Privacy-preserving model learning on a blockchain network-of-networks.

ObjectiveTo facilitate clinical/genomic/biomedical research, constructing generalizable predictive models using cross-institutional methods while protecting privacy is imperative. However, state-of-the-art methods assume a "flattened" topology, while real-world research networks may consist of "network-of-networks" which can imply practical issues including training on small data for rare diseases/conditions, prioritizing locally trained models, and maintaining models for each level of the hierarchy. In this study, we focus on developing a hierarchical approach to inherit the benefits of the privacy-preserving methods, retain the advantages of adopting blockchain, and address practical concerns on a research network-of-networks.Materials and methodsWe propose a framework to combine level-wise model learning, blockchain-based model dissemination, and a novel hierarchical consensus algorithm for model ensemble. We developed an example implementation HierarchicalChain (hierarchical privacy-preserving modeling on blockchain), evaluated it on 3 healthcare/genomic datasets, as well as compared its predictive correctness, learning iteration, and execution time with a state-of-the-art method designed for flattened network topology.ResultsHierarchicalChain improves the predictive correctness for small training datasets and provides comparable correctness results with the competing method with higher learning iteration and similar per-iteration execution time, inherits the benefits of the privacy-preserving learning and advantages of blockchain technology, and immutable records models for each level.DiscussionHierarchicalChain is independent of the core privacy-preserving learning method, as well as of the underlying blockchain platform. Further studies are warranted for various types of network topology, complex data, and privacy concerns.ConclusionWe demonstrated the potential of utilizing the information from the hierarchical network-of-networks topology to improve prediction

Centrally Banked Cryptocurrencies

Current cryptocurrencies, starting with Bitcoin, build a decentralized blockchain-based transaction ledger, maintained through proofs-of-work that also generate a monetary supply. Such decentralization has benefits, such as independence from national political control, but also significant limitations in terms of scalability and computational cost. We introduce RSCoin, a cryptocurrency framework in which central banks maintain complete control over the monetary supply, but rely on a distributed set of authorities, or mintettes, to prevent double-spending. While monetary policy is centralized, RSCoin still provides strong transparency and auditability guarantees. We demonstrate, both theoretically and experimentally, the benefits of a modest degree of centralization, such as the elimination of wasteful hashing and a scalable system for avoiding double-spending attacks.Comment: 15 pages, 4 figures, 2 tables in Proceedings of NDSS 201

The Bitcoin Backbone Protocol: Analysis and Applications

Bitcoin is the first and most popular decentralized cryptocurrency to date. In this work, we extract and analyze the core of the Bitcoin protocol, which we term the Bitcoin backbone, and prove two of its fundamental properties which we call common prefix and chain quality in the static setting where the number of players remains fixed. Our proofs hinge on appropriate and novel assumptions on the hashing power of the adversary relative to network synchronicity; we show our results to be tight under high synchronization. Next, we propose and analyze applications that can be built on top of the backbone protocol, specifically focusing on Byzantine agreement (BA) and on the notion of a public transaction ledger. Regarding BA, we observe that Nakamoto\u27s suggestion falls short of solving it, and present a simple alternative which works assuming that the adversary\u27s hashing power is bounded by 1/3. The public transaction ledger captures the essence of Bitcoin\u27s operation as a cryptocurrency, in the sense that it guarantees the liveness and persistence of committed transactions. Based on this notion we describe and analyze the Bitcoin system as well as a more elaborate BA protocol, proving them secure assuming high network synchronicity and that the adversary\u27s hashing power is strictly less than 1/2, while the adversarial bound needed for security decreases as the network desynchronizes. Finally, we show that our analysis of the Bitcoin backbone protocol for synchronous networks extends with relative ease to the recently considered partially synchronous model, where there is an upper bound in the delay of messages that is unknown to the honest parties

A Survey on Consensus Mechanisms and Mining Strategy Management in Blockchain Networks

