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Information Sciences Institute Viterbi School of Engineering University of Southern California Detecting and Tracking Hostile Plans in the Hats World Aram Galstyan and Paul Cohen Center for Research on Unexpected Events (CRUE) Information Sciences Institute University of Southern California U.S. Army Conference on Applied Statistics, Houston, 2007 Information Sciences Institute Viterbi School of Engineering University of Southern California Outline • Detection and tracking in symbolic spaces • Plan recognition in a virtual domain: The Hats Game • Probabilistic framework for plan recognition • Scalability issues – Approximate methods for tracking – Hybrid probabilistic/incremental search approach • Summary and future work Information Sciences Institute Viterbi School of Engineering University of Southern California Detection and Tracking of Plans • Goals define plans, plan execution results in observations Intentions Plans Actions Observations (Goals) • Inference problem: Given a stream of observations, and some behavioral model of the agent that generates those observation – What are their most likely intentions of the agent? – What kind of plan is he pursuing? – Stage of the plan execution? • Detect and track interesting plans/activities Information Sciences Institute Viterbi School of Engineering University of Southern California Classical Detection Theory and Plan Recognition • Large body of work on detection in metric spaces • Classical approach: detect and track – Applications:Tracking missiles, aircraft, submarines, etc. – Detection usually happens at relatively short time scales – Linear (Kalman) filters and various extension – Non-linear methods (HMM-s, particle filters) • Goal: Develop similar theory for symbolic spaces (plans, intentions) – Plans/intentions are not directly observable must be inferred – Detection might require long observation sequences – Interesting activities are often obscured by benign activities • Extremely low Signal to Noise Ratio (SNR) – In adversarial settings the agents might engage in deliberately deceptive behavior Information Sciences Institute Viterbi School of Engineering University of Southern California Plan Recognition for Intelligence Analysis • Requirements for Intelligence Analysis scenarios: – Should not assume known identities of hostile agents – Must handle clutter generated by large number of benign agents – Scalable • Terabytes of streaming data • Tens of thousands to millions of agents – Probabilistic, on-line, and recursive • Incorporating new evidence incrementally Information Sciences Institute Viterbi School of Engineering University of Southern California The Hats World • A simulation of ~105 agents (called hats) engaged in individual and collective activities. Most agents are benign, very few (e.g., 20) are known adversaries, some (e.g., 500) are covert adversaries. • Hats have shareable capabilities, passed from one hat to another at meetings. • Beacons have vulnerabilities. When a task force of hats possessing capabilities that match a beacon's vulnerabilities meet at the beacon, it is destroyed. • Adversarial plans are sequences of meetings of many hats to pass capabilities from one to another, culminating in destruction of a beacon • Hats belong to one or more organizations, which are not known and must be inferred from meetings. Some are adversarial organizations, most are not. Information Sciences Institute Viterbi School of Engineering University of Southern California The Hats Game • Goal: Given the record of meetings between hats, find and neutralize the adversarial task force before the attack, while minimizing the cost: – Penalty for “arresting” wrong hats – Information costs Information Sciences Institute Viterbi School of Engineering University of Southern California A Hierarchical Model for Hats The planner chooses an organization that will carry out the attack, and the beacon that will be attacked. Then a task--force (TF) for carrying out the attack is chosen. The planner then generates a meeting schedule (or a meeting tree) for each TF member, so that by the completion of those meetings each TF member carries the assigned capability. Highlighted nodes describe a particular plan instantiation: Information Sciences Institute Viterbi School of Engineering University of Southern California Plans in Hats • A plan is a sequence of meetings, structured as an inverted tree – Designed for passing around capabilities and confusing the observer • The observer’s goal is to find this meeting tree (target graph) before the final meeting takes place time Beacon attack Information Sciences Institute Viterbi School of Engineering University of Southern California Bayesian Filtering Approach • Treat the Hats world as a discrete time stochastic process, and develop methods for estimating this process • The state of each hat at time t, x(t), is characterized by a set of variables describing his hidden state, – “Adversary” indicator variable: {0,1} – Intention to acquire a capability: {0,1,..M} – Actual capability carried at