, Gowri Ramachandran, Jó Ueyama
Smart contracts allow application developers to automate business processes through a decentralized computation architecture. Contemporary blockchain platforms such as Ethereum and Hyperledger Fabric offer support for smart contracts through consensus mechanisms such as Proof-of-Work (PoW) or other types of transaction validation and ordering services. This article exploits smart contracts in the Byzantine Fault Tolerant (BFT) blockchain platforms. In particular, we explore Tendermint and Hyperledger Besu, BFT blockchain platforms, and apply them to a decentralized e-prescription case study to evaluate their effectiveness. We adopt Hyperledger Besu and Tendermint in this research, given that both are BFT-based blockchains. Also, it is noteworthy that smart contracts in BFT blockchain platforms such as Tendermint are not well established and not widely adopted yet. Our article empirically evaluates the performance of smart contracts in Tendermint and Hyperledger Besu using a decentralized medical prescription case study and compares their results with Ethereum, a PoW blockchain. Our results demonstrate that BFT blockchain platforms are efficient for multistakeholder applications such as e-prescription and supply chains. To the best of our knowledge, this is the first study investigating the implementation of smart contracts in BFT blockchain platforms, such as Tendermint and Hyperledger Besu.
Aimen Djari, Emmanuelle Anceaume, Sara Tucci-Piergiovanni
A recent evolution of the blockchain structure is emerging to address the performance issue of permissionless chain-based ledgers, in particular the small number of transactions confirmed per second. To address such an issue, new designs have been brought forward, including Bitcoin-NG, which favors an off-chain mechanism in which blocks refer to a leader in charge of validating transactions batched in micro-blocks out of the chain [6]; Lightning [10], which follows the same principle but only publishes the outcome of repeated transactions among a set of parties. Others propositions such as HashGraph [2], ByteBall [5], and Iota [4] leverage the presence of well known institutions to get rid of blocks, while Ghost [12] and Spectre [11] protocols family modifies the blockchain data structure from a totally ordered sequence of blocks to a directed graph of blocks. Blocks are built so that they commit the state of the directed graph at the time blocks were created which decreases the opportunity for powerful attackers to create blocks in advance. Regarding the graph-based approach, the absence of mechanisms to prevent the presence of conflicting records (i.e., blocks with conflicting transactions) or the presence of cycles in the directed graph (Spectre [11] organises blocks in a directed, but not acyclic, graph of blocks) require that participants execute a complex algorithm to extract from the graph the set of accepted (i.e., valid) transactions [11]. Sycomore1 is an immutable permissionless distributed ledger whose structure is a particular directed acyclic graph of blocks, called SYC-DAG [1]. Its design differs from existing distributed ledgers in that its graph structure dynamically adapts to fluctuations in transaction submission rates: When the leaf block of a chain (more precisely the last blocks appended to a chain) of the graph exceeds a maximal loading threshold (the load is measured in Bytes), subsequent blocks are partitioned over two sibling chains, and these blocks are mined in parallel (as will be described shortly, even if blocks are appended in parallel to the SYC-DAG they cannot be conflicting, i.e., each valid transaction cannot appear in more than one block). Conversely, when the leaf blocks of two sibling chains (again the last blocks of two sibling chains) fall short of a minimal loading threshold, subsequent blocks will belong to a unique chain. The decision to split a leaf chain of the SYC-DAG or to merge two sibling ones is locally taken by each miner, and soundness of this decision is verifiable by everyone at any time [1]. Actually, Sycomore has been designed to meet the following properties [1]: