A key-management service is built so that its key material can be reached only through three published operations — wrap, unwrap and rotate — with no operation that returns a key. Which principle does this most precisely illustrate?

Answer

Encapsulation

Options - A. Encapsulation - B. Isolation - C. Least privilege - D. Economy of mechanism Why - A. Correct — the internals are reachable only through designated entry points on the object itself, which is exactly encapsulation. - B. Not the most precise answer. Isolation is the general principle here and encapsulation is the specific object-oriented form of it; when a question describes designated entry points, it is asking for the specific one. - C. Wrong — least privilege concerns how few rights a principal holds. Nothing here is about the caller’s rights; the interface simply has no such operation. - D. Wrong — three operations is a small interface, but the property being described is where the boundary is, not how big the design is. Encapsulation is isolation scoped to an object’s internals and enforced by its interface. When a stem says "reachable only through its own procedures" or "designated entry points", it wants encapsulation; when it says "no route between these two networks" or "each process has its own memory", it wants isolation.

Stallings & Brown 5e ch1 §1.5; NCAE cyber-defence principles [NCAE13]