Jason Tsai

Computing optimal randomized resource allocations for massive security games

Predictable allocations of security resources such as police officers, canine units, or checkpoints are vulnerable to exploitation by attackers. Recent work has applied game-theoretic methods to find optimal randomized security policies, including a fielded application at the Los Angeles International Airport (LAX). This approach has promising applications in many similar domains, including police patrolling for subway and bus systems, randomized baggage screening, and scheduling for the Federal Air Marshal Service (FAMS) on commercial flights. However, the existing methods scale poorly when the security policy requires coordination of many resources, which is central to many of these potential applications. We develop new models and algorithms that scale to much more complex instances of security games. The key idea is to use a compact model of security games, which allows exponential improvements in both memory and runtime relative to the best known algorithms for solving general Stackelberg games. We develop even faster algorithms for security games under payoff restrictions that are natural in many security domains. Finally, introduce additional realistic scheduling constraints while retaining comparable performance improvements. The empirical evaluation comprises both random data and realistic instances of the FAMS and LAX problems. Our new methods scale to problems several orders of magnitude larger than the fastest known algorithm.

Security and Game Theory: IRIS – A Tool for Strategic Security Allocation in Transportation Networks

Security is a concern of major importance to governments and companies throughout the world. With limited resources, complete coverage of potential points of attack is not possible. Deterministic allocation of available law enforcement agents introduces predictable vulnerabilities that can be exploited by adversaries. Strategic randomization is a game theoretic alternative that we implement in Intelligent Randomization In Scheduling (IRIS) system, a software scheduling assistant for the Federal Air Marshals (FAMs) that provide law enforcement aboard U.S. commercial flights. In IRIS, we model the problem as a Stackelberg game, with FAMS as leaders that commit to a flight coverage schedule and terrorists as followers that attempt to attack a flight. The FAMS domain presents three challenges unique to transportation network security that we address in the implementation of IRIS. First, with tens of thousands of commercial flights per day, the size of the Stackelberg game we need to solve is tremendous. We use ERASERC, the fastest known algorithm for solving this class of Stackelberg games. Second, creating the game itself becomes a challenge due to number of payoffs we must enter for these large games. To address this, we create an attribute-based preference elicitation system to determine reward values. Third, the complex scheduling constraints in transportation networks make it computationally prohibitive to model the game by explicitly modeling all combinations of valid schedules. Instead, we model the leader’s strategy space by incorporating a representation of the underlying scheduling constraints. The scheduling assistant has been delivered to the FAMS and is currently undergoing testing and review for possible incorporation into their scheduling practices. In this paper, we discuss the design choices and challenges encountered during the implementation of IRIS.

Software Assistants for Randomized Patrol Planning for the LAX Airport Police and the Federal Air Marshal Service

The increasing threat of terrorism makes security at major locations of economic or political importance a major concern. Limited security resources prevent complete security coverage, allowing adversaries to observe and exploit patterns in patrolling or monitoring, and enabling them to plan attacks that avoid existing patrols. The use of randomized security policies that are more difficult for adversaries to predict and exploit can counter their surveillance capabilities. We describe two applications, ARMOR and IRIS, that assist security forces in randomizing their operations. These applications are based on fast algorithms for solving large instances of Bayesian Stackelberg games. Police at the Los Angeles International Airport deploy ARMOR to randomize the placement of checkpoints on roads entering the airport and the routes of canine unit patrols within the airport terminals. The Federal Air Marshal Service has deployed IRIS in a pilot program to randomize the schedules of air marshals on international flights. This paper examines the design choices, information, and evaluation criteria that were critical to developing these applications.

Empirical evaluation of computational emotional contagion models

In social psychology, emotional contagion describes the widely observed phenomenon of one persons emotions being influenced by surrounding peoples emotions. While the overall effect is agreed upon, the underlying mechanism of the spread of emotions has seen little quantification and application to computational agents despite extensive evidence of its impacts in everyday life. In this paper, we examine computational models of emotional contagion by implementing two models ([2] and [8]) that draw from two separate lines of contagion research: thermodynamics-based and epidemiological-based. We first perform sensitivity tests on each model in an evacuation simulation, ESCAPES, showing both models to be reasonably robust to parameter variations with certain exceptions. We then compare their ability to reproduce a real crowd panic scene in simulation, showing that the thermodynamics-style model ([2]) produces superior results due to the ill-suited contagion mechanism at the core of epidemiological models. We also identify that a graduated effect of fear and proximity-based contagion effects are key to producing the superior results. We then reproduce the methodology on a second video, showing that the same results hold, implying generality of the conclusions reached in the first scene.

A study of emotional contagion with virtual characters

In social psychology, emotional contagion describes the widely observed phenomenon of one persons emotions mimicking surrounding peoples emotions [10]. In this paper, we perform a battery of experiments to explore the existence of agent-human emotional contagion. The first study is a between-subjects design, wherein subjects were shown an image of a characters face with either a neutral or happy expression. Findings indicate that even a still image induces a very strong increase in self-reported happiness between Neutral and Happy conditions with all characters tested. In a second study, we examine the effect of a virtual characters presence in a strategic decision by presenting subjects with a modernized Stag Hunt game. Our experiments show that the contagion effect is substantially dampened and does not cause a consistent impact on behavior. A third study explores the impact of the strategic situation within the Stag Hunt and conducts the same experiment using a description of the same strategic situation with the decision already made. We find that the emotional impact returns, implying that the contagion effect is substantially lessened in the presence of a strategic decision.