© 2013 IEEE. The past decade has witnessed the rapid evolution in blockchain technologies, which has attracted tremendous interests from both the research communities and industries. The blockchain network was originated from the Internet financial sector as a decentralized, immutable ledger system for transactional data ordering. Nowadays, it is envisioned as a powerful backbone/framework for decentralized data processing and data-driven self-organization in flat, open-access networks. In particular, the plausible characteristics of decentralization, immutability, and self-organization are primarily owing to the unique decentralized consensus mechanisms introduced by blockchain networks. This survey is motivated by the lack of a comprehensive literature review on the development of decentralized consensus mechanisms in blockchain networks. In this paper, we provide a systematic vision of the organization of blockchain networks. By emphasizing the unique characteristics of decentralized consensus in blockchain networks, our in-depth review of the state-of-the-art consensus protocols is focused on both the perspective of distributed consensus system design and the perspective of incentive mechanism design. From a game-theoretic point of view, we also provide a thorough review of the strategy adopted for self-organization by the individual nodes in the blockchain backbone networks. Consequently, we provide a comprehensive survey of the emerging applications of blockchain networks in a broad area of telecommunication. We highlight our special interest in how the consensus mechanisms impact these applications. Finally, we discuss several open issues in the protocol design for blockchain consensus and the related potential research directions

Security Analysis of Filecoin's Expected Consensus in the Byzantine vs Honest Model

Filecoin is the largest storage-based open-source blockchain, both by storage capacity (>11EiB) and market capitalization. This paper provides the first formal security analysis of Filecoin's consensus (ordering) protocol, Expected Consensus (EC). Specifically, we show that EC is secure against an arbitrary adversary that controls a fraction $\beta$ of the total storage for $\beta m< 1- e^{-(1-\beta)m}$, where $m$ is a parameter that corresponds to the expected number of blocks per round, currently $m=5$ in Filecoin. We then present an attack, the $n$-split attack, where an adversary splits the honest miners between multiple chains, and show that it is successful for $\beta m \ge 1- e^{-(1-\beta)m}$, thus proving that $\beta m= 1- e^{-(1-\beta)m}$ is the tight security threshold of EC. This corresponds roughly to an adversary with $20\%$ of the total storage pledged to the chain. Finally, we propose two improvements to EC security that would increase this threshold. One of these two fixes is being implemented as a Filecoin Improvement Proposal (FIP).Comment: AFT 202

Blockchain security and applications

Cryptocurrencies, such as Bitcoin and Ethereum, have proven to be highly successful. In a cryptocurrency system, transactions and ownership data are stored digitally in a ledger that uses blockchain technology. This technology has the potential to revolutionize the future of financial transactions and decentralized applications. Blockchains have a layered architecture that enables their unique method of authenticating transactions. In this research, we examine three layers, each with its own distinct functionality: the network layer, consensus layer, and application layer. The network layer is responsible for exchanging data via a peer-to-peer (P2P) network. In this work, we present a practical yet secure network design. We also study the security and performance of the network and how it affects the overall security and performance of blockchain systems. The consensus layer is in charge of generating and ordering the blocks, as well as guaranteeing that everyone agrees. We study the existing Proof-of-stake (PoS) protocols, which follow a single-extension design framework. We present an impossibility result showing that those single-extension protocols cannot achieve standard security properties (e.g., common prefix) and the best possible unpredictability if the honest players control less than 73\% stake. To overcome this, we propose a new multi-extension design framework. The application layer consists of programs (e.g., smart contracts) that users can use to build decentralized applications. We construct a protocol on the application layer to enhance the security of federated learning

Compact storage of superblocks for NIPoPoW applications

Blocks in proof-of-work (PoW) blockchains satisfy the PoW equation H(B) ≤ T. If additionally a block satisfies H(B) ≤ T 2−μ, it is called a μ-superblock. Superblocks play an important role in the construction of compactblockchain proofs which allows the compression of PoW blockchains into so-called Non-Interactive Proofs of Proof-of-Work (NIPoPoWs). These certificates are essential for the construction of superlight clients, which are blockchain wallets thatcan synchronize exponentially faster than traditional SPV clients. In this work, we measure the distribution of superblocks in the Bitcoin blockchain. We find that the superblock distribution within the blockchain follows expectation, hence we empirically verify that the distribution of superblocks within the Bitcoin blockchain has not been adversarially biased. NIPoPoWs require that each block in a blockchain points to a sample of previous blocks in the blockchain. These pointers form a data structure called the interlink. We give efficient ways to store the interlink data structure. Repeated superblock references within an interlink can be omitted with no harm to security. Hence, it is more efficient to store a set of superblocks rather than a list. We show that, in honest executions, this simple observation reduces the number ofsuperblock references by approximately a half in expectation. We then verify our theoretical result by measuring the improvement over existing blockchains in terms of the interlink sizes (which we improve by 79%) and the sizes of succinct NIPoPoWs(which we improve by 25%). As such, we show that deduplication allows superlight clients to synchronize 25% faster