Access Control and the Bell-LaPadula Model

CS 4235

Historical Background

• Physical Access Control• No mixing of data (sensitive vs not)• Hardwired terminal access• No multiplexing of users and data• What happens when all the data is stored in

the same place and users with different trust levels are allowed to access?

• Multi-level security problem

Documents vs People

• Documents have classifications– Top Secret– Secret– Confidential– Unclassified

• Sensitive• Non sensitive

• People have Clearances– Top Secret– Secret– Q

There are also code words that are not classifications

• ULTRA identified information encrypted with Enigma machines• Categories – now material is handled

– Sensitive compartmented information (SCI) - Intelligence• Operations and methods• Nuclear secrets• Stealth

– Special Access Programs (SAP) -- Defense• Acknowledged• Unacknowledged• Waived

– Solves two logistical problems• Collateral clearances for everyone would be expensive• Need to limit information to those with need to know

– SIGMA (Department of Energy)– SAP/SCI requires Secure Compartmented Information Facility (SCIF)

Caveats and Other Codes

• NOFORN• RESTRICTED• NO CONTRACTOR• REL TO <Country Code>• ORCON• FOUO• PROPIN• SECRET//<compartment

name>//NOFORN//ORCON//25X1

People are cleared to

• Classification levels• Categories• Other Labels

Discretionary Access Control

• E.g., Unix permissions• Set access conditions on a file so that only a

group of your choosing can read it• Anyone with access can propagate the

information by resetting permissions

Mandatory Access Control

• Security authority sets permissions• Only security authority can propagate

information• Violations are very serious

Orderings• TS > S > C• How about – (S//NUC//NOFORN) vs TS?– (TS//EUR/25x1) vs (TS//CRYPTO//PROPIN)?

Access Control Models

O1 O2 O3 O4

S1

S2 operation

S3

S4

Read (observe)Write (observe, alter)

Execute (no observe, no alter)Append (alter, no observe)

(S,O,R) YES/NO

Accesses take system from state to state

(S,a, read)

(T,b, append)

σ1

σ2

σ3

If you start in a secure state do you end up in a secure state?

All accesses must be allowed by MAC rules

Granting Access Should Not Violate MAC

Object

Object

High Level

LowLevel

Subject

WRITE

? READ

Flow of information

Simple Security Property

• The current level of a subject dominates the level of every object that it observes

• Like paper systems• “No read up”

*-Property

• If S can observe a and alter b, then a ≤ b• “No write down”

Partial Orders

• S = {a1,a2,…,an}• P = (S, ≤) is a PO iff

– If a ≤ b and b ≤ a, then a = b (anti-symmetric)– If a ≤ b and b ≤ c, then a ≤ c (transitive)– a ≤ a (reflexive)

• Examples– Natural numbers under ≤– Subsets under

• How about– Choices on a ballot under “is preferred to”?– People under “trusts”?

Lattices

• A POSET S• Every subset of S has a greatest lower bound• Every subset of S has a least upper bound

x

x1 x2

x3 x4 x5

SSubset of S

LUBThese are all upper bounds

Security Levels

• A security level is a pair (c,s) where– c is a classification from a POSET of classifications

(e.g., U,S,TS but the exact classfications don’t matter)

– s is a set of categories (e.g., NUC,CRYPTO,… but the exact categories don’t matter)

• (c1,s1) ≥ (c2,s2) iff c1 ≥ c2 and s2s1

• Levels form a lattice

Assigning Security Levels to Subjects and Objects

• level(S), level(O) = security level of S,O• current-level(S) = levels at which S can operate• current-level(S) ≤ level(S)• level(S) = max(current-level(S)) is called S’s

clearance

Security Properties

• SS-property:For any (S,O,A) if A includes observation then

level(S) ≥ level(O)• *-property

For any (S,O,A)r A implies current-level(S) ≥ level(O) a A implies current-level(S)≤level(O)w A implies current-level(S) = level(O)

No read up

No write down

If a subject can observe O1 and modify O2 the level(O2)≥level(O1)

Lattice ModelInformation only flows up the latticeSystem enforces SS and * properties

A MAC Implementation

• Unix file system• Label all files and directory with levels• Assign level(u) to each user u• u is initially assigned the lowest current-level• Allow current-level(u) to float as higher level files

are observed• If level(u) ≥ current-level(u) issue kill(u)• If level(f) ≥ level(u) and u writes to f issue kill(u)• Is this secure?

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