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Front Matter, Table of Contents, Preface, Conference Organization, OASIcs, Vol. 110, 4th International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2022), pages 0:i-0:xii

OASIcs, Volume 110, Tokenomics 2022, Complete Volume, OASIcs, Vol. 110, 4th International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2022), pages 1-82

This paper presents Yggdrasil a sharding solution for permissionless blockchains that supports both payment transactions and general Ethereum-like smart contracts. Yggdrasil allows to split and merge shard dynamically leveraging decentralized mechanisms to assign nodes to shards in a secure way. A new 2PC protocol allows to guarantee the execution of smart contracts distributed across different shards even when shards dynamically re-organise. An experimental study confirms the capability of Yggdrasil to scale and to adapt to transaction load.

International audience; Nous étudions les comportements des participants d'un protocole de consensus lorsqu'ils présentent des comportements rationnels ou Byzantins. Nous nous inspirons des protocoles de blockchains tolérantes aux fautes Byzantines (comme Tendermint). Dans ces protocoles, les participants proposent des blocs et s'échangent des messages. Un bloc est accepté si une majorité de participants envoie le message correspondant à ce bloc (un vote), et les votants sont récompensés. Dans ce travail, nous étudions les conditions sous lesquelles ce protocole satisfait les deux propriétés suivantes : la terminaison (le système converge vers une décision) et la validité (toute décision est valide), quand certains participants sont rationnels et les autres Byzantins. Nous supposons que les participants Byzantins ont le comportement infligeant le plus de dégâts au système, tandis que les stratégies des participants rationnels forment un équilibre Bayésien parfait. Nous considérons les paramètres suivant : (i) le nombre de votes nécessaires, ν, pour qu'un bloc soit considéré comme accepté, et (ii) le nombre de participants Byzantins, noté f , dans le système. Nous obtenons les résultats suivants : Quand f ≥ ν, les blocs invalides sont acceptés, et donc la validité ne peut être garantie ; Quand f < ν, il existe un équilibre où la validité et la terminaison sont toutes deux satisfaites, par contre, il existe d'autres équilibres où la terminaison, et dans quelques cas la validité, ne sont pas satisfaites. Cela nous permet de conclure à l'existence de problèmes de coordination dans les protocoles étudiés.

Committee-based blockchains are among the most popular alternatives of proof-of-work based blockchains, such as Bitcoin. They provide strong consistency (no fork) under classical assumptions, and avoid using energy-consuming mechanisms to add new blocks in the blockchain. For each block, these blockchains use a committee that executes Byzantine-fault tolerant distributed consensus to decide the next block they will add in the blockchain. Unlike Bitcoin, where there is only one creator per block, in committee-based blockchain any block is cooperatively created. In order to incentivize committee members to participate in the creation of new blocks, rewarding schemes have to be designed. In this paper, we study the fairness of rewarding in committee-based blockchains and we provide necessary and sufficient conditions on the system communication under which it is possible to have a fair reward mechanism., OASIcs, Vol. 82, 2nd International Conference on Blockchain Economics, Security and Protocols (Tokenomics 2020), pages 4:1-4:15

International audience; Les blockchains basées sur le consensus sont considérées aujourd'hui comme étant parmi les alternatives les plus viables aux blockchains utilisant un mécanisme de Proof-of-work (Bitcoin, Ethereum,. . .), ces dernières étant très énergivores et ne garantissent pas une cohérence forte. Elles ont pour but d'offrir des garanties de cohérence forte (pas de fourches) dans un système ouvert grâce à : (i) un ensemble de validateurs qui produit un bloc via une variante du protocole de consensus Practical Byzantine Fault Tolerant (PBFT), et (ii) un mécanisme de sélection qui choisit dynamiquement les noeuds qui seront validateurs pour le bloc suivant. Dans cet article, nous caractérisons précisément le problème que tentent de résoudre ces protocoles de blockchains. Nous étudions Tendermint. Nos contributions sont les suivantes : nous formalisons pour la première fois le protocole Tendermint, puis nous présentons le modèle et les hypothèses précis sous lesquels il atteint son objectif. Nous prouvons que dans un système ultimement synchrone et avec une hypothèse supplémentaire, une légère modification du protocole résout une variante (i) du consensus pour la production d'un bloc, et une variante (ii) du consensus répété pour construire la chaîne de bloc ; cela si strictement moins d'un tiers des validateurs est atteint de fautes Byzantines. Nous nous sommes ensuite intéressés à l'étude de l'équité du mécanisme de récompense dans ces blockchains. Cette étude préliminaire permet d'établir que garantir (ultimement) l'équité de la récompense requiert une communication (ultimement) synchrone.

