DAC vs. MAC Bell-La Padula model [BL] Most people familiar with discretionary access View the system as subjects accessing objects • • control (DAC) - The system input is requests, the output is decisions - Unix permission bits are an example - Objects can be organized in one or more hierarchies, H - Might set a file private so only group friends can read it (a tree enforcing the type of decendents) Discretionary means anyone with access can Four modes of access are possible: • • propagate information: - execute – no observation or alteration - Mail
[email protected] < private - read – observation Mandatory access control - append – alteration • - Security administrator can restrict propagation - write – both observation and modification - Abbreviated MAC (NOT to be confused w. Message The current access set, b, is (subj, obj, attr) tripples Authentication Code or Medium Access Control) • An access matrix M encodes permissible access types • (as before, subjects are rows, objects columns) 1/39 2/39 Security levels Security properties A security level is a (c, s) pair: The simple security or ss-property: • • - c = classification – E.g., unclassified, secret, top secret - For any (S, O, A) b, if A includes observation, then level(S) ∈ - s = category-set – E.g., Nuclear, Crypto must dominate level(O) (c , s ) dominates (c , s ) iff c c and s s - E.g., an unclassified user cannot read a top-secret document • 1 1 2 2 1 ≥ 2 1 ⊇ 2 - L dominates L sometimes written L L or L L The star security or ⋆-property: 1 2 1 ⊒ 2 2 ⊑ 1 • - levels then form a lattice (partial order w.