740 research outputs found

    Incentivising Privacy in Cryptocurrencies

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    Privacy was one of the key points mentioned in Nakamoto's Bitcoin whitepaper, and one of the selling points of Bitcoin in its early stages. In hindsight, however, de-anonymising Bitcoin users turned out to be more feasible than expected. Since then, privacy focused cryptocurrencies such as Zcash and Monero have surfaced. Both of these examples cannot be described as fully successful in their aims, as recent research has shown. Incentives are integral to the security of cryptocurrencies, so it is interesting to investigate whether they could also be aligned with privacy goals. A lack of privacy often results from low user counts, resulting in low anonymity sets. Could users be incentivised to use the privacy preserving implementations of the systems they use? Not only is Zcash much less used than Bitcoin (which it forked from), but most Zcash transactions are simply transparent transactions, rather than the (at least intended to be) privacy-preserving shielded transactions. This paper and poster briefly discusses how incentives could be incorporated into systems like cryptocurrencies with the aim of achieving privacy goals. We take Zcash as example, but the ideas discussed could apply to other privacy-focused cryptocurrencies. This work was presented as a poster at OPERANDI 2018, the poster can be found within this short document

    Expertise médicolégale du handicap cognitif

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    Levels of Decentralization and Trust in Cryptocurrencies: Consensus, Governance and Applications

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    Since the apparition of Bitcoin, decentralization has become an ideal praised almost religiously. Indeed, removing the need for a central authority prevents many forms of abuse that could be performed by a trusted third party, especially when there are no transparency and accountability mechanisms in place. Decentralization is however a very subtle concept that has limits. In this thesis, we look at the decentralization of blockchains at three different levels. First we look at the consensus protocol, which is the heart of any decentralized system. The Nakamoto protocol, used by Bitcoin, has been shown to induce centralization through the shift to mining pools. Additionally, it is heavily criticized for the enormous amount of energy it requires. We propose a protocol, Fantômette, that incorporates incentives at its core and that consumes much less energy than Bitcoin and other proof-of-work based cryptocurrencies. If the consensus protocol makes it possible to decentralize the enforcement of rules in a cryptocurrency, there is still the question of who decides on the rules. Indeed, if a central authority is able to determine what those rules are then the fact that they are enforced in a decentralized way does not make it a decentralized system. We study the governance structure of Bitcoin and Ethereum by making measurements of their GitHub repositories and providing quantitative ways to compare their level of centralization by using appropriate metrics based on centrality measures. Finally, many applications are now built on top of blockchains. These can also induce or straightforwardly lead to centralization, for example by requiring that users register their identities to comply with regulations. We show how identities can be registered on blockchains in a decentralized and privacy-preserving way

    Winkle: Foiling Long-Range Attacks in Proof-of-Stake Systems

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    Winkle protects any validator-based byzantine fault tolerant consensus mechanisms, such as those used in modern Proof-of-Stake blockchains, against long-range attacks where old validators' signature keys get compromised. Winkle is a decentralized secondary layer of client-based validation, where a client includes a single additional field into a transaction that they sign: a hash of the previously sequenced block. The block that gets a threshold of signatures (confirmations) weighted by clients' coins is called a "confirmed"checkpoint. We show that under plausible and flexible security assumptions about clients the confirmed checkpoints can not be equivocated. We discuss how client key rotation increases security, how to accommodate for coins' minting and how delegation allows for faster checkpoints. We evaluate checkpoint latency experimentally using Bitcoin and Ethereum transaction graphs, with and without delegation of stake

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

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    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 β\beta of the total storage for βm<1−e−(1−β)m\beta m< 1- e^{-(1-\beta)m}, where mm is a parameter that corresponds to the expected number of blocks per round, currently m=5m=5 in Filecoin. We then present an attack, the nn-split attack, where an adversary splits the honest miners between multiple chains, and show that it is successful for βm≥1−e−(1−β)m\beta m \ge 1- e^{-(1-\beta)m}, thus proving that βm=1−e−(1−β)m\beta m= 1- e^{-(1-\beta)m} is the tight security threshold of EC. This corresponds roughly to an adversary with 20%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

    Histoire politique et culturelle de la philosophie moderne

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    François Azouvi, directeur d’études Les philosophes français et la République, 1848-1900 Le séminaire a été consacré à l’étude des premières philosophies de la République dans la France révolutionnaire et post-révolutionnaire. Il a paru indispensable de revenir d’abord, et longuement, sur la personne et l’œuvre de Condorcet, même si elles sont l’une et l’autre bien connues. Mais c’est que Condorcet a posé les bases d’une philosophie rationnelle de la République qui laissera sa trace très dura..
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