Contextual Integrity & its Logical Formalization

1. 18739A: Foundations of Security and Privacy Contextual Integrity & its Logical Formalization Anupam Datta Fall 2009

2. Privacy in Organizations • Huge amount of personal information available to various organizations • Hospitals, financial institutions, websites, universities, census bureau, social networks… • Information shared within and across organizational boundaries to enable useful tasks to be completed • Personal health information has to be shared to enable diagnosis, treatment, billing, … • Fundamental Question: • Do organizational processes respect privacy expectations while achieving purpose?

3. Approach What is Privacy? Contextual Integrity (CI): - Transfer of personal information governed by contextual norms Logic of Privacy and Utility: - Formalization of CI; automated methods for checking compliance with privacy regulations (HIPAA, GLBA, COPPA,…) Express and Enforce Privacy Policies

4. Contextual Integrity [Nissenbaum 2004] • Philosophical framework for privacy • Central concept: Context • Examples: Healthcare, banking, education • What is a context? • Set of interacting agents in roles • Roles in healthcare: doctor, patient, … • Informational norms • Doctors should share patient health information as per the HIPAA rules • Norms have a specific structure (descriptive theory) • Systematic method at arriving at contextual norms (prescriptive theory) • Purpose • Improve health • Some interactions should happen - patients should share personal health information with doctors

5. Outline • Motivating Example • Framework • Model • Logic of Privacy and Utility • Workflows and Responsibility • Algorithmic Results • Workflow Design assuming agents responsible • Auditing logs when agents irresponsible • Summary

6. Health Answer Yes! except broccoli Health Question Now that I have cancer, Should I eat more vegetables? MyHealth@Vanderbilt++ Workflow Humans + Electronic system Health Answer Appointment Request Secretary Doctor Patient Health Question Health Question Health Answer Nurse

7. Outline • Motivating Example • Framework • Model • Logic of Privacy and Utility • Workflows and Responsibility • Algorithmic Results • Workflow Design assuming agents responsible • Auditing logs when agents irresponsible • Conclusions

8. Informational Norms “In a context, the flow of information of a certain type about a subject (acting in a particular capacity/role) from one actor (could be the subject) to another actor (in a particular capacity/role) is governed by a particular transmission principle.” Contextual Integrity[Nis2004]

9. Health Question Now that I have cancer, Should I eat more vegetables? Model Bob Alice • Communication via send actions: • Sender: Bob in role Patient • Recipient: Alice in role Nurse • Subject of message: Bob • Tag: Health Question • Message: Now that …. • Data model & knowledge evolution: • Agents acquire knowledge by: • receiving messages • deriving additional attributes based on data model • Health Question Protected Health Information contents(msg) vs. tags (msg)

10. Model [Barth,Datta,Mitchell,Sundaram 2007] • State determined by knowledge of each agent • Transitions change state • Set of concurrent send actions • Send(p,q,m) possible only if agent p knows m K0 A13 A11 K13 A12 K11 ... K12 ... Concurrent Game Structure G = <k, Q, , , d, >

11. Logic of Privacy and Utility • Syntax  ::= send(p1,p2,m) p1 sends p2 message m | contains(m, q, t) m contains attrib t about q | tagged(m, q, t) m tagged attrib t about q | inrole(p, r) p is active in role r | t  t’ Attrib t is part of attrib t’ |    |  | x.  Classical operators | U | S | O Temporal operators | <<p>> Strategy quantifier • Semantics Formulas interpreted over concurrent game structure

12. Informational Norms “In a context, the flow of information of a certain type about a subject (acting in a particular capacity/role) from one actor (could be the subject) to another actor (in a particular capacity/role) is governed by a particular transmission principle.” Contextual Integrity[Nis2004]

13. Privacy Regulation Example (GLB Act) Sender role Subject role Financial institutions must notify consumers if they share their non-public personal information with non-affiliated companies, but the notification may occur either before or after the information sharing occurs Attribute Recipient role Exactly as CI says! Transmission principle

14. Structure of Privacy Rules [Barth,Datta,Mitchell,Nissenbaum 2006] • Allow message transmission if: • at least one positive norm is satisfied; and • all negative norms are satisfied

15. HIPAA – Healthcare Privacy • HIPAA consists primarily of positive norms: share phi if some rule explicitly allows it (2), (3), (5), (6) • Exception: negative norm about psychotherapy notes (4)

16. COPPA – Children Online Privacy • COPPA consists primarily of negative norms • children can share their protected info only if parents consent (7) (condition) • (8) (obligation – future requirements)

17. Specifying Privacy • MyHealth@Vanderbilt In all states, only nurses and doctors receive health questions G  p1, p2, q, m send(p1, p2, m)  contains(m, q, health-question)  inrole(p2, nurse)  inrole(p2, doctor)

18. Specifying Utility • MyHealth@Vanderbilt Patients have a strategy to get their health questions answered  p inrole(p, patient)  <<p>> F  q, m. send(q, p, m)  contains(m, p, health-answer)

19. Outline • Motivating Example • Framework • Model • Logic of Privacy and Utility • Workflows and Responsibility • Algorithmic Results • Workflow Design assuming agents responsible • Auditing logs when agents irresponsible • Conclusions • Differentially Private Workflows (WIP)

