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CS590U Access Control: Theory and Practice

CS590U Access Control: Theory and Practice. Lecture 21 (April 11) Distributed Credential Chain Discovery in Trust Management. What is RT?. RT is a family of Role-based Trust-management languages Publications on RT

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CS590U Access Control: Theory and Practice

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  1. CS590UAccess Control: Theory and Practice Lecture 21 (April 11) Distributed Credential Chain Discovery in Trust Management

  2. What is RT? • RT is a family of Role-based Trust-management languages • Publications on RT • Li, Winsborough & Mitchell: “Distributed Credential Chain Discovery in Trust Management”, CCS’01, JCS’03 • Li, Mitchell & Winsborough: “Design of a Role-Based Trust Management Framework”, S&P’02 • Li & Mitchell: “Datalog with Constraints: A Foundation for Trust Management Languages”, PADL’03 • Li & Mitchell: “RT: A Role-based Trust-management Framework”, DISCEX’03 • Li, Winsborough & Mitchell: “Beyond Proof-of-compliance: Safety and Availability Analysis in Trust Management”, S&P’03, JACM’05 2

  3. RT0: An Example • StateU.stuID  Alice • ABU.accredited  StateU • EPub.university  ABU.accredited • EPub.student  EPub.university.stuID • EPub.spdiscount  EPub.student  EOrg.preferred • EOrg.preferred  ACM.member • ACM.member  Alice • Together, the seven credentials prove that Alice is entitled to EPub’s spdiscount 3

  4. RT0: Concepts and Credentials • Concepts: • Entities (Principals): A, B, D • Role names: r, r1, r2, ... • Roles: A.r, B.r1,... e.g., StateU.stuID • Credentials: A.r e • Type-1: A.r D • Type-2: A.r  B.r1 • Type-3: A.r  A.r1.r2 • e.g., EPub.studentEPub.university.stuID • Type-4: A.r  B1.r1 B2.r2 ... Bk.rk 4

  5. RT0 and SDSI 2.0 • SDSI 2.0 (The SDSI part of SPKI/SDSI 2.0) • has arbitrarily long linked names, e.g., A.r1.r2.....rk, which can be broken up by introducing new role names • RT0 • has intersection (type-4 credentials) • is thus more expressive than SDSI 2.0 • algorithms for RT0 can be used for SDSI 2.0 5

  6. Goal-directed Chain Discovery • Three kinds of queries and algorithms for answering them: • Given A.r, determines its members • The backward search algorithm • Given D, determines the set of roles that D is a member of • The forward search algorithm • Given A.r and D, determines whether D is a member of A.r • The Bi-direction search algorithm 6

  7. Credential Graph GC • Nodes: • A.r and e for each credential A.r e in C • Credential edges: • e A.r for each credential A.r e in C • Summary edges: • B.r2 A.r1.r2 if there is a path from B to A.r1 • D  A1.r1 …  Ak.rk if there are paths from D to each Aj.rj • Reachability in the credential graph is sound and complete wrt. the set semantics of RT0 7

  8. EPub.spdiscount EPub.student EPub.university EPub.student  EOrg.preferred ABU.accredited EPub.university.stuID EOrg.preferred StateU ACM.member StateU.stuID Alice An Example Credential Graph Key Credential Summary 8

  9. The Forward Search Algorithm (Overview) • Starts with one entity node • Constructs a proof graph • Each node in the graph stores its solutions: • roles that this node can reach (is a member of ) • Maintains a work list of nodes need to be processed • Algorithm Outline: • keep processing nodes in the work list until it is empty 9

  10. 4: ABU.accredited.stuID 7: Epub.university.stuID 1: StateU.stuID 9: EPub.student 8: EPub 5: ABU 2: StateU 3: ABU.accredited 6: EPub.university Forward Search In Action StateU.stuID  Alice ABU.accredited  StateU EPub.university  ABU.accredited EPub.student  EPub.university.stuID EPub.student 0: Alice StateU.stuID StateU.stuID EPub.student EPub.student EPub.student ABU.accredited ABU.accredited EPub.university EPub.university EPub.university 10

  11. The Backward and Bi-direction Search Algorithms (Overview) • The backward algorithm differs from the forward algorithm in that: • each node stores outgoing edges, instead of incoming ones • each node stores entities that can reach it, instead of roles that it can reach • the processing of a node is different • traversing the other direction • The bi-direction search algorithm combines backward search and forward search 11

  12. Backward Search In Action 7: Alice 5: ACM.member 3: EOrg.preferred Alice Alice Alice 0: EPub.spdiscount 1: EPub.student  EOrg.preferred Alice Alice Alice 10: StateU.stuID 2: EPub.student 4: EPub.university.stuID Alice Alice Alice 9: StateU 8: ABU.accredited 6: EPub.university StateU StateU StateU 12

  13. Worst-Case Complexity • Backward: time O(N3+NM), space O(NM) • N is the number of rules • M is the sum of the sizes of all rules, • A.r  f1fk having size k, other credentials have size 1 • Forward: time O(N2M), space O(NM) • However, this is goal oriented, making it much better in practice 13

  14. Why Develop These Algorithms? • The queries can be answered using logic programs • however, this requires collection of all credentials in the system • The backward algorithm is a goal-directed top-down algorithm • The forward algorithm is a goal-directed bottom-up algorithm • Distributed discovery requires combination of both 14

