Explorer l’hétérogénéité dans la réplication de données décentralisées faiblement cohérentes

Decentralized systems are scalable by design but also difficult to coordinate due to their weak coupling. Replicating data in these geo-distributed systems is therefore a challenge inherent to their structure. The two contributions of this thesis exploit the heterogeneity of user requirements and enable personalizable quality of services for data replication in decentralized systems. Our first contribution Gossip Primary-Secondary enables the consistency criterion Update consistency Primary-Secondary to offer differentiated guarantees in terms of consistency and message delivery latency for large-scale data replication. Our second contribution Dietcoin enriches Bitcoin with diet nodes that can (i) verify the correctness of entire subchains of blocks while avoiding the exorbitant cost of bootstrap verification and (ii) personalize their own security and resource consumption guarantees.Les systèmes décentralisés sont par nature extensibles mais sont également difficiles à coordonner en raison de leur faible couplage. La réplication de données dans ces systèmes géo-répartis est donc un défi inhérent à leur structure. Les deux contributions de cette thèse exploitent l'hétérogénéité des besoins des utilisateurs et permettent une qualité de service personnalisable pour la réplication de données dans les systèmes décentralisés. Notre première contribution Gossip Primary-Secondary étend le critère de cohérence Update consistency Primary-Secondary afin d'offrir des garanties différenciées de cohérence et de latence de messages pour la réplication de données à grande échelle. Notre seconde contribution Dietcoin enrichit Bitcoin avec des nœuds diet qui peuvent (i) vérifier la validité de sous-chaînes de blocs en évitant le coût exorbitant de la vérification initiale et (ii) choisir leur propres garanties de sécurité et de consommation de ressources

Impact of peer-to-peer trading and flexibility on local energy systems

To meet the 2050 net zero emission targets, energy systems around the globe are being revisited to achieve multi-vector decarbonisation in terms of electricity, transport, heating and cooling. As energy systems become more decentralised and digitised, local energy systems will have greater potential to self-sustain and hence, decrease reliance on fossil-fuelled central generation. While the uptake of electric vehicles, heat pumps, solar and battery systems offer a solution, the increase in electricity demand poses challenges in terms of higher peak demand, imbalance and overloading. Additionally, the current energy market structure prevents these assets in the distribution network from reaching their true techno-economic potential in flexibility services and energy trading. Peer-to-peer energy trading and community-level control algorithms achieve better matching of local demand and supply through the use of transactive energy markets, load shifting and peak shaving techniques. Existing research addresses the challenges of local energy markets and others investigate the effect of increased distributed assets on the network. However, the combined techno-economic effect requires the co-simulation of both market and network levels, coupled with simultaneous system balance, cost and carbon intensity considerations. Using bottom-up coordination and user-centric optimisation, this project investigated the potential of network-aware peer-to-peer trading and community-level control to increase self-sufficiency and self-consumption in energy communities. The techno-economic effects of these strategies are modelled while maintaining user comfort levels and healthy operation of the network and assets. The proposed strategies are evaluated according to their economic benefit, environmental impact and network stress. A case study in Scotland was employed to demonstrate the benefits of peer-to-peer trading and community self-consumption using future projections of demand, generation and storage. Additionally, the concept of energy smart contracts, embedded in blockchains, are proposed and demonstrated to overcome the major challenges of monitoring and contracting. The results indicate benefits for various energy systems stakeholders. Distribution system end-users benefit from lower energy costs while system operators obtain better visibility of the local-level flexibility along with the associated technical challenges in terms of losses, imbalance and loading. From a commercial perspective, community energy companies may utilise this study to inform investment decisions regarding storage, distributed generation and transactive market solutions. Additionally, the insights about the energy smart contracts allow blockchain and relevant technology sectors to recognise the opportunities and challenges of smart contracts and distributed ledger technologies that are specific to the energy sector. On the broader scale, energy system operators, regulators and high-level decision-makers can compare the simulated impact of community-led energy transition on the net zero goals with large-scale top-down initiatives

