Information Security Principles & Applications

1. Information SecurityPrinciples & Applications Topic 6: Security Policy Models 虞慧群 yhq@ecust.edu.cn

2. Introduction • Primary mission of information security is to ensure that systems and contents stay the same. • If no threats, we could focus on improving systems, resulting in vast improvements in ease of use and usefulness. • Attacks on information systems are a daily occurrence.

3. Security Policy • Defines what it means for a system to be secure • Formally: Partitions a system into • Set of secure (authorized) states • Set of non-secure (unauthorized) states • Secure system is one that • Starts in authorized state • Cannot enter unauthorized state

4. Secure System - Example Unauthorized states A B C D Authorized states • Is this Finite State Machine Secure? • A is start state ? • B is start state ? • C is start state ? • How can this be made secure if not? • Suppose A, B, and C are authorized states ?

5. Additional Definitions: • Security breach: system enters an unauthorized state • Let X be a set of entities, I be information. • I has confidentiality with respect to X if no member of X can obtain information on I • I has integrity with respect to X if all members of X trust I • Trust I, its conveyance and protection (data integrity) • I maybe origin information or an identity (authentication) • I is a resource – its integrity implies it functions as it should (assurance) • I has availability with respect to X if all members of X can access I • Time limits (quality of service)

6. Confidentiality Policy • Also known as information flow policy • Transfer of rights • Transfer of information without transfer of rights • Temporal context • Highly developed in Military/Government

7. Integrity Policy • Defines how information can be altered • Entities allowed to alter data • Conditions under which data can be altered • Limits to change of data • Examples: • Purchase over $1000 requires signature • Check over $10,000 must be approved by one person and cashed by another • Separation of duties : for preventing fraud • Highly developed in commercial world

8. Availability Policy • An availability policy describes what services must be provided. • It may present parameters within which the service will be accessible. • It may require a level of service.

9. Security Mechanism • Policy describes what is allowed and/or what is not. • Mechanism • An entity/procedure that enforces (part of) policy. • Example Policy: Students should not copy homework. • Mechanism: Disallow access to files owned by other users. • Does mechanism enforce policy?

10. Security Model • Security Policy: What is/isn’t authorized • Problem: Policy specification often informal • Implicit vs. Explicit • Ambiguity • Security Model: Model that represents a particular policy (policies) • Model must be explicit, unambiguous • Abstract details for analysis

11. High-Level Policy Languages • High-level: Independent of mechanisms • Constraints expressed independent of enforcement mechanism • Constraints restrict entities, actions • Constraints expressed unambiguously • Requires a precise language, usually a mathematical, logical, or programming-like language • Example: Domain-Type Enforcement Language • Subjects partitioned into domains • Objects partitioned into types • Each domain has a set of rights over each type

12. Example: Web Browser • Goal: restrict actions of Java programs that are downloaded and executed under control of web browser • Language specific to Java programs • Expresses constraints as conditions restricting invocation of entities

13. Expressing Constraints • Entities are classes, methods • Class: set of objects that an access constraint constrains • Method: set of ways an operation can be invoked • Operations • Instantiation: s creates instance of class c: s├c • Invocation: s1 executes object s2: s1 |→s2 • Access constraints • deny(sopx) whenb • when b is true, subject s cannot perform op on (subject or class) x; empty s means all subjects

14. Sample Constraints • Downloaded program cannot access password database file on UNIX system • Program’s class and methods for files: class File { public file(String name); public String getfilename(); public char read(); …. • Constraint: deny(|→ file.read) when (file.getfilename() == “/etc/passwd”)

15. Low-Level Policy Languages • Low-level: close to mechanisms • A set of inputs or arguments to commands that set, or check, constraints on a system • Example: Tripwire: Flags what has changed • Configuration file specifies settings to be checked • History file keeps old (good) example

16. Confidentiality Policy • Also known as information flow policy • Integrity is secondary objective • Eg. Military mission date • Bell-LaPadula Model • Formally models military requirements • Information has sensitivity levels or classification • Subjects have clearance • Subjects with clearance are allowed access • Multi-level access control or mandatory access control

17. Bell-LaPadula: Basics • Mandatory access control • Entities are assigned security levels • Subject has security clearance L(s) = ls • Object has security classification L(o) = lo • Simplest case: Security levels are arranged in a linear order li< li+1 • Example Top secret > Secret > Confidential >Unclassified

18. “No Read Up” • Information is allowed to flow up, not down • Simple security property: • s can read o if and only if • lo≤ lsand • s has read access to o • Combines mandatory (security levels) and discretionary (permission required) • Prevents subjects from reading objects at higher levels (No Read Up rule)

19. “No Write Down” • Information is allowed to flow up, not down • *property • s can write o if and only if • ls≤ lo and • s has write access to o • Combines mandatory (security levels) and discretionary (permission required) • Prevents subjects from writing to objects at lower levels (No Write Down rule)

20. Example • Tamara can read which objects? And write? • Claire cannot read which objects? And write? • Ulaley can read which objects? And write?

