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Mitra, Sayan.  2014.  Proving Abstractions of Dynamical Systems Through Numerical Simulations. Proceedings of the 2014 Symposium and Bootcamp on the Science of Security. :12:1–12:9.

A key question that arises in rigorous analysis of cyberphysical systems under attack involves establishing whether or not the attacked system deviates significantly from the ideal allowed behavior. This is the problem of deciding whether or not the ideal system is an abstraction of the attacked system. A quantitative variation of this question can capture how much the attacked system deviates from the ideal. Thus, algorithms for deciding abstraction relations can help measure the effect of attacks on cyberphysical systems and to develop attack detection strategies. In this paper, we present a decision procedure for proving that one nonlinear dynamical system is a quantitative abstraction of another. Directly computing the reach sets of these nonlinear systems are undecidable in general and reach set over-approximations do not give a direct way for proving abstraction. Our procedure uses (possibly inaccurate) numerical simulations and a model annotation to compute tight approximations of the observable behaviors of the system and then uses these approximations to decide on abstraction. We show that the procedure is sound and that it is guaranteed to terminate under reasonable robustness assumptions.

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Liberzon, Daniel, Mitra, Sayan.  2016.  Entropy and Minimal Data Rates for State Estimation and Model Detection. Proceedings of the 19th International Conference on Hybrid Systems: Computation and Control. :247–256.

We investigate the problem of constructing exponentially converging estimates of the state of a continuous-time system from state measurements transmitted via a limited-data-rate communication channel, so that only quantized and sampled measurements of continuous signals are available to the estimator. Following prior work on topological entropy of dynamical systems, we introduce a notion of estimation entropy which captures this data rate in terms of the number of system trajectories that approximate all other trajectories with desired accuracy. We also propose a novel alternative definition of estimation entropy which uses approximating functions that are not necessarily trajectories of the system. We show that the two entropy notions are actually equivalent. We establish an upper bound for the estimation entropy in terms of the sum of the system's Lipschitz constant and the desired convergence rate, multiplied by the system dimension. We propose an iterative procedure that uses quantized and sampled state measurements to generate state estimates that converge to the true state at the desired exponential rate. The average bit rate utilized by this procedure matches the derived upper bound on the estimation entropy. We also show that no other estimator (based on iterative quantized measurements) can perform the same estimation task with bit rates lower than the estimation entropy. Finally, we develop an application of the estimation procedure in determining, from the quantized state measurements, which of two competing models of a dynamical system is the true model. We show that under a mild assumption of exponential separation of the candidate models, detection is always possible in finite time. Our numerical experiments with randomly generated affine dynamical systems suggest that in practice the algorithm always works.

D
Qi, Bolun, Fan, Chuchu, Jiang, Minghao, Mitra, Sayan.  2018.  DryVR 2.0: A Tool for Verification and Controller Synthesis of Black-box Cyber-physical Systems. Proceedings of the 21st International Conference on Hybrid Systems: Computation and Control (Part of CPS Week). :269–270.
We present a demo of DryVR 2.0, a framework for verification and controller synthesis of cyber-physical systems composed of black-box simulators and white-box automata. For verification, DryVR 2.0 takes as input a black-box simulator, a white-box transition graph, a time bound and a safety specification. As output it generates over-approximations of the reachable states and returns "Safe" if the system meets the given bounded safety specification, or it returns "Unsafe" with a counter-example. For controller synthesis, DryVR 2.0 takes as input black-box simulator(s) and a reach-avoid specification, and uses RRTs to find a transition graph such that the combined system satisfies the given specification.
C
Huang, Zhenqi, Wang, Yu, Mitra, Sayan, Dullerud, Geir.  2016.  Controller Synthesis for Linear Dynamical Systems with Adversaries. Proceedings of the Symposium and Bootcamp on the Science of Security. :53–62.

We present a controller synthesis algorithm for a reach-avoid problem in the presence of adversaries. Our model of the adversary abstractly captures typical malicious attacks envisioned on cyber-physical systems such as sensor spoofing, controller corruption, and actuator intrusion. After formulating the problem in a general setting, we present a sound and complete algorithm for the case with linear dynamics and an adversary with a budget on the total L2-norm of its actions. The algorithm relies on a result from linear control theory that enables us to decompose and compute the reachable states of the system in terms of a symbolic simulation of the adversary-free dynamics and the total uncertainty induced by the adversary. With this decomposition, the synthesis problem eliminates the universal quantifier on the adversary's choices and the symbolic controller actions can be effectively solved using an SMT solver. The constraints induced by the adversary are computed by solving second-order cone programmings. The algorithm is later extended to synthesize state-dependent controller and to generate attacks for the adversary. We present preliminary experimental results that show the effectiveness of this approach on several example problems.

Huang, Zhenqi, Wang, Yu, Mitra, Sayan, Dullerud, Geir.  2016.  Controller Synthesis for Linear Dynamical Systems with Adversaries. Proceedings of the {Symposium} and {Bootcamp} on the {Science} of {Security}. :53–62.
We present a controller synthesis algorithm for a reach-avoid problem in the presence of adversaries. Our model of the adversary abstractly captures typical malicious attacks envisioned on cyber-physical systems such as sensor spoofing, controller corruption, and actuator intrusion. After formulating the problem in a general setting, we present a sound and complete algorithm for the case with linear dynamics and an adversary with a budget on the total L2-norm of its actions. The algorithm relies on a result from linear control theory that enables us to decompose and compute the reachable states of the system in terms of a symbolic simulation of the adversary-free dynamics and the total uncertainty induced by the adversary. With this decomposition, the synthesis problem eliminates the universal quantifier on the adversary's choices and the symbolic controller actions can be effectively solved using an SMT solver. The constraints induced by the adversary are computed by solving second-order cone programmings. The algorithm is later extended to synthesize state-dependent controller and to generate attacks for the adversary. We present preliminary experimental results that show the effectiveness of this approach on several example problems.