Empirical evaluation of computational fear contagion models in crowd dispersions

In social psychology, emotional contagion describes the widely observed phenomenon of one person’s emotions being influenced by surrounding people’s emotions. While the overall effect is agreed upon, the underlying mechanism of the spread of emotions has seen little quantification and application to computational agents despite extensive evidence of its impacts in everyday life. In this paper, we examine computational models of emotional contagion by implementing two models (Bosse et al., European council on modeling and simulation, pp. 212–218, 2009) and Durupinar, From audiences to mobs: Crowd simulation with psychological factors, PhD dissertation, Bilkent University, 2010) that draw from two separate lines of contagion research: thermodynamics-based and epidemiological-based. We first perform sensitivity tests on each model in an evacuation simulation, ESCAPES, showing both models to be reasonably robust to parameter variations with certain exceptions. We then compare their ability to reproduce a real crowd panic scene in simulation, showing that the thermodynamics-style model (Bosse et al., European council on modeling and simulation, pp. 212–218, 2009) produces superior results due to the ill-suited contagion mechanism at the core of epidemiological models. We also identify that a graduated effect of fear and proximity-based contagion effects are key to producing the superior results. We then reproduce the methodology on a second video, showing that the same results hold, implying generality of the conclusions reached in the first scene.

Security applications: lessons of real-world deployment

Game theory has played an important role in security decisions. Recent work using Stackelberg games [Fudenberg and Tirole 1991] to model security domains has been particularly influential [Basilico et al. 2009; Kiekintveld et al. 2009; Paruchuri et al. 2008; Pita et al. 2008; Pita et al. 2009]. In a Stackelberg game, a leader (in this case the defender) acts first and commits to a randomized security policy. The follower (attacker) optimizes its reward considering the strategy chosen by the leader. These games are well-suited to representing the problem security forces face in allocating limited resources, such as officers, canine units, and checkpoints. In particular, the fact that the attacker is able to observe the policy reflects the way real terrorist organizations plan attacks using extensive surveillance and long planning cycles.

Bayesian Security Games for Controlling Contagion

Influence blocking games have been used to model adversarial domains with a social component, such as counterinsurgency. In these games, a mitigator attempts to minimize the efforts of an influencer to spread his agenda across a social network. Previous work has assumed that the influence graph structure is known with certainty by both players. However, in reality, there is often significant information asymmetry between the mitigator and the influencer. We introduce a model of this information asymmetry as a two-player zero-sum Bayesian game. Nearly all past work in influence maximization and social network analysis suggests that graph structure is fundamental in strategy generation, leading to an expectation that solving the Bayesian game exactly is crucial. Surprisingly, we show through extensive experimentation on synthetic and real-world social networks that many common forms of uncertainty can be addressed near optimally by ignoring the vast majority of it and simply solving an abstracted game with a few randomly chosen types. This suggests that optimal strategies of games that do not model the full range of uncertainty in influence blocking games are typically robust to uncertainty about the influence graph structure.

Deployed Security Games for Patrol Planning

Nations and organizations need to secure locations of economic, military, or political importance from groups or individuals that can cause harm. The fact that there are limited security resources prevents complete security coverage, which allows adversaries to observe and exploit patterns in patrolling or monitoring and enables them to plan attacks that avoid existing patrols. The use of randomized security policies that are more difficult for adversaries to predict and exploit can counter their surveillance capabilities and improve security. In this chapter we describe the recent development of models to assist security forces in randomizing their patrols and their deployment in real applications. The systems deployed are based on fast algorithms for solving large instances of Bayesian Stackelberg games that capture the interaction between security forces and adversaries. Here we describe a generic mathematical formulation of these models, present some of the results that have allowed these systems to be deployed in practice, and outline remaining future challenges. We discuss the deployment of these systems in two real-world security applications: (1) The police at the Los Angeles International Airport uses these models to randomize the placement of checkpoints on roads entering the airport and the routes of canine unit patrols within the airport terminals. (2) The Federal Air Marshal Service (FAMS) uses these models to randomize the schedules of air marshals on international flights.

Game-Theoretic Target Selection in Contagion-based Domains

Many strategic actions carry a ‘contagious’ component beyond the immediate locale of the effort itself. Viral marketing and peacekeeping operations have both been observed to have a spreading effect. In this work, we use counterinsurgency as our illustrative domain. Defined as the effort to block the spread of support for an insurgency, such operations lack the manpower to defend the entire population and must focus on the opinions of a subset of local leaders. As past researchers of security resource allocation have done, we propose using game theory to develop such policies and model the interconnected network of leaders as a graph. Unlike this past work in security games, actions in these domains possess a probabilistic, nonlocal impact. To address this new class of security games, we combine recent research in influence blocking maximization with a double oracle approach and create novel heuristic oracles to generate mixed strategies for a real-world leadership network from Afghanistan, synthetic leadership networks, and scale-free graphs. We find that leadership networks that exhibit highly interconnected clusters can be solved equally well by our heuristic methods, but our more sophisticated heuristics outperform simpler ones in less interconnected scale-free graphs.