Themelio: a new blockchain paradigm

Public blockchains hold great promise in building protocols that uphold security properties like transparency and consistency based on internal, incentivized cryptoeconomic mechanisms rather than preexisting trust in participants. Yet user-facing blockchain applications beyond "internal" immediate derivatives of blockchain incentive models, like cryptocurrency and decentralized finance, have not achieved widespread development or adoption. We propose that this is not primarily due to "engineering" problems in aspects such as scaling, but due to an overall lack of transferable endogenous trust—the twofold ability to uphold strong, internally-generated security guarantees and to translate them into application-level security. Yet we argue that blockchains, due to their foundation on game-theoretic incentive models rather than trusted authorities, are uniquely suited for building transferable endogenous trust, despite their current deficiencies. We then engage in a survey of existing public blockchains and the difficulties and crises that they have faced, noting that in almost every case, problems such as governance disputes and ecosystem inflexibility stem from a lack of transferable endogenous trust. Next, we introduce Themelio, a decentralized, public blockchain designed to support a new blockchain paradigm focused on transferable endogenous trust. Here, the blockchain is used as a low-level, stable, and simple root of trust, capable of sharing this trust with applications through scalable light clients. This contrasts with current blockchains, which are either applications or application execution platforms. We present evidence that this new paradigm is crucial to achieving flexible deployment of blockchain-based trust. We then describe the Themelio blockchain in detail, focusing on three areas key to its overall theme of transferable, strong endogenous trust: a traditional yet enhanced UTXO model with features that allow powerful programmability and light-client composability, a novel proof-of-stake system with unique cryptoeconomic guarantees against collusion, and Themelio's unique cryptocurrency "mel", which achieves stablecoin-like low volatility without sacrificing decentralization and security. Finally, we explore the wide variety of novel, partly off-chain applications enabled by Themelio's decoupled blockchain paradigm. This includes Astrape, a privacy-protecting off-chain micropayment network, Bitforest, a blockchain-based PKI that combines blockchain-backed security guarantees with the performance and administration benefits of traditional systems, as well as sketches of further applications

Enter the Hydra: Towards Principled Bug Bounties and Exploit-Resistant Smart Contracts

Bug bounties are a popular tool to help prevent software exploits. Yet, they lack rigorous principles for setting bounty amounts and require high payments to attract economically rational hackers. Rather than claim bounties for serious bugs, hackers often sell or exploit them. We present the *Hydra Framework*, the first general, principled approach to modeling and administering bug bounties that incentivize bug disclosure. Our key idea is an *exploit gap*, a program transformation that enables runtime detection, and rewarding, of critical bugs. Our framework transforms programs via *N-of-N-version programming*, a variant of classical N-version programming that runs multiple independent program instances. We apply the Hydra Framework to *smart contracts*, small programs that execute on blockchains. We show how Hydra contracts greatly amplify the power of bounties to incentivize bug disclosure by economically rational adversaries, establishing the first framework for rigorous economic evaluation of smart contract security. We also model powerful adversaries capable of *bug withholding*, exploiting race conditions in blockchains to claim bounties before honest users can. We present *Submarine Commitments*, a countermeasure of independent interest that conceals transactions on blockchains. We design a simple, automated version of the Hydra Framework for Ethereum (ethereum.org) and implement two Hydra contracts, an ERC20 standard token and a Monty-Hall game. We evaluate our implementation for completeness and soundness with the official Ethereum virtual machine test suite and live blockchain data

CHURP: Dynamic-Committee Proactive Secret Sharing

We introduce CHURP (CHUrn-Robust Proactive secret sharing). CHURP enables secure secret-sharing in dynamic settings, where the committee of nodes storing a secret changes over time. Designed for blockchains, CHURP has lower communication complexity than previous schemes: $O(n)$ on-chain and $O(n^2)$ off-chain in the optimistic case of no node failures. CHURP includes several technical innovations: An efficient new proactivization scheme of independent interest, a technique (using asymmetric bivariate polynomials) for efficiently changing secret-sharing thresholds, and a hedge against setup failures in an efficient polynomial commitment scheme. We also introduce a general new technique for inexpensive off-chain communication across the peer-to-peer networks of permissionless blockchains. We formally prove the security of CHURP, report on an implementation, and present performance measurements

Towards Sustainable Blockchains:Cryptocurrency Treasury and General Decision-making Systems with Provably Secure Delegable Blockchain-based Voting

The blockchain technology and cryptocurrencies, its most prevalent application, continue to gain acceptance and wide traction in research and practice within academia and the industry because of its promise in decentralised and distributed computing. Notably, the meteoric rise in the value and number of cryptocurrencies since the creation of Bitcoin in 2009 have ushered in newer innovations and interventions that addressed some of the prominent issues that affect these platforms. Despite the increased privacy, security, scalability, and energy-saving capabilities of new consensus protocols in newer systems, the development and management of blockchains, mostly, do not reflect the decentralisation principle despite blockchains being decentralised and distributed in their architecture. The concept of treasury has been identified as a tool to address this problem. We explore the idea of blockchain treasury systems within literature and practice, especially with relation to funding and decision-making power towards blockchain development and maintenance. Consequently, we propose a taxonomy for treasury models within cryptocurrencies. Thereafter, we propose an efficient community-controlled and decentralised collaborative decision-making mechanism to support the development and management of blockchains. Our proposed system incentivises participants and is proven secure under the universally composable (UC) framework while also addressing gaps identified from our investigation of prior systems e.g. non-private ballots and insecure voting. Furthermore, we adapt our system and propose a privacy-preserving general decision making system for blockchain governance that supports privacy-centric cryptocurrencies. Besides, using a set of metrics, we introduce a consensus analysis mechanism to enhance the utility of decision-making of the systems by evaluating individual choices against collective (system-wide) decisions. Finally, we provide pilot system implementations with benchmark results confirming the efficiency and practicality of our constructions