arXiv subjects

Emmanuelle Anceaume

Publications and source records attributed to Emmanuelle Anceaume.

12 recordsLinked to original sources

arXiv2025

Ethical Risk Analysis of L2 Rollups

Layer 2 rollups improve throughput and fees, but can reintroduce risk through operator discretion and information asymmetry. We ask which operator and governance designs produce ethically problematic user risk. We adapt Ethical Risk Analysis to rollup architectures, build a role-based taxonomy of decision authority and exposure, and pair the framework with two empirical signals, a cross sectional snapshot of 129 projects from L2BEAT and a hand curated incident set covering 2022 to 2025. We analyze mechanisms that affect risks to users funds, including upgrade timing and exit windows, proposer liveness and whitelisting, forced inclusion usability, and data availability choices. We find that ethical hazards rooted in L2 components control arrangements are widespread: instant upgrades without exit windows appear in about 86 percent of projects, and proposer controls that can freeze withdrawals in about 50 percent. Reported incidents concentrate in sequencer liveness and inclusion, consistent with these dependencies. We translate these findings into ethically grounded suggestions on mitigation strategies including technical components and governance mechanisms.

cs.DC↗

arXiv2025

Asynchronous BFT Asset Transfer: Quasi-Anonymous, Light, and Consensus-Free

This paper introduces a new asynchronous Byzantine-tolerant asset transfer system (cryptocurrency) with three noteworthy properties: quasi-anonymity, lightness, and consensus-freedom. Quasi-anonymity means no information is leaked regarding the receivers and amounts of the asset transfers. Lightness means that the underlying cryptographic schemes are \textit{succinct} (\textit{i.e.}, they produce short-sized and quickly verifiable proofs) and each process only stores its own transfers while keeping communication cost as low as possible. Consensus-freedom means the system does not rely on a total order of asset transfers. The proposed algorithm is the first asset transfer system that simultaneously fulfills all these properties in the presence of asynchrony and Byzantine processes. To obtain them, the paper adopts a modular approach combining a new distributed object called ``agreement proof'' and well-known techniques such as commitments, universal accumulators, and zero-knowledge proofs.

cs.DC↗

arXiv2020

On Finality in Blockchains

There exist many forms of Blockchain finality conditions, from deterministic to probabilistic terminations. To favor availability against consistency in the face of partitions, most blockchains only offer probabilistic eventual finality: blocks may be revoked after being appended to the blockchain, yet with decreasing probability as they sink deeper into the chain. Other blockchains favor consistency by leveraging the immediate finality of Consensus-a block appended is never revoked-at the cost of additional synchronization. In this paper, we focus on necessary and sufficient conditions to implement a blockchain with deterministic eventual finality, which ensures that selected main chains at different processes share a common increasing prefix. This is a much weaker form of finality that allows us to provide a solution in an asynchronous system subject to unlimited number of byzantine failures. We study stronger forms of eventual finality as well and show that it is unfortunately impossible to provide a bounded displacement. By bounded displacement we mean that the (unknown) number of blocks that can be revoked from the current blockchain is bounded. This problem reduces to consensus or eventual consensus depending on whether the bound is known or not. We also show that the classical selection mechanism, such as in Bitcoin, that appends blocks at the longest chain is not compliant with a solution to eventual finality.

cs.DC↗

arXiv2020

Byzantine Generalized Lattice Agreement

The paper investigates the Lattice Agreement (LA) problem in asynchronous systems. In LA each process proposes an element $e$ from a predetermined lattice, and has to decide on an element $e'$ of the lattice such that $e \leq e'$. Moreover, decisions of different processes have to be comparable (no two processes can decide two elements $e$ and $e'$ such that $(e \not\leq e') \land (e' \not\leq e)$). It has been shown that Generalized LA (i.e., a version of LA proposing and deciding on sequences of values) can be used to build a Replicated State Machine (RSM) with commutative update operations. The key advantage of LA and Generalized LA is that they can be solved in asynchronous systems prone to crash-failures (this is not the case with standard Consensus). In this paper we assume Byzantine failures. We propose the Wait Till Safe (WTS) algorithm for LA, and we show that its resilience to $f \leq (n-1)/3$ Byzantines is optimal. We then generalize WTS obtaining a Generalized LA algorithm, namely GWTS. We use GWTS to build a RSM with commutative updates. Our RSM works in asynchronous systems and tolerates $f \leq (n-1)/3$ malicious entities. All our algorithms use the minimal assumption of authenticated channels. When the more powerful public signatures are available, we discuss how to improve the message complexity of our results (from quadratic to linear, when $f={\cal O}(1)$). At the best of our knowledge this is the first paper proposing a solution for Byzantine LA that works on any possible lattice, and it is the first work proposing a Byzantine tolerant RSM built on it.

