Romaric Ludinard

PeerCube: A Hypercube-Based P2P Overlay Robust against Collusion and Churn

This paper presents PeerCube, a DHT-based system aiming at minimizing performance penalties caused by high churn while preventing malicious peers from subverting the system through collusion. This is achieved by i) applying a clustering strategy to support quorum-based operations; ii) using a randomized insertion algorithm to reduce the probability with which colluding Byzantine peers corrupt clusters, and; iii) leveraging on the properties of PeerCubes hypercube structure to allow operations to be successfully handled despite the corruption of some clusters. In spite of a powerful adversary that can inspect the whole system and issue malicious join requests as often as it wishes, PeerCube guarantees robust operations in O(logN) messages, with N the number of peers in the system. Extended simulations validate PeerCube robustness.

Safety analysis of Bitcoin improvement proposals

Decentralized cryptocurrency systems offer a medium of exchange secured by cryptography, without the need of a centralized banking authority. Among others, Bitcoin is considered as the most mature one. Its popularity lies on the introduction of the concept of the blockchain, a public distributed ledger shared by all participants of the system. Double spending attacks and blockchain forks are two main issues in blockchain-based protocols. The first one refers to the ability of an adversary to use the very same bitcoin more than once, while blockchain forks cause transient inconsistencies in the blockchain. We show through probabilistic analysis that the reliability of recent solutions that exclusively rely on a particular type of Bitcoin actors, called miners, to guarantee the consistency of Bitcoin operations, drastically decreases with the size of the blockchain.

DEPENDABILITY EVALUATION OF CLUSTER-BASED DISTRIBUTED SYSTEMS

Awerbuch and Scheideler have shown that peer-to-peer overlay networks can survive Byzantine attacks only if malicious nodes are not able to predict what will be the topology of the network for a given sequence of join and leave operations. In this paper we investigate adversarial strategies by following specific protocols. Our analysis demonstrates first that an adversary can very quickly subvert overlays based on distributed hash tables by simply never triggering leave operations. We then show that when all nodes (honest and malicious ones) are imposed on a limited lifetime, the system eventually reaches a stationary regime where the ratio of polluted clusters is bounded, independently from the initial amount of corruption in the system.

Handling bitcoin conflicts through a glimpse of structure

Double spending and blockchain forks are two main issues that the Bitcoin crypto-system is confronted with. The former refers to an adversarys ability to use the very same coin more than once while the latter reflects the occurrence of transient inconsistencies in the history of the blockchain distributed data structure. We present a new approach to tackle these issues: it consists in adding some local synchronization constraints on Bitcoins validation operations, and in making these constraints independent from the native blockchain protocol. Synchronization constraints are handled by nodes which are randomly and dynamically chosen in the Bitcoin system. We show that with such an approach, content of the blockchain is consistent with all validated transactions and blocks which guarantees the absence of both double-spending attacks and blockchain forks.

Bitcoin a Distributed Shared Register

Distributed Ledgers (e.g. Bitcoin) occupy currently the first lines of the economical and political media and many speculations are done with respect to their level of coherence and their computability power. Interestingly, there is no consensus on the properties and abstractions that fully capture the behaviour of distributed ledgers. The interest in formalising the behaviour of distributed ledgers is twofold. Firstly, it helps to prove the correctness of the algorithms that implement existing distributed ledgers and explore their limits with respect to an unfriendly environment and target applications. Secondly, it facilitates the identification of the minimal building blocks necessary to implement the distributed ledger in a specific environment.

Brief Announcement: Distributed Ledger Technology meets Distributed Shared Register Theory

This paper introduces Distributed Ledger Register that mimics the behavior of most popular ledgers such as Bitcoin or Ethereum. Our work is the first to make the connection between the Distributed Ledger Register and the classical theory of shared registers. We furthermore, propose an algorithm that emulates the distributed ledger register.We study a natural extension to the well-known convex hull problem by introducing multiplicity: if we are given a set of convex polygons, and we are allowed to partition the set into multiple components and take the convex hull of each individual component, what is the minimum total sum of the perimeters of the convex hulls? We show why this problem is intriguing, and then introduce a novel algorithm with a run-time cubic in the total number of vertices. In the case that the input polygons are disjoint, we show an optimization that achieves a run-time that, in most cases, is cubic in the total number of polygons, within a logarithmic factor.The knapsack problem is a classic optimisation problem that has been recently extended in the setting of groups. Its study reveals to be interesting since it provides many different behaviours, depending on the considered class of groups. In this paper we deal with groups generated by Mealy automata—a class that is often used to study group-theoretical conjectures—and prove that the knapsack problem is undecidable for this class. In a second time, we construct a graph that, if finite, provides a solution to the knapsack problem. We deduce that the knapsack problem is decidable for the so-called bounded automaton groups, a class where the order and conjugacy problems are already known to be decidable.