Natural Strategic Ability

International audienc

Dynamic escape game

We introduce Dynamic Escape Game (DEC), a tool that provides emergency evacuation plans in situations where some of the escape paths may become unavailable at runtime. We formalize the setting as a reachability two-player turn-based game where the universal player has the power of inhibiting at runtime some moves to the existential player. Thus, the universal player can change the structure of the game arena along a play. DEC uses a graphical interface to depict the game and displays a winning play whenever it exists

Natural Strategic Abilities in Voting Protocols

Security properties are often focused on the technological side of the system. One implicitly assumes that the users will behave in the right way to preserve the property at hand. In real life, this cannot be taken for granted. In particular, security mechanisms that are difficult and costly to use are often ignored by the users, and do not really defend the system against possible attacks. Here, we propose a graded notion of security based on the complexity of the user's strategic behavior. More precisely, we suggest that the level to which a security property $\varphi$ is satisfied can be defined in terms of (a) the complexity of the strategy that the voter needs to execute to make $\varphi$ true, and (b) the resources that the user must employ on the way. The simpler and cheaper to obtain $\varphi$, the higher the degree of security. We demonstrate how the idea works in a case study based on an electronic voting scenario. To this end, we model the vVote implementation of the \Pret voting protocol for coercion-resistant and voter-verifiable elections. Then, we identify "natural" strategies for the voter to obtain receipt-freeness, and measure the voter's effort that they require. We also look at how hard it is for the coercer to compromise the election through a randomization attack

Strategy Logic with Simple Goals: Tractable Reasoning about Strategies

peer reviewe

Enforcing equilibria in multi-agent systems

We introduce and investigate Normative Synthesis: a new class of problems for the equilibrium verification that counters the absence of equilibria by purposely constraining multi-agent systems. We show that norms are powerful enough to ensure a positive answer to every instance of the equilibrium verification problem. Subsequently, we focus on two optimization versions, that aim at providing a solution in compliance with implementation costs. We show that the complexities of our procedures range between 2exptime and 3exptime, thus that the problems are no harder than the corresponding equilibrium verification ones

Strategic Reasoning in Game Theory

Game theory in AI is a powerful mathematical framework largely applied in the last three decades for the strategic reasoning in multi-agent systems. Seminal works along this line started with turn-based two-player games (under perfect and imperfect information) to check the correctness of a system against an unpredictable environment. Then, large effort has been devoted to extend those works to the multi-agent setting and, specifically, to efficiently reasoning about important solution concepts such as Nash Equilibria and the like. Breakthrough contributions along this direction concern the introduction of logics for the strategic reasoning such as Alternating-time Temporal Logic (ATL), Strategy Logic (SL), and their extensions. Two-player games and logics for the strategic reasoning are nowadays very active areas of research. In this thesis we significantly advance the work along both these two lines of research by providing fresh studies and results of practical application. We start with two-player reachability games and first investigate the problem of checking whether a designed player has more than a winning strategy to reach a target. We investigate this question under both perfect and imperfect information. We provide an automata-based solution that requires linear-time, in the perfect information setting, and exponential-time, in the imperfect one. In both cases, the results are tight. Then, we move to multi-player concurrent games and study the following specific setting: (i) Player_0's objective is to reach a target W, and (ii) the opponents are trying to stop this but have partial observation about Player_0's actions. We study the problem of deciding whether the opponents can prevent Player_0 to reach W. We show, using an automata-theoretic approach that, assuming the opponents have the same partial observation and play under uniformity, the problem is in ExpTime. We recall that, in general, multi-player reachability games with imperfect information are undecidable. Then, we move to the more expressive framework of logics for the strategic reasoning. We first introduce and study two graded extensions of SL, namely GSL and GradedSL. By the former, we define a graded version over single strategy variables, i.e. "there exist at least g different strategies", where the strategies are counted semantically. We study the model checking-problem for GSL and show that for its fragment called vanilla GSL[1G] the problem is PTime-complete. By GradedSL, we consider a graded version over tuple of strategy variables and use a syntactic counting over strategies. By means of GradedSL we show how to count the number of different strategy profiles that are Nash equilibria (NE). By analyzing the structure of the specific formulas involved, we conclude that the important problem of checking for the existence of a unique NE can be solved in 2ExpTime, which is not harder than merely checking for the existence of such an equilibrium. Finally, we adopt the view of bounded rationality, and look only at "simple" strategies in specifications of agents’ abilities. We formally define what "simple" means, and propose a variant of plain ATL, namely NatATL, that takes only such strategies into account. We study the model checking problem for the resulting semantics of ability and obtain tight results. The positive results achieved with NatATL encourage for the investigation of simple strategies over more powerful logics, including SL