21. Access Rules • Secure system: • One in which both the properties hold • Theorem: Let Σ be a system with secure initial state σ0, T be a set of state transformations • If every element of T follows rules, every state σi secure • Proof - induction

22. Categories • Total order of classifications not flexible enough • Alice cleared for missiles; Bob cleared for warheads; Both cleared for targets • Solution: Categories • Use set of compartments (from power set of compartments) • Enforce “need to know” principle • Security levels (security level, category set) • (Top Secret, {Nuc, Eur, Asi}) • (Top Secret, {Nuc, Asi}) • Combining with clearance: • (L,C) dominates (L’,C’)  L’ ≤L and C’ C • Induces lattice of security levels

23. {Nuc, Eur, Us} {Nuc, Eur} {Nuc, Us} {Eur, Us} {Us} {Nuc} {Eur} {} Lattice of categories • Examples of levels • (Top Secret, {Nuc,Asi}) dom (Secret, {Nuc}) • (Secret, {Nuc, Eur}) dom (Confidential, {Nuc,Eur}) • (Top Secret, {Nuc}) dom (Confidential, {Eur}) • Bounds • Greatest lower, • Lowest upper • glbof {X, Nuc, Us} & {X, Eur, Us}? • lubof {X, Nuc, Us} & {X, Eur, Us}?

24. Access Rules • Simple Security Condition: S can read O if and only if • S dominate O and • S has read access to O • *-Property: S can write O if and only if • O dom S and • S has write access to O • Secure system: One with above properties • Theorem: Let Σ be a system with secure initial state σ0, T be a set of state transformations • If every element of T follows rules, every state σi secure

25. Problem: No write-down Cleared subject can’t communicate to non-cleared subject • Any write from li to lk, i > k, would violate *-property • Subject at li can only write to li and above • Any read from lk to li, i > k, would violate simple security property • Subject at lk can only read from lk and below • Subject at level i can’t write something readable by subject at k • Not very practical A solution: each subject has a maximum security level and a current security level. A subject may decrease its security level from maximum in order to communicate with entities at lower security levels.

26. Integrity Policy Requirements Commercial requirements differ from military requirements in their emphasis on preserving data integrity. • Users will not write their own programs, but will use existing production programs and databases. • Programmers will develop and test programs on a nonproduction system; if they need access to actual data, they will be given production data via a special process, but will use it on their development system. • A special process must be followed to install a program from the development system onto the production system. • The special process in requirement 3 must be controlled and audited. • The managers and auditors must have access to both the system state and the system logs that are generated.

27. Integrity Policy: Principles of operation • These requirements induce principles of operation: • Separation of Duty: Single person should not be allowed to carry out all steps of a critical function • Moving a program from Dev. to Prod. system • Developer and Certifier (installer) of a program • Authorizing checks and cashing it • Separation of function • Do not process production data on development system • Auditing • Emphasis on recovery and accountability • Controlled/audited process for updating code on production system

28. Biba’s Integrity Policy Model • Based on Bell-LaPadula (a mathematical dual of BL) • Subject, Objects • Integrity Levels with dominance relation • Higher levels • more reliable/trustworthy • More accurate • Information transfer path:Sequence of subjects, objects where • siroi • siwoi+1

29. Policies • Low-Water-Mark Policy • swo i(o) ≤i(s) prevents writing to higher level • sro  i’(s) = min(i(s), i(o)) drops subject’s level • s1xs2  i(s2) ≤i(s1) prevents executing higher level objects • Ring Policy • sro allows any subject to read any object • swo  i(o) ≤i(s) (same as above) • s1xs2  i(s2) ≤i(s1) • Biba’s Model: Strict Integrity Policy (dual of Bell-LaPadula) • sro  i(s) ≤i(o) (no read-down) • swo i(o) ≤i(s) (no write-up) • s1xs2  i(s2) ≤i(s1) • Theorem for each: • If there is an information transfer path from object o1 to object on+1, then the enforcement of the policy requires that i(on+1) ≤i(o1) for all n>1

30. Chinese Wall Model • Supports confidentiality and integrity, i.e. a hybrid policy • Information can’t flow between items in a Conflict of Interest set • Applicable to environment of stock exchange or investment house • Models conflict of interest • Objects: items of information related to a company • Company dataset (CD): contains objects related to a single company • Written CD(O) • Conflict of interestclass (COI): contains datasets of companies in competition • Written COI(O) • Assume: each object belongs to exactly one COI class

31. Example Bank COI Class Gasoline Company COI Class Bank of America Shell Oil Standard Oil a Bank of the West Union ’76 ARCO Citibank a

32. CW-Simple Security Property (Read rule) • CW-Simple Security Property • s can read o one of the following holds •  o’  PR(s) such that CD(o’) = CD(o) •  o’, o’  PR(s)  COI(o’)  COI(o), or • o has been “sanitized” (o’  PR(s) indicates o’ has been previously read by s) • Public information may belong to a CD • As is publicly available, no conflicts of interest arise • So, should not affect ability of analysts to read • Typically, all sensitive data removed from such information before it is released publicly (called sanitization)

33. Writing • Anthony, Susan work in the same trading house • Anthony can read BankOfAmercia’s CD, • Susan can read Bank CitiBanks’s CD, • Both can read ARCO’s CD • If Anthony could write to Gas’ CD, Susan can read it • Hence, indirectly, she can read information from BankOfAmercia’s CD, a clear conflict of interest

34. CW-*-Property (Write rule) • CW-*- Property • s can write oboth of the following conditions hold. • The CW-simple security condition permits S to read O. • For all unsanitized objects o’, s can read o’  CD(o’) = CD(o) Says that s can write to an object if all the (unsanitized) objects it can read are in the same dataset • Anthony can read both CDs hence condition 1 is met • He can read unsanitized objects of BankOfAmercia, hence condition 2 is false • Hence Anthony can’t write to objects in ARCO’s CD.

