A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> C. Proof of Concepts Attached to Useful Resources <s> Abstract : The Tor network relies on volunteer relay operators for relay bandwidth, which may limit its growth and scaling potential. We propose an incentive scheme for Tor relying on two novel concepts. We introduce TorCoin, an altcoin that uses the Bitcoin protocol to reward relays for contributing bandwidth. Relays mine TorCoins, then sell them for cash on any existing altcoin exchange. To verify that a given TorCoin represents actual bandwidth transferred, we introduce TorPath a decentralized protocol for forming Tor circuits such that each circuit is privately-addressable but publicly veriable. Each circuit's participants may then collectively mine a limited number of TorCoins, in proportion to the end-to-end transmission goodput they measure on that circuit. <s> BIB001 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> C. Proof of Concepts Attached to Useful Resources <s> One of the current problems of peer-to-peer-based file storage systems like Freenet is missing participation, especially of storage providers. Users are expected to contribute storage resources but may have little incentive to do so. In this paper we propose KopperCoin, a token system inspired by Bitcoin’s blockchain which can be integrated into a peer-to-peer file storage system. In contrast to Bitcoin, KopperCoin does not rely on a proof of work (PoW) but instead on a proof of retrievability (PoR). Thus it is not computationally expensive and instead requires participants to contribute file storage to maintain the network. Participants can earn digital tokens by providing storage to other users, and by allowing other participants in the network to download files. These tokens serve as a payment mechanism. Thus we provide direct reward to participants contributing storage resources. <s> BIB002 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> C. Proof of Concepts Attached to Useful Resources <s> We introduce the novel notion of a Proof of Human-work PoH and present the first distributed consensus protocol from hard Artificial Intelligence problems. As the name suggests, a PoH is a proof that a human invested a moderate amount of effort to solve some challenge. A PoH puzzle should be moderately hard for a human to solve. However, a PoH puzzle must be hard for a computer to solve, including the computer that generated the puzzle, without sufficient assistance from a human. By contrast, CAPTCHAs are only difficult for other computers to solve -- not for the computer that generated the puzzle. We also require that a PoH be publicly verifiable by a computer without any human assistance and without ever interacting with the agent who generated the proof of human-work. We show how to construct PoH puzzles from indistinguishability obfuscation and from CAPTCHAs. We motivate our ideas with two applications: HumanCoin and passwords. We use PoH puzzles to construct HumanCoin, the first cryptocurrency system with human miners. Second, we use proofs of human work to develop a password authentication scheme which provably protects users against offline attacks. <s> BIB003 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> C. Proof of Concepts Attached to Useful Resources <s> Cryptocurrency and blockchain technologies are recently gaining wide adoption since the introduction of Bitcoin, being distributed, authority-free, and secure. Proof of Work (PoW) is at the heart of blockchain's security, asset generation, and maintenance. Although simple and secure, a hash-based PoW like Bitcoin's puzzle is often referred to as “useless”, and the used intensive computations are considered “waste” of energy. A myriad of Proof of “something” alternatives have been proposed to mitigate energy consumption; however, they either introduced new security threats and limitations, or the “work” remained far from being really “useful”. In this work, we introduce Proof of eXercise (PoX): a sustainable alternative to PoW where an eXercise is a real world matrix-based scientific computation problem. We provide a novel study of the properties of Bitcoin's PoW, the challenges of a more “rational” solution as PoX, and we suggest a comprehensive approach for PoX. <s> BIB004 </s> A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks <s> C. Proof of Concepts Attached to Useful Resources <s> Blockchain technologies are gaining massive momentum in the last few years. Blockchains are distributed ledgers that enable parties who do not fully trust each other to maintain a set of global states. The parties agree on the existence, values and histories of the states. As the technology landscape is expanding rapidly, it is both important and challenging to have a firm grasp of what the core technologies have to offer, especially with respect to their data processing capabilities. In this paper, we first survey the state of the art, focusing on private blockchains (in which parties are authenticated). We analyze both in-production and research systems in four dimensions: distributed ledger, cryptography, consensus protocol and smart contract. We then present BLOCKBENCH, a benchmarking framework for understanding performance of private blockchains against data processing workloads. We conduct a comprehensive evaluation of three major blockchain systems based on BLOCKBENCH, namely Ethereum, Parity and Hyperledger Fabric. The results demonstrate several trade-offs in the design space, as well as big performance gaps between blockchain and database systems. Drawing from design principles of database systems, we discuss several research directions for bringing blockchain performance closer to the realm of databases. <s> BIB005
Under the framework of Nakamoto protocol, a number of alternative PoX schemes have been proposed to replace the original PoW scheme in permissionless blockchain networks. Generally, these PoX schemes aim at two major designing goals, i.e., to incentivize useful resource provision, e.g., [37], BIB002 ,  , BIB001 ,  and to improve the performance, e.g., in terms of security, fairness and eco-friendliness  ,  , BIB003 of the blockchain networks. Starting from this subsection, we will focus on the principles of puzzle design discussed in Section III-A and provide a close examination on different PoX schemes in the literature. With the purpose of useful resource provision, the idea of "Proof of Useful Resources" (PoUS) has been proposed to tackle the resource wasting problem of PoW. Instead of enforcing the consumption of computational cycles for merely hash queries, a number of studies are devoted to the design of puzzles that are attached to useful work. An early attempt, i.e., Primecoin  , proposed to replace the PoW puzzle in (1) by the puzzle of searching three types of prime number chains, i.e., the Cunningham chain of the first/second kind or the bi-twin chain  . However, the verification stage of Primecoin puzzle is based on classical Fermat test of base two (pseudoprime)  , hence violates the principle of soundness in non-interactive ZKP. Meanwhile, since the induced solution arrival does not follow the i.i.d. Bernoulli model in (3), the Primecoin puzzle does not simulate the random distribution for leader selection as required by BIB005 . In BIB004 , a similar scheme, i.e., the proof of exercise is proposed to replace the preimage searching problem in PoW with the useful "exercise" of matrix product problems. The scheme uses a pool of task proposals to replace the PoW-based puzzle solving processes by the computation tasks offered by non-authenticated clients. Each consensus node needs to bid for a specific task to determine its puzzle. For this reason, the puzzle solution-generating scheme behaves more like a Computation as a Service (CaaS) platform. Since the matrix problems in the task pool may present different complexity levels, the puzzle competition does not fully simulate on the network level the random distribution in BIB005 . Also, the solution verification can only be done probabilistically due to the lack of O(n) verification schemes. Therefore, the proposed scheme in BIB004 suffers from the same problems as in the Primecoin  . In  , a new puzzle framework, i.e., useful Proof of Work (uPoW) is designed to replace the primitive PoW puzzle in (1) with a specific set of problems satisfying not only the properties of completeness, soundness and non-invertibility (hardness), but also the additional requirement of usefulness. Here, the usefulness is implied in the execution stage of the puzzle (cf . Table I) . Formally, by assuming completeness and soundness, the properties of usefulness can be defined as follows (cf. [ 