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CS363

Week 8 - Wednesday. CS363. Last time. What did we talk about last time? Authentication Challenge response Biometrics Started Bell-La Padula model. Questions?. Project 2. Security Presentation. Yuki Gage. Bell-La Padula Model. Bell-La Padula overview.

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CS363

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  1. Week 8 - Wednesday CS363

  2. Last time • What did we talk about last time? • Authentication • Challenge response • Biometrics • Started Bell-La Padula model

  3. Questions?

  4. Project 2

  5. Security Presentation Yuki Gage

  6. Bell-La Padula Model

  7. Bell-La Padula overview • Confidentiality access control system • Military-style classifications • Uses a linear clearance hierarchy • All information is on a need-to-know basis • It uses clearance (or sensitivity) levels as well as project-specific compartments

  8. Security clearances • Both subjects (users) and objects (files) have security clearances • Below are the clearances arranged in a hierarchy

  9. Simple security condition • Let levelO be the clearance level of object O • Let levelS be the clearance level of subject S • The simple security condition states that S can read O if and only if the levelO≤ levelS and S has discretionary read access to O • In short, you can only read down • Example? • In a few slides, we will expand the simple security condition to make the concept of level

  10. *-Property • The *-property states that S can write O if and only if the levelS≤ levelO and S has discretionary write access to O • In short, you can only write up • Example?

  11. Basic security theorem • Assume your system starts in a secure initial state • Let T be all the possible state transformations • If every element in T preserves the simple security condition and the *-property, every reachable state is secure • This is sort of a stupid theorem, because we define “secure” to mean a system that preserves the security condition and the *-property

  12. Adding compartments • We add compartments such as NUC = Non-Union Countries, EUR = Europe, and US = United States • The possible sets of compartments are: •  • {NUC} • {EUR} • {US} • {NUC, EUR} • {NUC, US} • {EUR, US} • {NUC, EUR, US} • Put a clearance level with a compartment set and you get a security level • The literature does not always agree on terminology

  13. Romaine lattice • The subset relationship induces a lattice {NUC, EUR, US} {NUC, EUR} {NUC, US} {EUR, US} {NUC} {EUR} {US} 

  14. Updated properties • Let L be a clearance level and C be a category • Instead of talking about levelO≤ levelS, we say that security level (L, C) dominates security level (L’, C’) if and only if L’ ≤ L and C’ C • Simple security now requires (LS, CS) to dominate (LO, CO) and S to have read access • *-property now requires (LO, CO) to dominate (LS, CS) and S to have write access • Problems?

  15. Clark-Wilson Model

  16. Clark-Wilson model • Commercial model that focuses on transactions • Just like a bank, we want certain conditions to hold before a transaction and the same conditions to hold after • If conditions hold in both cases, we call the system consistent • Example: • D is the amount of money deposited today • W is the amount of money withdrawn today • YB is the amount of money in accounts at the end of business yesterday • TB is the amount of money currently in all accounts • Thus, D + YB – W = TB

  17. Clark-Wilson definitions • Data that has to follow integrity controls are called constrained data items or CDIs • The rest of the data items are unconstrained data items or UDIs • Integrity constraints (like the bank transaction rule) constrain the values of the CDIs • Two kinds of procedures: • Integrity verification procedures (IVPs) test that the CDIs conform to the integrity constraints • Transformation procedures (TPs) change the data in the system from one valid state to another

  18. Clark-Wilson rules • Clark-Wilson has a system of 9 rules designed to protect the integrity of the system • There are five certification rules that test to see if the system is in a valid state • There are four enforcement rules that give requirements for the system

  19. Certification Rules 1 and 2 • CR1: When any IVP is run, it must ensure that all CDIs are in a valid state • CR2: For some associated set of CDIs, a TP must transform those CDIs in a valid state into a (possibly different) valid state • By inference, a TP is only certified to work on a particular set of CDIs

  20. Enforcement Rules 1 and 2 • ER1: The system must maintain the certified relations, and must ensure that only TPs certified to run on a CDI manipulate that CDI • ER2: The system must associate a user with each TP and set of CDIs. The TP may access those CDIs on behalf of the associated user. If the user is not associated with a particular TP and CDI, then the TP cannot access that CDI on behalf of that user. • Thus, a user is only allowed to use certain TPs on certain CDIs

  21. Certification Rule 3 and Enforcement Rule 3 • CR3: The allowed relations must meet the requirements imposed by the principle of separation of duty • ER3: The system must authenticate each user attempting to execute a TP • In theory, this means that users don't necessarily have to log on if they are not going to interact with CDIs

  22. Certification Rules 4 and 5 • CR4: All TPs must append enough information to reconstruct the operation to an append-only CDI • Logging operations • CR5: Any TP that takes input as a UDI may perform only valid transformations, or no transformations, for all possible values of the UDI. The transformation either rejects the UDI or transforms it into a CDI • Gives a rule for bringing new information into the integrity system

  23. Enforcement Rule 4 • ER4: Only the certifier of a TP may change the list of entities associated with that TP. No certifier of a TP, or of any entity associated with that TP, may ever have execute permission with respect to that entity. • Separation of duties

