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.