Question: What happens if the BSF finds that not all required USSs identified by GSIDs are present in a subscriber's GUSS during a NAF request for shared keys?
Option 1: The BSF derives the NAF-specific shared key(s) anyway and sends them along with available USSs to the NAF.
Option 2: The BSF omits only the missing USSs from the response but still sends the derived NAF-specific shared key(s) and available USSs to the NAF.
Option 3: The BSF sends an error message to the NAF indicating that not all required USSs are present, without sending any shared keys or USSs.
Option 4: The BSF requests additional subscriber information from external sources to try and fulfill the NAF's request.
Explanation: According to security principles, if the BSF cannot fulfill a NAF's request due to missing required USSs in the subscriber's GUSS, it prioritizes security. The BSF communicates the issue to the NAF through an error message, ensuring no shared keys are improperly distributed, which could potentially weaken the security setup between the UE and the NAF.