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S S symmetry Review Blockchain Consensus: An Overview of Alternative Protocols Damilare Peter Oyinloye 1,2 , Je Sen Teh 1,* , Norziana Jamil 3,* and Moatsum Alawida 1,4 1 School of Computer Sciences, Universiti Sains Malaysia, Pulau Pinang 11800, Malaysia; [email protected] (D.P.O.); [email protected] (M.A.) 2 Department of Computer Science, Kwara State University, Malete 241104, Nigeria 3 Department of Computing, College of Computing and Informatics, Universiti Tenaga Nasional, Selangor 43000, Malaysia 4 Center of Cyber-Physical Systems, Khalifa University, Abu Dhabi 127788, United Arab Emirates * Correspondence: [email protected] (J.S.T.); [email protected] (N.J.) Abstract: Blockchain networks are based on cryptographic notions that include asymmetric-key encryption, hash functions and consensus protocols. Despite their popularity, mainstream protocols, such as Proof of Work or Proof of Stake still have drawbacks. Efforts to enhance these protocols led to the birth of alternative consensus protocols, catering to specific areas, such as medicine or transporta- tion. These protocols remain relatively unknown despite having unique merits worth investigating. Although past reviews have been published on popular blockchain consensus protocols, they do not include most of these lesser-known protocols. Highlighting these alternative consensus protocols contributes toward the advancement of the state of the art, as they have design features that may be useful to academics, blockchain practitioners and researchers. In this paper, we bridge this gap by providing an overview of alternative consensus protocols proposed within the past 3 years. We evaluate their overall performance based on metrics such as throughput, scalability, security, energy consumption, and finality. In our review, we examine the trade-offs that these consensus protocols have made in their attempts to optimize scalability and performance. To the best of our knowledge, Citation: Oyinloye, D.P.; Teh, J.S..; this is the first paper that focuses on these alternative protocols, highlighting their unique features Alawida, M.; Jamil, N. Blockchain that can be used to develop future consensus protocols. Consensus: An Overview of Alternative Protocols. Symmetry 2021, Keywords: blockchain; consensus protocol; distributed ledger technology; energy; finality; scalability; 13, 1363. https://doi.org/10.3390/ security; throughput sym13081363 Academic Editor: Kuo-Hui Yeh 1. Introduction Received: 21 June 2021 Since its inception, blockchain technology and its applications have been of great Accepted: 9 July 2021 Published: 27 July 2021 interest in the financial sector. One of the most popular use cases of blockchain, cryptocur- rencies, relies on cryptographic algorithms, such as asymmetric-key encryption and hash Publisher’s Note: MDPI stays neutral functions, to facilitate secure financial transactions between various parties or nodes [1]. with regard to jurisdictional claims in Blockchain can be viewed as a fully replicated distributed database system that keeps a published maps and institutional affil- record of all transactions in a network. All of these transactions are also stored by every iations. contributing node or unit within the network itself [2]. It has a distributed consensus protocol running on every participating node, managing message exchanges and local decision making to enforce network consistency. Consensus protocols are a set of rules that participating nodes in a network use to decide whether a transaction is valid [3]. This ensures that all participants collectively maintain a common transaction ledger. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. On cryptocurrency platforms, blockchain keeps track of currency transactions in a This article is an open access article successive and orderly form to achieve a tamper-proof record [4]. With the rise of Ethereum distributed under the terms and and other blockchain-based distributed computing platforms, transactions also include conditions of the Creative Commons data that are required to execute smart contracts. Blockchain as a term connotes that Attribution (CC BY) license (https:// transaction records between participating nodes in a system or network are stored in a creativecommons.org/licenses/by/ data format known as a “block”. A sequential set of these connected blocks arranged in 4.0/). ascending order is referred to as the chain of blocks (i.e., “blockchain”). Permission-less Symmetry 2021, 13, 1363. https://doi.org/10.3390/sym13081363 https://www.mdpi.com/journal/symmetry Symmetry 2021, 13, 1363 2 of 35 blockchain networks do not need a centralized authorizing figure; interested nodes can join without restrictions, and it permits a large number of participating nodes in the consensus process [5,6]. Bitcoin is without a doubt the most mainstream or well-known use case of blockchain technology. It was introduced as an alternative form of currency with the aim of overcoming the limitations of fiat money [7]. Bitcoin’s underlying blockchain mechanism keeps a record of cryptocurrency transactions among participating nodes. Each node involved in the transaction possesses two keys: a private key and a public key [8]. The transaction address of a participating node is the hash value of the public key, which also serves as the node’s identity. As multiple private–public key pairs can be generated, nodes can thus have more than one identity, leading to attacks such as a Sybil attack [9]. Transactions are signed with a node’s private key and are sent to all other nodes in the network for verification. With a decentralized consensus architecture, the participating nodes come to an agreement on both the sequence and validity of all transactions. The records of these transactions are stored in blocks. The current block of any block sequence has a timestamp and is connected to prior blocks by cryptographic hash values. Participating nodes cannot delete a block but can append new ones. The chaining of these blocks results in a shared, distributed database with an immensely growing record of transactions that are irreversible and immutable. It is difficult for anyone to tamper with block information without other nodes detecting the changes [10]. Unlike the days of centralized ledgers in the custody of a single point of authority, blockchains are essentially decentralized databases that are collaboratively man- aged by multiple participating entities. One important element of blockchain technology is its underlying consensus mechanism, known as the consensus protocol. These protocols decide which node is allowed to add a new block to the chain. Blockchain consensus protocols can be classified into two distinct classes: (1) proof-based consensus protocols, which require that entities provide proof of effort or resource expenditure, and (2) voting- based consensus protocols, which require participating entities in the network to exchange their results of verifying new blocks or transactions before making the final decision about which node is allowed to commit a new block to the chain [11]. Well-established consensus protocols include Proof of Work (PoW) [12], Proof of Stake (PoS) [13] and their variants [14]. 1.1. Contribution and Scope Over the years, numerous lesser-known alternative blockchain consensus protocols have been proposed, some of which are for specific areas of application, such as medicine or electric vehicles. Although many of these protocols are currently of academic interest rather than for practical applications, they have unique features that may be useful for the advancement of blockchain technologies. Past reviews have mostly covered conventional consensus protocols, evaluating them based on various metrics, such as throughput, scala- bility and security. In this paper, we present a survey of alternative consensus protocols that have been proposed within the past three years (as of April 2021), evaluating their overall performance based on metrics depicted in Figure1. Our sources include academic papers, white papers, and official documentation of cryptocurrencies. White papers and official documentation were used mainly for the mainstream protocols, whereas alternative protocols were taken from academic papers. These academic papers were extracted from Scopus, using the search keywords “blockchain, consensus, protocol”, and protocols that were solely based on mainstream protocols (PoW, PoS) were eliminated. We define alternative protocols as those that are lesser known and do not fall directly into the same category as the popular consensus protocols listed in Section3 of our paper. Symmetry 2021, 13, 1363 3 of 35 Throughput Scalability Security Consensus Protocol Properties Finality Energy Figure 1. Consensus protocol properties. The aim of this paper is to provide, at a glance, the advances in alternative consensus protocols to both new and experienced researchers alike. The paper also contains an introduction to widely adopted, conventional consensus protocols, such as PoW and PoS, for the benefit of readers who are new to the area, and also to define the scope of what we consider to be popular or conventional protocols as opposed to alternative protocols. As such, coverage of these well-known protocols will not delve too deeply into their details, which are already widely available in many other sources. In addition, the purpose of this paper is not to advocate the adoption of these alternative protocols, but rather to highlight their unique features
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