Logiques pour la représentation et la conception d'enchères

Une enchère est un mécanisme compétitif qui permet d’allouer un ensemble de ressources auprès d’un ensemble d’agents. Ce mécanisme agrège les offres effectuées par les participants à l’enchère dans le but de produire une décision sociale caractérisée en termes d'allocations et de paiements. Les agents automatisés sont largement utilisés sur les marchés basés sur les enchères, mais ils sont généralement conçus pour agir dans un contexte spécifique. Pour passer d'un type de marché à un autre, les agents doivent être capables de "comprendre" les règles de l'enchère et de raisonner sur leurs valuations ainsi que sur les valeurs privées des autres joueurs. De fait, il est nécessaire de définir un langage simple permettant de représenter les règles d'un marché aux enchères, qui permettra ensuite à des joueurs généraux automatiques de raisonner stratégiquement dans différents environnements. Cet aspect stratégique, notion centrale de la théorie des jeux et des systèmes multi-agents, est de première importance dans la caractérisation des mécanismes d’enchères. Un problème de première importance concerne la conception de nouvelles enchères, ou plus généralement, de nouveaux mécanismes. En effet, un des principaux objectifs consiste à agréger les offres individuelles tout en garantissant un résultat socialement souhaitable. La dimension stratégique est donc au cœur de la conception de mécanismes. Alors que les langages logiques ont été largement considérés dans le contexte des Systèmes Multi-Agents (SMA), l'utilisation de méthodes formelles et de raisonnement stratégique pour la Conception Automatique de Mécanismes a été à peine étudiée. Cette thèse explore l'application des logiques pour la description et la conception de mécanismes d’enchères. Ces derniers placent la dimension stratégique en leur cœur et nous proposons l'utilisation de méthodes formelles pour la spécification, la conception et l'évaluation de mécanismes intégrant cette dimension. Dans un premier temps, afin de fournir une fondation pour les joueurs d'enchères généraux et automatisés dans les SMA, nous proposons un formalisme pour représenter les enchères, appelé Auction Description Language (ADL). ADL traite des dimensions importantes des marchés basés sur des enchères et est suffisamment général pour représenter la plupart des contextes d'enchères. Nous montrons qu’en enrichissant ADL avec un opérateur de connaissance et une modalité d'action pour caractériser le comportement rationnel limité des enchérisseurs, les agents enchérisseurs peuvent raisonner sur l'effet des actions ainsi que sur la rationalité des autres agents. Dans un second temps, nous proposons une nouvelle approche pour le raisonnement et la conception de nouvelles enchères basée sur des méthodes formelles. Cette approche vise à générer des mécanismes à partir de leurs spécifications et à les vérifier par rapport à des propriétés économiques objectives. Nous proposons une nouvelle variante de Strategy Logic (SL) avec une sémantique quantitative et des opérateurs épistémiques. Nous montrons comment elle permet d'exprimer des concepts essentiels de la théorie de l'économie, notamment l'équilibre de Nash et la manipulation stratégique, qui sont de première importance lors de la conception de nouvelles enchères et lorsque les agents doivent raisonner sur leurs propriétés. Nous introduisons aussi SL avec des stratégies naturelles, qui permet de raisonner sur les mécanismes en fonction de la complexité des stratégies. L'analyse des mécanismes et des stratégies se résume donc à la vérification de formule en SL dans des modèles représentant des enchères. Enfin nous proposons la reformulation du problème de la conception de mécanismes en termes de synthèse de spécifications logiques. Cette approche permet de générer automatiquement des mécanismes optimaux à partir d'une spécification, qui peut inclure non seulement les règles du jeu mais aussi des exigences sur le comportement stratégique des participants.An auction is a popular mechanism that aggregates participants' bids into a social decision, usually expressed in terms of allocations and payments. Automated agents are widely used in auction-based markets, but they are usually designed to act on a specific context. Those agents cannot switch between different kinds of markets. For doing so, they should be able to ``understand'' the auction rules and reason about their own valuations and about other players’ private information valuations. This limitation inspires the development of a lightweight logic-based language for representing the rules of an auction market, which will then allow automated general players to reason strategically in different environments. Another important problem is the design of new auctions and, more generally, mechanisms. The challenge here is to aggregate individual preferences, while choosing a socially desirable outcome and reaching an equilibrium even though agents can lie about their preferences. Although logic-based languages have been widely considered in the context of Multi-Agent Systems (MAS), the use of formal methods and strategic reasoning for Automated Mechanism Design (AMD) has not been much explored yet. This thesis investigates an application of logics and strategic reasoning for Game Theory and MAS. In particular, we propose the use of formal methods for the specification, design and evaluation of mechanisms, with focus on auctions. This thesis addresses such challenges by introducing logic-based approaches for representing and designing auction-based markets with strategic players. Firstly, for providing a foundation for general and automated auction playing in MAS, we propose a framework for representing auctions, denoted Auction Description Language (ADL). ADL addresses important dimensions of auction-based markets and is general enough to represent most auction settings. We illustrate the generality of ADL by modeling a number of representative auctions. We extend ADL with knowledge operators and an action modality for characterizing bounded rational behavior of bidders when reasoning about the effect of actions and other agents' rationality. Second, we propose a novel approach for reasoning and designing new auctions (and, in general, preference aggregation mechanisms) based on formal methods. Such approach for AMD aims to automatically generate mechanisms from their specifications and verify them in relation to target economical properties. For verifying mechanisms, we propose a new variant of Strategy Logic (SL) with quantitative semantics and epistemic operators. We demonstrate how it can express key concepts from Economic Theory, including Nash equilibrium, strategyproofness and individual rationality, which are at first importance when designing new auctions and when agents need to reason about their properties. We also introduce a quantitative semantics for SL with natural strategies and imperfect information which enables reasoning about mechanisms based on the complexity of strategies. The analysis of mechanisms and their strategies boils-down to model checking formulas from those SL variants. Finally, we offer a novel perspective on the design of mechanisms by rephrasing the AMD problem in terms of synthesis from specifications in Quantitative SL. This approach enables automatically generating optimal mechanisms from a quantitative logical specification, which may include not only game rules but also requirements over the strategic behavior of participants and quality of the outcome