  24. Clark-Wilson summary • Designed close to real commercial situations • No rigid multilevel scheme • Enforces separation of duty • Certification and enforcement are separated • Enforcement in a system depends simply on following given rules • Certification of a system is difficult to determine

  25. Chinese Wall Model

  26. Chinese Wall overview • The Chinese Wall model respects both confidentiality and integrity • It's very important in business situations where there are conflict of interest issues • Real systems, including British law, have policies similar to the Chinese Wall model • Most discussions around the Chinese Wall model are couched in business terms

  27. Chinese Wall definitions • We can imagine the Chinese Wall model as a policy controlling access in a database • The objects of the database are items of information relating to a company • A company dataset (CD) contains objects related to a single company • A conflict of interest (COI) class contains the datasets of companies in competition • Let COI(O) be the COI class containing object O • Let CD(O) be the CD that contains object O • We assume that each object belongs to exactly one COI

  28. COI Examples Bank COI Class Gasoline Company COI Class Bank of America a Citibank c Bank of the West b Shell Oil s Standard Oil e Union '76 u ARCO n

  29. CW-Simple Security Condition • Let PR(S) be the set of objects that S has read • Subject S can read O if and only if any of the following is true • There is an object O' such that S has accessed O' and CD(O') = CD(O) • For all objects O', O' PR(S) COI(O')  COI(O) • O is a sanitized object • Give examples of objects that can and cannot be read

  30. CW-*-Property • Subject S may write to an object O if and only if 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)

  31. Biba Model

  32. Biba overview • Integrity based access control system • Uses integrity levels, similar to the clearance levels of Bell-LaPadula • Precisely the dual of the Bell-LaPadula Model • That is, we can only read up and write down • Note that integrity levels are intended only to indicate integrity, not confidentiality • Actually a measure of accuracy or reliability

  33. Formal rules • S is the set of subjects and O is the set of objects • Integrity levels are ordered • i(s) and i(o) gives the integrity level of s or o, respectively • Rules: • s S can read o  O if and only if i(s) ≤ i(o) • s S can write to o  O if and only if i(o) ≤ i(s) • s1 S can execute s2  S if and only if i(s2) ≤ i(s1)

  34. Extensions • Rules 1 and 2 imply that, if both read and write are allowed, i(s) = i(o) • By adding the idea of integrity compartments and domination, we can get the full dual of the Bell-La Padula lattice framework • Real systems (for example the LOCUS operating system) usually have a command like run-untrusted • That way, users have to recognize the fact that a risk is being made • What if you used the same levels for integrity AND security, could you implement both Biba and Bell-La Padula on the same system?

  35. Theoretical Limitations on Access Control

  36. Determining security • How do we know if something is secure? • We define our security policy using our access control matrix • We say that a right is leaked if it is added to an element of the access control matrix that doesn’t already have it • A system is secure if there is no way rights can be leaked • Is there an algorithm to determine if a system is secure?

  37. Mono-operational systems • In a mono-operational system, each command consists of a single primitive command: • Create subject s • Create object o • Enter r into a[s,o] • Delete r from a[s,o] • Destroy subject s • Destroy object o • In this system, we could see if a right is leaked with a sequence of k commands

  38. Proof • Delete and Destroy commands can be ignored • No more than one Create command is needed (in the case that there are no subjects) • Entering rights is the trouble • We start with set S0 of subjects and O0 of objects • With n generic rights, we might add all n rights to everything before we leak a right • Thus, the maximum length of the command sequence that leaks a right is k ≤ n(|S0|+1)(|O0|+1) + 1 • If there are m different commands, how many different command sequences are possible?

  39. Turing machine • A Turing machine is a mathematical model for computation • It consists of a head, an infinitely long tape, a set of possible states, and an alphabet of characters that can be written on the tape • A list of rules saying what it should write and should it move left or right given the current symbol and state A

  40. Turing machine example • 3 state, 2 symbol “busy beaver” Turing machine: • Starting state A

  41. Church-Turing thesis • If an algorithm exists, a Turing machine can perform that algorithm • In essence, a Turing machine is the most powerful model we have of computation • Power, in this sense, means the ability to compute some function, not the speed associated with its computation

  42. Halting problem • Given a Turing machine and input x, does it reach the halt state? • It turns out that this problem is undecidable • That means that there is no algorithm that can be to determine if any Turing machine will go into an infinite loop • Consequently, there is no algorithm that can take any program and check to see if it goes into an infinite loop

  43. Leaking Undecidable

  44. Simulate a Turing machine • We can simulate a Turing machine using an access control matrix • We map the symbols, states and tape for the Turing machine onto the rights and cells of an access control matrix • Discovering whether or not the right leaks is equivalent to the Turing machine halting with a 1 or a 0

  45. The bad news • Without heavy restrictions on the rules for an access control, it is impossible to construct an algorithm that will determine if a right leaks • Even for a mono-operational system, the problem might take an infeasible amount of time • But, we don’t give up! • There are still lots of ways to model security • Some of them offer more practical results

  46. Upcoming

  47. Next time… • Finish theoretical limitations • Trusted system design elements • Common OS features and flaws • OS assurance and evaluation • Taylor Ryan presents

  48. Reminders • Read Sections 5.4 and 5.5 • Keep working on Project 2 • Finish Assignment 3

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