35. Access control in organizations is based on “roles that individual users take on as part of the organization” A role is “is a collection of permissions” Role Based Access Control http://csrc.nist.gov/groups/SNS/rbac/

36. RBAC • Access depends on function, not identity • Example: Allison is bookkeeper for Math Dept. She has access to financial records. If she leaves and Betty is hired as the new bookkeeper, Betty now has access to those records. The role of “bookkeeper” dictates access, not the identity of the individual.

37. Advantages of RBAC • Allows Efficient Security Management • Administrative roles, Role hierarchy • Principle of least privilege allows minimizing damage • Separation of Duties constraints to prevent fraud • Allows grouping of objects • Policy-neutral - Provides generality • Encompasses DAC and MAC policies

38. RBAC

39. RBAC (NIST Standard) Permissions PA UA Users Roles Operations Objects user_sessions (one-to-many) role_sessions (many-to-many) Sessions An important difference from classical models is that Subject in other models corresponds to a Session in RBAC

40. Core RBAC (relations) • Permissions = 2Operations x Objects • UA ⊆ Users x Roles • PA ⊆ Permissions x Roles • assigned_users: Roles  2Users • assigned_permissions: Roles  2Permissions • Op(p): set of operations associated with permission p • Ob(p): set of objects associated with permission p • user_sessions: Users  2Sessions • session_user: Sessions  Users • session_roles: Sessions  2Roles • session_roles(s) = {r | (session_user(s), r)  UA)} • avail_session_perms: Sessions  2Permissions

41. RBAC with General Role Hierarchy RH (role hierarchy) Permissions PA UA Users Roles Operations Objects user_sessions (one-to-many) role_sessions (many-to-many) Sessions

42. RBAC with General Role Hierarchy • authorized_users: Roles 2Users authorized_users(r) = {u | r’ ≥ r &(r’, u) UA) • authorized_permissions: Roles 2Permissions authorized_permissions(r) = {p | r’ ≥ r &(p, r’) PA) • RH ⊆ Roles x Roles is a partial order • called the inheritance relation • written as ≥. (r1 ≥ r2) authorized_users(r1) ⊆ authorized_users(r2) & authorized_permisssions(r2) ⊆ authorized_permisssions(r1)

43. px, py px, py px, py px, py e1, e2 e3, e4 e5 x Example pa, pb authorized_users(Employee)? authorized_users(Administrator)? authorized_permissions(Employee)? authorized_permissions(Administrator)? px, py p1, p2

44. Constrained RBAC RH (role hierarchy) Static Separation of Duty Permissions PA UA Users Roles Operations Objects user_sessions (one-to-many) Dynamic Separation of Duty Sessions

45. Static Separation of Duty • SSD ⊆2Roles x N • In absence of hierarchy • Collection of pairs (RS, n) where RS is a role set, n ≥ 2; for all (RS, n) SSD, for allt ⊆RS: |t| ≥ n ∩rtassigned_users(r)=  • In presence of hierarchy • Collection of pairs (RS, n) where RS is a role set, n ≥ 2; for all (RS, n) SSD, for allt ⊆RS: |t| ≥ n ∩rtauthorized_uers(r)= 

46. Dynamic Separation of Duty • DSD ⊆2Roles x N • Collection of pairs (RS, n) where RS is a role set, n ≥ 2; • A user cannot activate n or more roles from RS • Formally?? [HW3?] • What if both SSD and DSD contains (RS, n)? • Consider (RS, n) = ({r1, r2, r3}, 2)? • If SSD – can r1, r2 and r3 be assigned to u? • If DSD – can r1, r2 and r3 be assigned to u?

47. MAC using RBAC HR LW H Read Roles (same lattice) Write Roles (inverse lattice) M1R M2R M1W M2W M1 M2 BLP LR HW L Transformation rules • R = {L1R, L2R,…, LnR, L1W, L2W,…, LnW} • Two separate hierarchies for {L1R, L2R,…, LnR} and { L1W, L2W,…, LnW} • Each user is assigned to exactly two roles: xR and LW • Each session has exactly two roles yR and yW • Permission (o, r) is assigned to xR iff (o, w) is assigned to xW)

48. RBAC’s Benefits

49. Summary • Policy describes what is allowed in a system. • Confidentiality policies • Bell-LaPadula model • Integrity policies • Biba’s model • Hybrid policies • Chinese Wall model • Role-Based Access Control (RBAC) Model