We study the rational behaviors of participants in committee-based blockchains. Committee-based blockchains rely on specific blockchain consensus that must be guaranteed in presence of rational participants. We consider a simplified blockchain consensus algorithm based on existing or proposed committee-based blockchains that encapsulate the main actions of the participants: voting for a block, and checking its validity. Knowing that those actions have costs, and achieving the consensus gives rewards to committee members, we study using game theory how strategic participants behave while trying to maximize their gains. We consider different reward schemes, and found that in each setting, there exist equilibria where blockchain consensus is guaranteed; in some settings however, there can be coordination failures hindering consensus. Moreover, we study equilibria with trembling participants, which is a novelty in the context of committee-based blockchains. Trembling participants are rational that can do unintended actions with a low probability. We found that in presence of trembling participants, there exist equilibria where blockchain consensus is guaranteed; however, when only voters are rewarded, there also exist equilibria where validity can be violated., LIPIcs, Vol. 184, 24th International Conference on Principles of Distributed Systems (OPODIS 2020), pages 12:1-12:16

Blockchains are one of the most appealing technologies over the last years, both for scientists and for the general public. Blockchains are distributed ledgers that aim to offer transparency, integrity and many more advantages over their centralised counterparts. Blockchains were “revealed” and became popular thanks to the creation and rise of the cryptocurrency Bitcoin. Over the years, blockchain technologies become more and more popular with an exceptional peak in 2017. Blockchains are becoming mainstream technologies, as there is an observatory for blockchains established by the European Commission, blockchain forums in many countries, blockchain start-ups are flourishing, scientific conferences are discussing the topic, and even some scientific conferences are now specifically dedicated to the technology, etc. The blockchain technology promises, thanks to its integrity and transparency properties to be useful and interesting in various domains, and not only for financial systems. However, many questions and doubts float around it. Is it environmentally viable? Is the technology even ensuring its promises? Can they be used in real-life settings, etc. In this thesis, using the lens of distributed systems, we study and define the properties of committee-based blockchains and their fairness; that definition allows formalising and helping correct one of the most used blockchain of that class. Furthermore, adding lenses from game theory, we propose a methodology to analyse the rational behaviours of participants in a blockchains system. Using that methodology, we show, under different mechanisms of rewards, the necessary conditions needed to ensure the blockchain properties.; Les blockchains (ou chaîne de blocs) font partie des technologies les plus attrayantes, autant dans le domaine scientifique que pour le grand public. Une blockchain est un registre distribué dont l’objectif est de garantir de la transparence, de l’intégrité des données et d’autres avantages par rapport aux registres centralisés. La technologie blockchain a été révélée et est devenue populaire grâce à l’avènement de la cybermonnaie Bitcoin. Depuis 2008 et la création de Bitcoin, les blockchains sont devenue extrêmement populaires, avec un intérêt à son pic en 2017. Les blockchains sont quasiment une technologie “à la mode”: il existe un observatoire de la blockchain initié par la Commission Européenne, des événements blockchains dans plusieurs pays, etc. La plupart des blockchains ont comme application la finance, mais les propriétés qu’elles promettent sont intéressantes et utiles pour bien des domaines (ex : médecine, notariat, etc.). Malgré les promesses et intérêts, afin d’espérer une meilleure adoption, il est nécessaire de s'assurer que la technologie est viable du point de vue environnemental, que les garanties promises sont satisfaites, qu'elle puisse être utilisée dans les environnements réels, etc. Dans cette thèse, nous définissons le problème résolu par les blockchains à comité ainsi que leur équité ; cela a permis de corriger une des blockchains les plus utilisées. Notre second apport majeur est une méthodologie permettant d’étudier le comportement des participants dans les blockchains. Grâce à cela, nous montrons les conditions nécessaires où le comportement rationnel des participants mène à des situations où les propriétés des blockchains sont satisfaites.

Blockchains are one of the most appealing technologies over the last years, both for scientists and for the general public. Blockchains are distributed ledgers that aim to offer transparency, integrity and many more advantages over their centralised counterparts. Blockchains were “revealed” and became popular thanks to the creation and rise of the cryptocurrency Bitcoin. Over the years, blockchain technologies become more and more popular with an exceptional peak in 2017. Blockchains are becoming mainstream technologies, as there is an observatory for blockchains established by the European Commission, blockchain forums in many countries, blockchain start-ups are flourishing, scientific conferences are discussing the topic, and even some scientific conferences are now specifically dedicated to the technology, etc. The blockchain technology promises, thanks to its integrity and transparency properties to be useful and interesting in various domains, and not only for financial systems. However, many questions and doubts float around it. Is it environmentally viable? Is the technology even ensuring its promises? Can they be used in real-life settings, etc. In this thesis, using the lens of distributed systems, we study and define the properties of committee-based blockchains and their fairness; that definition allows formalising and helping correct one of the most used blockchain of that class. Furthermore, adding lenses from game theory, we propose a methodology to analyse the rational behaviours of participants in a blockchains system. Using that methodology, we show, under different mechanisms of rewards, the necessary conditions needed to ensure the blockchain properties., Les blockchains (ou chaîne de blocs) font partie des technologies les plus attrayantes, autant dans le domaine scientifique que pour le grand public. Une blockchain est un registre distribué dont l’objectif est de garantir de la transparence, de l’intégrité des données et d’autres avantages par rapport aux registres centralisés. La technologie blockchain a été révélée et est devenue populaire grâce à l’avènement de la cybermonnaie Bitcoin. Depuis 2008 et la création de Bitcoin, les blockchains sont devenue extrêmement populaires, avec un intérêt à son pic en 2017. Les blockchains sont quasiment une technologie “à la mode”: il existe un observatoire de la blockchain initié par la Commission Européenne, des événements blockchains dans plusieurs pays, etc. La plupart des blockchains ont comme application la finance, mais les propriétés qu’elles promettent sont intéressantes et utiles pour bien des domaines (ex : médecine, notariat, etc.). Malgré les promesses et intérêts, afin d’espérer une meilleure adoption, il est nécessaire de s'assurer que la technologie est viable du point de vue environnemental, que les garanties promises sont satisfaites, qu'elle puisse être utilisée dans les environnements réels, etc. Dans cette thèse, nous définissons le problème résolu par les blockchains à comité ainsi que leur équité ; cela a permis de corriger une des blockchains les plus utilisées. Notre second apport majeur est une méthodologie permettant d’étudier le comportement des participants dans les blockchains. Grâce à cela, nous montrons les conditions nécessaires où le comportement rationnel des participants mène à des situations où les propriétés des blockchains sont satisfaites.