Reasoning about natural strategic ability

In game theory, as well as in the semantics of game logics, a strategy can be represented by any function from states of the game to the agent's actions. That makes sense from the mathematical point of view, but not necessarily in the context of human behavior. This is because humans are quite bad at executing complex plans, and also rather unlikely to come up with such plans in the first place. In this paper, we adopt the view of bounded rationality, and look only at "simple" strategies in specifications of agents' abilities. We formally define what "simple" means, and propose a variant of alternating-Time temporal logic that takes only such strategies into account. We also study the model checking problem for the resulting semantics of ability

Natural strategic ability under imperfect information

Strategies in game theory and multi-agent logics are mathematical objects of remarkable combinatorial complexity Recently, the concept of natural strategies has been proposed to model more human-like reasoning about simple plans and their outcomes So far, the theory of such simple strategic play was only considered in scenarios where all the agents have perfect information about the state of the game In this paper, we extend the notion of natural strategies to games with imperfect information We also show that almost all the complexity results for model checking carry over from the perfect to imperfect information setting That is, verification of natural strategies is usually no more complex for agents with uncertainty This tells games of natural strategic ability clearly apart from most results in game theory and multi-agent logics. © 2019 International Foundation for Autonomous Agents and Multiagent Systems (www.ifaamas.org) Ail rights reserved