cs.DC↗

arXiv2020

Synchronous Byzantine Lattice Agreement in ${\cal O}(\log (f))$ Rounds

In the Lattice Agreement (LA) problem, originally proposed by Attiya et al. \cite{Attiya:1995}, a set of processes has to decide on a chain of a lattice. More precisely, each correct process proposes an element $e$ of a certain join-semi lattice $L$ and it has to decide on a value that contains $e$. Moreover, any pair $p_i,p_j$ of correct processes has to decide two values $dec_i$ and $dec_j$ that are comparable (e.g., $dec_i \leq dec_j$ or $dec_j < dec_i$). LA has been studied for its practical applications, as example it can be used to implement a snapshot objects \cite{Attiya:1995} or a replicated state machine with commutative operations \cite{Faleiro:2012}. Interestingly, the study of the Byzantine Lattice Agreement started only recently, and it has been mainly devoted to asynchronous systems. The synchronous case has been object of a recent pre-print \cite{Zheng:aa} where Zheng et al. propose an algorithm terminating in ${\cal O}(\sqrt f)$ rounds and tolerating $f < \lceil n/3 \rceil$ Byzantine processes. In this paper we present new contributions for the synchronous case. We investigate the problem in the usual message passing model for a system of $n$ processes with distinct unique IDs. We first prove that, when only authenticated channels are available, the problem cannot be solved if $f=n/3$ or more processes are Byzantine. We then propose a novel algorithm that works in a synchronous system model with signatures (i.e., the {\em authenticated message} model), tolerates up to $f$ byzantine failures (where $f<n/3$) and that terminates in ${\cal O}(\log f)$ rounds. We discuss how to remove authenticated messages at the price of algorithm resiliency ($f < n/4$). Finally, we present a transformer that converts any synchronous LA algorithm to an algorithm for synchronous Generalised Lattice Agreement.

cs.DC↗

arXiv2019

StakeCube: Combining Sharding and Proof-of-Stake to build Fork-free Secure Permissionless Distributed Ledgers

Our work focuses on the design of a scalable permissionless blockchain in the proof-of-stake setting. In particular, we use a distributed hash table as a building block to set up randomized shards, and then leverage the sharded architecture to validate blocks in an efficient manner. We combine verifiable Byzantine agreements run by shards of stakeholders and a block validation protocol to guarantee that forks occur with negligible probability. We impose induced churn to make shards robust to eclipse attacks, and we rely on the UTXO coin model to guarantee that any stakeholder action is securely verifiable by anyone. Our protocol works against adaptive adversary, and makes no synchrony assumption beyond what is required for the byzantine agreement.

cs.DC↗

arXiv2018

Blockchain Abstract Data Type

The presented work continues the line of recent distributed computing communityefforts dedicated to the theoretical aspects of blockchains. This paper is the rst tospecify blockchains as a composition of abstract data types all together with a hierarchyof consistency criteria that formally characterizes the histories admissible for distributedprograms that use them. Our work is based on an original oracle-based constructionthat, along with new consistency deffnitions, captures the eventual convergence processin blockchain systems. The paper presents as well some results on implementability ofthe presented abstractions and a mapping of representative existing blockchains fromboth academia and industry in our framework.

cs.DC↗

arXiv2015

Optimization results for a generalized coupon collector problem

We study in this paper a generalized coupon collector problem, which consists in analyzing the time needed to collect a given number of distinct coupons that are drawn from a set of coupons with an arbitrary probability distribution. We suppose that a special coupon called the null coupon can be drawn but never belongs to any collection. In this context, we prove that the almost uniform distribution, for which all the non-null coupons have the same drawing probability, is the distribution which stochastically minimizes the time needed to collect a fixed number of distinct coupons. Moreover, we show that in a given closed subset of probability distributions, the distribution with all its entries, but one, equal to the smallest possible value is the one, which stochastically maximizes the time needed to collect a fixed number of distinct coupons. An computer science application shows the utility of these results.

cs.DS↗

arXiv2014

New results on a generalized coupon collector problem using Markov chains

We study in this paper a generalized coupon collector problem, which consists in determining the distribution and the moments of the time needed to collect a given number of distinct coupons that are drawn from a set of coupons with an arbitrary probability distribution. We suppose that a special coupon called the null coupon can be drawn but never belongs to any collection. In this context, we obtain expressions of the distribution and the moments of this time. We also prove that the almost-uniform distribution, for which all the non-null coupons have the same drawing probability, is the distribution which minimizes the expected time to get a fixed subset of distinct coupons. This optimization result is extended to the complementary distribution of that time when the full collection is considered, proving by the way this well-known conjecture. Finally, we propose a new conjecture which expresses the fact that the almost-uniform distribution should minimize the complementary distribution of the time needed to get any fixed number of distinct coupons.

math.PR↗

arXiv2012

Sketch \star-metric: Comparing Data Streams via Sketching

In this paper, we consider the problem of estimating the distance between any two large data streams in small- space constraint. This problem is of utmost importance in data intensive monitoring applications where input streams are generated rapidly. These streams need to be processed on the fly and accurately to quickly determine any deviance from nominal behavior. We present a new metric, the Sketch \star-metric, which allows to define a distance between updatable summaries (or sketches) of large data streams. An important feature of the Sketch \star-metric is that, given a measure on the entire initial data streams, the Sketch \star-metric preserves the axioms of the latter measure on the sketch (such as the non-negativity, the identity, the symmetry, the triangle inequality but also specific properties of the f-divergence). Extensive experiments conducted on both synthetic traces and real data allow us to validate the robustness and accuracy of the Sketch \star-metric.

cs.DS↗

arXiv2010

A framework for proving the self-organization of dynamic systems

This paper aims at providing a rigorous definition of self- organization, one of the most desired properties for dynamic systems (e.g., peer-to-peer systems, sensor networks, cooperative robotics, or ad-hoc networks). We characterize different classes of self-organization through liveness and safety properties that both capture information re- garding the system entropy. We illustrate these classes through study cases. The first ones are two representative P2P overlays (CAN and Pas- try) and the others are specific implementations of Ω(the leader oracle) and one-shot query abstractions for dynamic settings. Our study aims at understanding the limits and respective power of existing self-organized protocols and lays the basis of designing robust algorithm for dynamic systems.

cs.DC↗

arXiv2009

Analytical Study of Adversarial Strategies in Cluster-based Overlays

Scheideler has shown that peer-to-peer overlays networks can only survive Byzantine attacks if malicious nodes are not able to predict what is going to 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 games. Our analysis demonstrates first that an adversary can very quickly subvert DHT-based overlays 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.

cs.GT↗