International audience; Tendermint-core blockchains (e.g. Cosmos) are considered today one of the most viable alternatives for the highly energy consuming proof-of-work blockchains such as Bitcoin and Ethereum. Their particularity is that they aim at offering strong consistency (no forks) in an open system combining two ingredients (i) a set of validators that generate blocks via a variant of Practical Byzantine Fault Tolerant (PBFT) consensus protocol and (ii) a selection strategy that dynamically selects nodes to be validators for the next block via a proof-of-stake mechanism. The exact assumptions on the system model under which Tendermint underlying algorithms are correct and the exact properties Tendermint verifies, however, have never been formally analyzed. The contribution of this paper is as follows. First, while formalizing Tendermint algorithms we precisely characterize the system model and the exact problem solved by Tendermint, then, we prove that in eventual synchronous systems a modified version of Tendermint solves (i) under additional assumptions, a variant of one-shot consensus for the validation of one single block and (ii) a variant of the repeated consensus problem for multiple blocks. These results hold even if the set of validators is hit by Byzantine failures, provided that for each one-shot consensus instance less than one third of the validators is Byzantine.

In this paper we address partial information games restricted to memoryless strategies.Our contribution is threefold. First, we prove that for partial information games, deciding the existence of memoryless strategies is NP-complete, even for games with only reachability objectives. %This result is interesting in itself since in the case of games with total information it is already proven that memory is not necessary to win the game. The second contribution of this paper is a SAT/SMT-based encoding of a partial information game altogether with the correctness proof of this encoding. Finally, we apply our methodology to automatically synthesize strategies for oblivious mobile robots. We first prove that synthesizing memoryless strategies is equivalent to providing a distributed protocol for the robots. Then, we use an SMT-solver to synthesize strategies for the gathering problem in a particular configuration ($SP_4$), open case in distributed computing theory for more than a decade. Interestingly, our work is the first that combines two-player games theory and SMT-solvers in the context of mobile robots with promising results and therefore it is highly valuable for distributed computing theory where a broad class of open problems are still to be investigated.

Tendermint-core blockchains (e.g. Cosmos) are considered today one of the most viable alternatives for the highly energy consuming proof-of-work blockchains such as Bitcoin and Ethereum. Their particularity is that they aim at offering strong consistency (no forks) in an open system combining two ingredients (i) a set of validators that generate blocks via a variant of Practical Byzantine Fault Tolerant (PBFT) consensus protocol and (ii) a selection strategy that dynamically selects nodes to be validators for the next block via a proof-of-stake mechanism. However,the exact assumptions on the system model under which Tendermint underlying algorithms are correct and the exact properties Tendermint verifies have never been formally analyzed. The contribution of this paper is two-fold. First, while formalizing Tendermint algorithms we precisely characterize the system model and the exact problem solved by Tendermint. We prove that in eventual synchronous systems a modified version of Tendermint solves (i) under additional assumptions, a variant of one-shot consensus for the validation of one single block and (ii) a variant of the repeated consensus problem for multiple blocks. These results hold even if the set of validators is hit by Byzantine failures, provided that for each one-shot consensus instance less than one third of the validators is Byzantine. Our second contribution relates to the fairness of the rewarding mechanism. It is common knowledge that in permisionless blockchain systems the main threat is the tragedy of commons that may yield the system to collapse if the rewarding mechanism is not adequate. Ad minimum the rewarding mechanism must be fair, i.e.distributing the rewards in proportion to the merit of participants. We prove, for the first time in blockchain systems, that in repeated-consensus based blockchains there exists an (eventual) fair rewarding mechanism if and only if the system is (eventual) synchronous. We also show that the original Tendermint rewarding is not fair, however, a modification of the original protocol makes it eventually fair.