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Regret Minimizing Audits: A Learning-theoretic Basis for Privacy Protection Jeremiah Blocki, Nicolas Christin, Anupam Datta, Arunesh Sinha Carnegie Mellon University

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Motivation Goal: treatment Rigid access control hinders treatment Permissive access control ⇒ privacy violations Breach 2

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A real problem 3

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Audits Audits: one way to address the problem ◦ Permissive access control If in doubt allow access ◦ Log the accesses ◦ Human auditors review the accesses later and find violations Adhoc approaches in practice ◦ FairWarning audit tool implements simple heuristics, e.g., flag all celebrity access 4

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Desiderata Principled study of the audit process ◦ A model for audit process ◦ Properties of the audit mechanism ◦ Audit mechanism which provably satisfies the property 5

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Auditing Challenges Organization’s economic tradeoff Employee’s incentives unknown How to optimally allocate budget for auditing, with no knowledge about adversary incentives? 6 Reputation loss Audit cost

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Audit Algorithm by Example OverviewAudit ModelLow Regret Algorithm Auditing budget: $3000/ cycle Cost for one inspection: $ 1 00 Only 30 inspections per cycle Auditor 1 00 accesses 30 accesses 70 accesses Access divided into 2 types Loss from 1 violation (internal, external) $500, $ 1 000 $250, $500

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Audit Algorithm Choices 8 Only 30 inspections 0 10102030 20 10100 Consider 4 possible allocations of the available 30 inspections 1.0 Weights Choose allocation probabilistically based on weights OverviewAudit ModelLow Regret Algorithm

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No. of Access Audit Algorithm Run 9 0 10102030 20 10100 0.5 2.01.5 Updated weights Observed Loss $2000$ 1 500$ 1 000 $750$ 1 250 $ 1 500 Learn from experience: weights updated using observed and estimated loss 2 4 Actual Violation Ext. Caught Int. Caught 1 1 1 2 30 70 OverviewAudit ModelLow Regret Algorithm Estimated Loss

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Main Contributions A game model for the audit process Defining a desirable property of audit mechanisms, namely low regret An efficient audit mechanism RMA that provably achieves low regret o Better bound on regret than existing algorithms that achieve low regret 10 OverviewAudit ModelLow Regret Algorithm

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Repeated Game Model Game model The interaction repeats for each audit cycle (typically called rounds of repeated game) Typical actions in one round ◦ Emp action: (access, violate) = ([30,70], [2,4]) ◦ Org action: inspection = ([ 1 0,20]) Inspect Reputation loss Audit Cost Access, Violate One audit cycle (round) 11 Imperfection OverviewAudit ModelLow Regret Algorithm

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Game Payoffs Organization’s payoff ◦ Audit cost depends on the number of inspections ◦ Reputation loss depends on the number of violations caught Employee’s payoff unknown Reputation loss Audit cost 12 OverviewAudit ModelLow Regret Algorithm

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Regret Intuition 13 Is it possible to audit as well as the best strategy in hindsight ? 0 10102030 20 10100 OverviewAudit ModelLow Regret Algorithm

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Regret by Example $5 $6 $0$5 1 2 3,1 3, 2 Payoff of Org only Players Emp Org: s Round 1 3, 1 2 ( 1 ) Round 2 3,2 1 (-5) Total Payoff Unknown -4 Org : s 1 1 (2) 1 (-5) -3 Total regret(s, s 1 ) = (–5) – (–6) = 1 regret(s, s 1 ) = 1 /2 Strategy: outputs an action for every round Emp Org 14 Players Emp Org:s Round 1 3, 1 2 ( $6 ) Round 2 3, 2 1 ( $0) Total Payoff Unknown $6 Org:s 1 1 ($5) 1( $0) $5 OverviewAudit ModelLow Regret Algorithm

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Meaning of Regret Low regret of s w.r.t. s 1 means s performs as well as s 1 Desirable property of an audit mechanism ◦ Low regret w.r.t all strategies in a given set of strategies ◦ regret → 0 as T → ∞ 15 OverviewAudit ModelLow Regret Algorithm

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Regret minimization Multiplicative weight update (MWU) ◦ is a standard algorithm that achieves low regret w.r.t. to all strategies in a given set The regret bound of MWU is ◦ N: number of strategies in the given set ◦ T: number of rounds of the game ◦ All payoffs scaled to lie in [0, 1 ] 16 OverviewAudit ModelLow Regret Algorithm

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Why not MWU? Imperfect information ◦ Org never learns the true action (violation) of the employee ◦ RMA regret bound: O((ln N)/T) Best known bounds [ACFS03] : O((N 1 /3 ln N)/T 1 /3 ) Idea: estimate the payoff that would have been received Sleeping strategies: unavailable strategies ◦ Some inspections unavailable due to budgetary constraints ◦ We use techniques from [BM05] 17 [ACFS03] P. Auer, N. Cesa-Bianchi, Y. Freund, R. Schapire, “The nonstochastic multiarmed bandit problem,” SIAM Journal on Computing, 2003 [BM05] A. Blum and Y. Mansour, “From external to internal regret,”in COLT 2005 OverviewAudit ModelLow Regret Algorithm

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Regret Minimizing Audits (RMA) 18 New audit cycle starts. Find AWAKE Pick s in AWAKE with probability D t (s) ∝ w s Update weight* of strategies s in AWAKE Estimate payoff vector Pay using Pay(s) Violation caught; obtain payoff Pay(s) w s = 1 for all strategies s * OverviewAudit ModelLow Regret Algorithm

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Guarantees of RMA With probability RMA achieves the regret bound ◦ N is the set of strategies ◦ T is the number of rounds ◦ All payoffs scaled to lie in [0, 1 ] 19 OverviewAudit ModelLow Regret Algorithm

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Related Work Authorization proof recorded in audit log [Vaughan et al. 2008] Analyze audit logs to detect and resolve access control policy misconfigurations [Bauer et al. 2008] Mechanically checkable complaince proof constructed using evidence from audit logs [Cederquist et al. 2007] Mechanically checking policy compliance over incomplete audit logs [Garg et al. 2011] 20

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Take Away Message Future Work ◦ Evaluation over real hospital audit logs ◦ Analyze performance with more complex adversary models Worst case + rational Learning technique for effective auditing with imperfect information

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