  15. Distributed Storage of Credentials • Example: • EOrg.preferred  ACM.member • ACM.member  Alice • Who should store a credential? • either issuer or subject • It is not reasonable to require that • all credentials are stored by issuers, or, • all are stored by subjects. 15

  16. Who stores these statements? Alice EPub 1. COE.stuID  Alice 4. EPub.university  ABU.accredited 5. EPub.student  EPub.university.stuID COE 2. StateU.stuID  COE.stuID 3. ABU.accredited  StateU StateU ABU

  17. EPub.student EPub.university.stuID EPub.university Backward (Issuer stored) StateU.stuID Key ABU.accredited Forward (Subject stored) Alice StateU Confluent Traversability of Edges and Paths (con’d) An edge B.r2 A.r1.r2 has the same traversability as B  A.r1 17

  18. How to Ensure that Every Path is Confluent? • Goal: using constraints local to each credential to ensure that every path is confluent • Approach: • give each role name a traceability type • introduce a notion of well-typed credentials • Main idea: • by requiring consistent storage strategy at role name level, we guarantee chains using well-typed credentials are confluent 18

  19. Types of Role Names • A role name has two types: • Issuer side: • issuer-traces-all • issuer-traces-def • issuer-traces-none • Subject side: • subject-traces-all • subject-traces-none 19

  20. A Typing Scheme Alice EPub 1. COE.stuID Alice 4. EPub.university ABU.accredited 5. EPub.student EPub.university.stuID COE 2. StateU.stuID COE.stuID 3. ABU.accredited StateU StateU ABU

  21. Agreement on Types and Meaning of Role Names • An approach inspired by XML namespaces • Use an Application Domain Specification Document (ADSD) to define a vocabulary • Each role has a storage type • Credentials have a preamble • Which defines vocabulary identifier to correspond to an ADSD • When using a role name, add a vocabulary identifier as prefix 21

  22. Benefits of the Storage Type System • Guarantees that chains of well-typed credentials can be discovered • Enables efficient chain discovery by telling the algorithm whether forward or backward search should be used for an intermediate query • Communicates the application domain knowledge to the algorithm 22

  23. Design of A Role-based Trust-management Framework Ninghui Li, John C. Mitchell & William H. Winsborough IEEE S&P 2002

  24. Features of the RT family of TM languages • Expressive delegation constructs • Permissions for structured resources • A tractable logical semantics based on (Constraint) Datalog • Strongly-typed credentials and vocabulary agreement • Efficient deduction with large number of distributedpolicy statements • Security analysis 24

  25. Expressive Features (part one) • Simple attribute assignmentStateU.stuID  Alice • Delegation of attribute authority StateU.stuID  COE.stuID • Attribute inferencing EPub.access  EPub.student • Attribute-based delegation of authority EPub.student  EPub.university.stuID 25

  26. Expressive Features (part two) • Conjunction EPub.access  EPub.student  ACM.member • Attributes with fields • StateU.stuID (name=.., program=.., …)  Alice • EPub.access  StateU.stuID(program=“graduate”) • Permissions for structured resources • e.g., allow connection to any host in a domain and at any port in a range 26

  27. The Languages in the RT Framework RT0: Decentralized Roles RTD: forSelective Use of Role memberships RTT : forSeparation of Duties RT1: Parameterized Roles RT2: Logical Objects RT1C: structured resources RT2C: structured resources RTT and RTD can be used (either together or separately) with any of the five base languages: RT0, RT1, RT2, RT1C, and RT2C 27

  28. RT1 = RT0 + Parameterized Roles • Motivations: to represent • attributes that have fields, e.g., digital ids, diplomas • relationships between principals, e.g., physicianOf, advisorOf • role templates, e.g., project leaders • Approach: • a role term R has a role name and a list of fields 28

  29. RT1 (Examples) • Example 1: Alpha allows manager of an employee to evaluate the employee: Alpha.evaluatorOf(employee=y) Alpha.managerOf(employee=y) • Example 2: EPub allows CS students to access certain resources: EPub.access(action=‘read’, resource=‘file1’)  EPub.university.stuID(dept=‘CS’) 29

  30. RTT: Supporting Threshold and Separation-of-Duty • Threshold: require agreement among k principals drawn from a given list • SoD: requires two or more different persons be responsible for the completion of a sensitive task • want to achieve SoD without mutual exclusion, which is nonmonotonic • Though related, neither subsumes the other • RT T introduces a primitive that supports both: manifold roles 30

  31. Manifold Roles • While a standard role is a set of principals, a manifold role is a set of sets of principals • A set of principals that together occupy a manifold role can collectively exercise privileges of that role • Two operators: ⊙, ⊗ • K1.R1 ⊗ K2.R2 contains sets of two distinct principals, one a member of K1.R1, the other of K2.R2 • K1.R1 ⊙ K2.R2 does not require them to be distinct 31

  32. RTT (Examples) • Example 1: require a manager and an accountant • K.approval  K.manager  K.accountant • members(K.approval) {{x,y} | x  K.manager, y  K.accountant} • Example 2: require a manager and a different accountant • K.approval  K.manager  K.accountant • members(K.approval) {{x,y} | x  y, x  K.manager, y  K.accountant} 32

  33. RTT (Examples) • Example 3: require three different managers • K.approval  K.manager  K.manager  K.manager • members(K.approval) {{x,y,z} | x  y  z  K.manager} 33

  34. Next Lecture • Database Access Control • Review of the course 34

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