time t: t {0,1,..M} n n 1 Yn Yn 1 DBN model of agent activity Information Sciences Institute Viterbi School of Engineering University of Southern California Collective Dynamics • The collective state is characterized by a vector X(t) = (x1(t), x2(t),.., xN(t)) • Our beliefs about the system are expressed in a pdf Pr[X(t) X] • Dynamics is governed by some type of hidden Markov process, – HMM with observation dependent state transitions Hidden state Observations – Given the current state and observation, the next state is conditionally independent of the past history Pr[X(t 1) | X(t),Y(t),..,X(1),Y(1)] Pr[X(t 1) | X(t),Y(t)] Information Sciences Institute Viterbi School of Engineering University of Southern California HMM Definitions • Transition Model: M(x, x P(Xt x | Xt1 x t1 y) ;y) ,Y • Evidence Generating (or Emission) Model (y, x) P(Yt y | Xt x) • These functions are chosen based on expert knowledge, e.g., – Agents from the same organization are more likely to meet – An agent intending to acquire a capability will try to meet with agents carrying that capability Information Sciences Institute Viterbi School of Engineering University of Southern California Bayesian Filtering • Filtering distribution (joint distribution over the hidden state at time t and observations up to time t): (x, y 0 ) P(X t x,Y0t y 0 ) t t Y0t {Y0 ,Y1,..,Yt } • Bayesian recursive update formula t (x, y 0 ) (x, y t ) M(x, x t1 ) t1 ( x y 0 ) t ;y , t1 x • Given the state transition and evidence generation models, as well as priors (initial beliefs), we can use this formula to update out beliefs as new observations arrive. Information Sciences Institute Viterbi School of Engineering University of Southern California Example of a Transition Function • The adversarial status and intention variable do not change • The only change is due to capability trades, which are specified as follows: – If our hat has no intention of acquiring a specific capability, then, with probability 1 he will acquire the capability c of the known hat, and with probability 1 will keep his old one. 1 – If the hat has already acquired the capability he intended to, then it will remain unchanged. – If the hat has not yet acquired its intended capability, then, if the second carries that capability, our hat will acquire it with probability 2 hat Information Sciences Institute Viterbi School of Engineering University of Southern California Example of an Evidence Generating Function • Agents from the same group meet with probability p1>0.5, while agents from different groups meet with probability 1- p1 (homophily, guilt-by-association) (1, 2 ) p1 1 , 2 (1 p1)(1 1 , 2 ) Information Sciences Institute Viterbi School of Engineering University of Southern California Results for the Bayesian GBA model • Given some known adversaries, find the covert adversaries using the sequence of meetings between different hats covert benign • Able to correctly identify the coverts (high posteriors after number of iterations) if p1 is sufficiently large Information Sciences Institute Viterbi School of Engineering University of Southern California Scaling Issues • For sufficiently large systems it is not feasible to maintain and update beliefs about all possible joint (collective) states – E.g., need a table with 2N entries for N agents, binary states • Flat state representation will not work, need alternatives Information Sciences Institute Viterbi School of Engineering University of Southern California Factorial HMM-s • Models simultaneous dynamics of multiple chains (hats) • Different chains contribute to the observation process • Note: Conditioned on the observation sequence, the state dynamics across the chains are not independent Information Sciences Institute Viterbi School of Engineering University of Southern California Coupled Factorial HMM-s • Different chains coupled not only through observations, but also explicitly Information Sciences Institute Viterbi School of Engineering University of Southern California Coupled HMM-s for Hats • Hats are coupled via observations (meetings) • Observation is an act of interaction between two (or more) stochastic processes Information Sciences Institute Viterbi School of Engineering University of Southern California Approximation for the Belief State • After long enough time all the hats will become “coupled” – E.g., construct a graph by linking any two hats the have met with each other. Then complexity is determined by the largest connected component of the graph • Exact inference becomes infeasible for long observation sequences • Use factorization (Boyen-Koller approximation) C P(X n | y ) P (X n | y ) P(X c,n | y 0 ) n 0 ˜ n 0 n c1 • X c,n are subsets (clusters) of variables (need not to be disjoint) • Exact inference is recovered if each connected component is a cluster Information Sciences Institute Viterbi School of Engineering University of Southern California Factorization • Full factorization: Each cluster contains a single agent N P(X n | y ) P(X c,n | y 0 ) n 0 n c1 • Projection to the factorized distributions after the update Pni (x n ) Pn1 (x n1 ) M(x n , x n1, x n1) P j (i j | x n1, x n1 ) P j i i i i j i j n1 n1 i x n1 • Intuition: To update our beliefs for the agent i after its meeting with j, we average the state transition and evidence generating functions over the state of j (e.g., treat {i,j} as a noisy observation) • Goodness of the approximation depends on the mixing properties of the Markov chain – E.g., works well if the transition matrix is very stochastic Information Sciences Institute Viterbi School of Engineering University of Southern California Summary of Initial Results • Able to correctly infer the intention of an agent if – The behavior is sufficiently repetitive (e.g., a hat needs to attend many meetings to acquire the intended capability) – The priors on capabilities are sufficiently accurate • Not so good at inferring the adversarial/benign status, – For sufficiently large probability of observing a meeting between a benign and adversarial hats – For small fraction of initially known hats Information Sciences Institute Viterbi School of Engineering University of Southern California Other Scaling Issues • Even if we have an efficient representation for P(X), testing certain hypotheses might be computationally prohibitive (combinatorial complexity) – E.g., evaluating a hypothesis about a task force with m members requires summing up ~Nm terms • For sufficiently large problems, the simple strategy of constructing and scoring all the hypotheses is prohibitively expensive Information Sciences Institute Viterbi School of Engineering University of Southern California Solution: Hybrid Approach • Combine the Bayesian tracking framework with incremental search – Start with simple hypotheses – Build more complex hypotheses incrementally – Prune irrelevant hypotheses and expand the promising ones Information Sciences Institute Viterbi School of Engineering University of Southern California Finding the Covert Plan using the Hybrid Approach • Start from a single meeting nodes • Score these nodes using the Bayesian state estimator • Expand promising nodes by adding another meeting to the graph • This is a search problem with costs determined by the Bayesian estimator Information Sciences Institute Viterbi School of Engineering University of Southern California Open Research Questions • What are the appropriate search strategies? – Breadth first: should we commit our resources to analyze low level data, hoping that something will flag a malicious intent? – Depth first: or should we quickly build more complex hypotheses, assuming that the global picture will emerge as the hypotheses graph gets larger? – Other heuristic search methods : A*, beam search. • Initial results – The choice of the optimal search strategy depends on the accuracy of the Bayesian state estimator Information Sciences Institute Viterbi School of Engineering University of Southern California Summary • A general probabilistic framework for detecting and tracking adversarial activities in a virtual environment – Mines dynamical patterns (processes) rather than static structures (highly connected sub-graphs, communities) – Not domain specific, and not limited to Hats • Underlying assumptions are very generic • Agent activity is modeled through some type of HMM • Hypotheses on coordinated activities are represented as graphs • Initial results on approximate filtering methods • Initial steps towards a hybrid approach for plan recognition in large scale systems – Bayesian framework for non-linear tracking – Incremental Search for early hypotheses pruning Information Sciences Institute Viterbi School of Engineering University of Southern California Future Work • Experiments with the new factorial approximation schemes – Factorization over larger clusters – Augmenting the algorithm with a better GBA model • Other approximations of the belief states (particle filters) • Developing increasingly complex models of behavior – Accounting for recognizing higher level policies • Recruitment, change of tactics • Formation, evolution and transformation of groups • Information Fusion at different levels of abstraction, e.g., – “Hat #29 behaves suspiciously” – “Organization #2 is up to something” • Theoretical issues:“Trackability”: – Under what circumstances one can track certain processes? – Bounds on accuracy • Examining optimal search strategies for hypotheses building Information Sciences Institute Viterbi School of Engineering University of Southern California Detection on Networks • Understanding and analyzing adversarial networks – Static vs. dynamical properties of networks • Most network analysis research is concerned with static patterns – Strongly connected sub-graphs, communities – Degree distribution, clustering, hubs, etc • Certain patterns are inherently dynamical – Can not be revealed by analyzing static structures • Need approaches for mining dynamical patterns – Detecting processes that represent hostile activities – Tracking and predicting the evolution of such processes

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posted: | 10/20/2012 |

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