20. Health Answer Yes! except broccoli Health Question Now that I have cancer, Should I eat more vegetables? MyHealth@Vanderbilt Improved Doctor should answer health questions Health Answer Appointment Request Secretary Doctor Patient Health Question Health Question Assign responsibilities to roles & workflow engine Health Answer Nurse

21. Graph-based Workflow • Graph (R, R x R), where R is the set of roles • Edge-labeling function permit: R x R  2T, where T is the set of attributes • Responsibility of workflow engine Allow msg from role r1 to role r2 iff tags(msg)  permit(r1, r2) • Responsibility of human agents in roles Tagging responsibilities • ensure messages are correctly tagged Progress responsibilities • ensure messages proceed through workflow

22. MyHealth Responsibilities • Tagging Nurses should tag health questions G p, q, s, m. inrole(p, nurse)  send(p, q, m)  contains(m, s, health-question)  tagged(m, s, health-question) • Progress • Doctors should answer health questions G p, q, s, m. inrole(p, doctor)  send(q, p, m)  contains(m, s, health-question)  F m’. send(p, s, m’)  contains(m’, s, health-answer)

23. Abstract Workflow • Responsibility of workflow engine • LTL formula  • Feasible (enforceable) if  is asafety formula without the contains() predicate • Responsibility of each role r • LTL formula r • Feasible if agents have a strategy to discharge their responsibilities  p.   inrole(p, r)  <<p>> r Graph-based workflows are a special case

24. Outline • Motivating Example • Framework • Model • Logic of Privacy and Utility • Workflows and Responsibility • Algorithmic Results • Workflow Design assuming agents responsible • Abstract workflows • Auditing logs when agents irresponsible • Only graph-based workflows • Conclusions

25. Workflow Design Results • Theorems: Assuming all agents act responsibly, checking whether workflow achieves • Privacy is in PSPACE (in the size of the formula describing the workflow) • Utility is decidable Algorithms implemented in model-checkers, e.g. SPIN, MOCHA Potential for small model theorems

26. Outline • Motivating Example • Framework • Model • Logic of Privacy and Utility • Workflows and Responsibility • Algorithmic Results • Workflow Design assuming agents responsible • Abstract workflows • Auditing logs when agents irresponsible • Only graph-based workflows • Summary

27. Auditing Results: Summary • Definitions • Accountability = Causality + Irresponsibility • Design of audit log • Captures causality relation • Algorithm • Finding agents accountable for policy violation in graph-based workflows using audit log • Algorithm uses oracle: • O(msg) = contents(msg)

28. Policy compliance/violation Contemplated Action Judgment Policy Future Reqs History • Strong compliance [BDMN2006, BDMS 2007] • Action does not violate current policy requirements • Future policy requirements after action can be met • Formally,

29. Local Compliance • Locally compliant policy • Agents can determine strong compliance based on their local view of history

30. Causality • Lamport Causality [1978]

31. Accountability & Audit Log • Accountability • Causality + Irresponsibility • Audit log design • Records all Send(p,q,m) and Receive(p,q,m) events executed • Maintains causality structure • O(1) operation per event logged

32. Auditing Algorithm

33. Central Lemma

34. Main Theorem

35. MyHealth Example • Policy violation: Secretary Candy receives health-question mistagged as appointment-request • Construct causality graph G and search backwards using BFS Candy received message m from Patient Jorge. • O(m) = health-question, but tags(m) = appointment-request. • Patient responsible for health-question tag. • Jorge identified as accountable

36. Summary • Framework • Concurrent game model • Logic of Privacy and Utility • Temporal logic (LTL, ATL*) • Organizational Process as Workflow • Role-based responsibility for human and mechanical agents • Algorithmic Results • Workflow design assuming agents responsible • Privacy, utility decidable (model-checking) • Auditing logs when agents irresponsible • From policy violation to accountable agents Automated Using oracle

37. Thanks Questions?

39. Deciding Privacy • PLTL model-checking problem is PSPACE decidable G|= tags-correct U agents-responsible  privacy-policy G: concurrent game structure Result applies to finite models (#agents, msgs,…)

40. Deciding Utility • ATL* model-checking of concurrent game structures is • Decidable with perfect information • Undecidable with imperfect information • Theorem: There is a sound decision procedure for deciding whether workflow achieves utility • Intuition: • Translate imperfect information into perfect information by considering possible actions from one player’s point of view

41. Local communication game • Quotient structure under invisible actions, Gp • States: Smallest equivalence relation K1 ~p K2 if K1 K2 and a is invisible to p • Actions: [K]  [K’] if there exists K1 in [K] and K2 in [K’] s.t. K1 K2 • Lemma: For all LTL formulas  visible to p, Gp |= <<p>> implies G |= <<p>> • The truth value of formulas visible to p depends only on actions that p can observe, e.g. send (*, p, *), send(p, *, *) Locality again!

42. Related Languages • Legend:  unsupported o partially supported  fully supported • LPU fully supports attributes, combination, temporal conditions, and utility