Signature-Free Asynchronous Binary Byzantine Consensus with t<<n/3, O(n²) Messages, and O(1) Expected Time

International audienceThis paper is on broadcast and agreement in asynchronous message-passing systems made up of n processes, and where up to t processes may have a Byzantine Behavior. Its first contribution is a powerful , yet simple, all-to-all broadcast communication abstraction suited to binary values. This abstraction, which copes with up to t < n/3 Byzantine processes, allows each process to broadcast a binary value, and obtain a set of values such that (1) no value broadcast only by Byzantine processes can belong to the set of a correct process, and (2) if the set obtained by a correct process contains a single value v, then the set obtained by any correct process contains v. The second contribution of the paper is a new round-based asynchronous consensus algorithm that copes with up to t < n/3 Byzantine processes. This algorithm is based on the previous binary broadcast abstraction and a weak common coin. In addition of being signature-free and optimal with respect to the value of t, this consensus algorithm has several noteworthy properties: the expected number of rounds to decide is constant; each round is composed of a constant number of communication steps and involves O(n²) messages; each message is composed of a round number plus a constant number of bits. Moreover , the algorithm tolerates message reordering by the adversary (i.e., the Byzantine processes)

Time-Free Authenticated Byzantine Consensus

International audienceThis paper presents a simple protocol that solves the authenticated ByzantineConsensus problem in asynchronous distributed systems. To circumvent the FLP impossibility result in a deterministic way, synchrony assumptions should be added. In the context of Byzantine failures for systems where at most t processes may exhibit a Byzantine behavior and where not all the system is assumed eventually synchronous, Moumen et al. provide the main result. They assume at least one correct process, called 2t-bisource, connected with 2t privileged neighbors with eventually timely outgoing and incoming links. The present paper shows that a deterministic solution for the authenticated byzantine consensus problem is possible if the system model satisfies an additional assumption that does not rely on physical time but on the pattern of messages that are exchanged. The basic message exchange between processes is the query-response mechanism. To solve the Consensus problem, we assume a correct process p, called eventual 2t-winning process, and a set Q of 2t processes such that, eventually, for each query issued by p, any process q of Q receives a response from p among the (n-t) first responses to that query. The processes in the set Q can exhibit a Byzantine behavior and this set may change over time. Whereas many time-free solutions have been designed for the consensus problem in the crash model, this is, to our knowledge, the first time-free deterministic solution to the Byzantine consensus problem.Cet article présente un protocole de consensus dans un système asynchrone où un certain nombre de processus (au plus t) peuvent exhiber un comportement byzantin. Il est connu que ce problème n'a pas de solution dans un tel contexte c'est pour cela qu'il faut renforcer le système asynchrone de base avec des contraintes temporelles ou structurelles qui puisse rendre le problème décidable. La démarche choisie dans cet article puisque c'est la première fois que la contrainte rajoutée n'est pas temorelles mais repose sur la pattern de message échangé par les processus

Time-Free Authenticated Byzantine Consensus

International audienceThis paper presents a simple protocol that solves the authenticated ByzantineConsensus problem in asynchronous distributed systems. To circumvent the FLP impossibility result in a deterministic way, synchrony assumptions should be added. In the context of Byzantine failures for systems where at most t processes may exhibit a Byzantine behavior and where not all the system is assumed eventually synchronous, Moumen et al. provide the main result. They assume at least one correct process, called 2t-bisource, connected with 2t privileged neighbors with eventually timely outgoing and incoming links. The present paper shows that a deterministic solution for the authenticated byzantine consensus problem is possible if the system model satisfies an additional assumption that does not rely on physical time but on the pattern of messages that are exchanged. The basic message exchange between processes is the query-response mechanism. To solve the Consensus problem, we assume a correct process p, called eventual 2t-winning process, and a set Q of 2t processes such that, eventually, for each query issued by p, any process q of Q receives a response from p among the (n-t) first responses to that query. The processes in the set Q can exhibit a Byzantine behavior and this set may change over time. Whereas many time-free solutions have been designed for the consensus problem in the crash model, this is, to our knowledge, the first time-free deterministic solution to the Byzantine consensus problem.Cet article présente un protocole de consensus dans un système asynchrone où un certain nombre de processus (au plus t) peuvent exhiber un comportement byzantin. Il est connu que ce problème n'a pas de solution dans un tel contexte c'est pour cela qu'il faut renforcer le système asynchrone de base avec des contraintes temporelles ou structurelles qui puisse rendre le problème décidable. La démarche choisie dans cet article puisque c'est la première fois que la contrainte rajoutée n'est pas temorelles mais repose sur la pattern de message échangé par les processus

Modular randomized byzantine k-set agreement in asynchronous message-passing systems

International audiencek-Set agreement is a central problem of fault-tolerant distibuted computing. Considering a set of n processes, where up to t may commit failures, let us assume that each process proposes a value. The problem consists in defining an algorithm such that each non-faulty process decides a value, at most k dfferent values are decided, and the decided values satisfy some context-depending validity condition. Synchronous message-passing algorithms solving k-set agreement have been proposed for different failure models (mainly process crashes, and process Byzantine failures). Differently, k-set agreement cannot be solved in failure-prone asynchronous message-passing systems when t ≥ k. To circumvent this impossibility an asynchronous system must be enriched with additional computational power. Assuming t ≥ k, this paper presents a distributed algorithm that solves k-set agreement in an asynchronous message-passing system wher up to t processes may commit Byzantine failures. To that end, each process is enriched with randomization power. While randomized k-set agreement algorithms exist for the asynchronous process crash failure model where t ≥ k, to our knowledge the proposed algorithm is the first that solves k-set agreement in the presence of up to t ≥ k Byzantine processes. Interestingly, this algorithm is signature-free, and ensures that no value proposed only by Byzantine processes can be decided by a non-faulty process. Its design is based on a modular construction which rests on a "no-duplicity" one-to-all broadcast abstraction, and two all-to-all communication abstractions

Randomized k -set agreement in crash-prone and Byzantine asynchronous systems

International audienc

Byzantine Consensus with Few Synchronous Links

International audienc

Tracking COVID-19 by Tracking Infectious Trajectories

Nowadays, the coronavirus pandemic has and is still causing large numbers of deaths and infected people. Although governments all over the world have taken severe measurements to slow down the virus spreading (e.g., travel restrictions, suspending all sportive, social, and economic activities, quarantines, social distancing, etc.), a lot of persons have died and a lot more are still in danger. Indeed, a recently conducted study [1] has reported that 79% of the confirmed infections in China were caused by undocumented patients who had no symptoms. In the same context, in numerous other countries, since coronavirus takes several days before the emergence of symptoms, it has also been reported that the known number of infections is not representative of the real number of infected people (the actual number is expected to be much higher). That is to say, asymptomatic patients are the main factor behind the large quick spreading of coronavirus and are also the major reason that caused governments to lose control over this critical situation. To contribute to remedying this global pandemic, in this article, we propose an IoTa investigation system that was specifically designed to spot both undocumented patients and infectious places. The goal is to help the authorities to disinfect high-contamination sites and confine persons even if they have no apparent symptoms. The proposed system also allows determining all persons who had close contact with infected or suspected patients. Consequently, rapid isolation of suspicious cases and more efficient control over any pandemic propagation can be achieved