Session 2 - Security Models and Architecture

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OverviewOverview

• Basic concepts

• The Models

– Bell-LaPadula (BLP)

– Biba

– Clark-Wilson

– Chinese Wall

• Systems Evaluation

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Basic Concepts

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TerminologyTerminology

• Trusted Computing Base (TCB) – combination of protection mechanisms within a computer system

• Subjects / Objects

– Subjects are active (e.g., users / programs)

– Objects are passive (e.g., files)

• Reference Monitor – abstract machine that mediates subject access to objects

• Security Kernel – core element of TCB that enforces the reference monitor’s security policy

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Types of Access ControlTypes of Access Control

• Discretionary Access Control (DAC) – data owners can create and modify matrix of subject / object relationships (e.g., ACLs)

• Mandatory Access Control (MAC) – “insecure” transactions prohibited regardless of DAC

• Cannot enforce MAC rules with DAC security kernel

– Someone with read access to a file can copy it and build a new “insecure” DAC matrix because he will be an owner of the new file.

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Information Flow ModelsInformation Flow Models

• Pour cement over a PC and you have a secure system

• In reality, there are state transitions

• Key is to ensure transitions are secure

• Models provide rules for how information flows from state to state.

• Information flow models do not address covert channels

– Trojan horses

– Requesting system resources to learn about other users

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Access Control Models

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ModelsModels

• Bell-LaPadula

• Biba

• Clark-Wilson

• Chinese Wall

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Bell-LaPadula (BLP) ModelBell-LaPadula (BLP) Model

• BLP is formal (mathematical) description of mandatory access control

• Three properties:

– ds-property (discretionary security)

– ss-property (simple security – no “read up”)

– *-property (star property – no “write down”)

• A secure system satisfies all of these properties

• BLP includes mathematical proof that if a system is secure and a transition satisfies all of the properties, then the system will remain secure.

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Bell-LaPadula Model (Continued)Bell-LaPadula Model (Continued)

• Honeywell Multics kernel was only true implementation of BLP, but it never took hold

• DOD information security requirements currently achieved via discretionary access control and segregation of systems rather than BLP-compliant computers

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Bell-LaPadula Model, Step 1Bell-LaPadula Model, Step 1

• Security levels arranged in linear ordering

– Top Secret: highest

– Secret

– Confidential

– Unclassified: lowest

• Levels consist of security clearance L(s)

– Objects have security classification L(o)

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ExampleExample

security level subject object

Top Secret Tamara Personnel Files

Secret Samuel E-Mail Files

Confidential Claire Activity Logs

Unclassified Larry Telephone Lists

• Tamara can read all files• Claire cannot read Personnel or E-Mail Files• Larry can only read Telephone Lists

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Reading InformationReading Information

• Information flows up, not down

– “Reads up” disallowed, “reads down” allowed

• Simple Security Condition (Step 1)

– Subject s can read object o iff, L(o) ≤ L(s) and s has permission to read o

– Sometimes called “no reads up” rule

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Writing InformationWriting Information

• Information flows up, not down

– “Writes up” allowed, “writes down” disallowed

• *-Property (Step 1)

– Subject s can write object o iff L(s) ≤ L(o) and s has permission to write o

– Sometimes called “no writes down” rule

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Example ScenarioExample Scenario

Role User Clearance Projects

Project Manager

Alice High Proj1,Proj2,Proj3

Intern Bob Low Proj1,Proj2

Dev Manager Charles High Proj1

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Foundation Sensitivity LabelsFoundation Sensitivity Labels

User Sensitivity Label

Alice High:Proj1,Proj2,Proj3

Bob Low:Proj1,Proj2

Charles High:Proj1

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OperationsOperations

• What is the highest Proj1 file label such that

– Alice and Bob can both read?

– Alice and Charles can both read?

– All three can read

• What about write?

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Biba ModelBiba Model

• Similar to BLP but focus is on integrity, not confidentiality

• Result is to turn the BLP model upside down

– High integrity subjects cannot read lower integrity objects (no “read down”)

– Subjects cannot move low integrity data to high-integrity environment (no “write up”)

• McLean notes that ability to flip models essentially renders their assurance properties useless

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Clark-Wilson ModelClark-Wilson Model

• Reviews distinction between military and commercial policy

– Military policy focus on confidentiality

– Commercial policy focus on integrity

• Mandatory commercial controls typically involve who gets to do what type of transaction rather than who sees what (Example: cut a check above a certain dollar amount)

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Clark-Wilson Model (Continued)Clark-Wilson Model (Continued)

• Two types of objects:

– Constrained Data Items (CDIs)

– Unconstrained Data Items (UDIs)

• Two types of transactions on CDIs in model

– Integrity Verification Procedures (IVPs)

– Transformation Procedures (TPs)

• IVPs certify that TPs on CDIs result in valid state

• All TPs must be certified to result in valid transformation

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Clark-Wilson Model (Continued)Clark-Wilson Model (Continued)

• System maintains list of valid relations of the form:{UserID, TP, CDI/UDI}

• Only permitted manipulation of CDI is via an authorized TP

• If a TP takes a UDI as an input, then it must result in a proper CDI or the TP will be rejected

• Additional requirements

– Auditing: TPs must write to an append-only CDI (log)

– Separation of duties

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Clark-Wilson versus BibaClark-Wilson versus Biba

• In Biba’s model, UDI to CDI conversion is performed by trusted subject only (e.g., a security officer), but this is problematic for data entry function.

• In Clark-Wilson, TPs are specified for particular users and functions. Biba’s model does not offer this level of granularity.

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Chinese WallChinese Wall

Focus is on conflicts of interest.

• Principle: Users should not access the confidential information of both a client organization and one or more of its competitors.

• How it works

– Users have no “wall” initially.

– Once any given file is accessed, files with competitor information become inaccessible.

– Unlike other models, access control rules change with user behavior

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ConclusionConclusion

• In practice, DAC is widely used.

• Other models are too stringent and expensive.

• Access control list is common application of DAC.