Question: In a 3GPP SAE security architecture, when using the 'Subscriber key separation' mechanism for multiple local HSSs (Home Subscriber Servers), what is the primary purpose of deriving a unique key `K_n` from the master key `MK` for each local HSS_n?
Option 1: To enable seamless handovers between different local HSSs.
Option 2: To reduce computational overhead in authentication procedures.
Option 3: To enhance security by using a single, global key across all HSSs.
Option 4: To limit the impact of a compromise to only the affected local HSS or subclass of HSSs.
Explanation: Deriving a unique `K_n` from `MK` for each local HSS_n ensures that if one local HSS is compromised, the impact is contained to that specific HSS or, in cases of grouping into subclasses, to only those within the same subclass, thereby enhancing security by minimizing the scope of potential breaches.