Question: In a Generic Authentication Architecture (GAA) scenario where a Network Application Function (NAF) does not use User Security Settings (USSs) and the Bootstrapping Server Function (BSF) has a local policy for this NAF, what action does the BSF take if all required USSs are **not** present in the subscriber's Generic User Security Settings (GUSS)?
Option 1: The BSF derives the NAF-specific shared key(s) and sends them to the NAF with a warning.
Option 2: The BSF sends an error message to the NAF indicating "not authorized".
Option 3: The BSF requests additional USSs from the NAF over the Zn reference point.
Option 4: The BSF updates its local policy to accommodate the missing USSs for this NAF.
Explanation: According to the scenario, if the BSF checks and finds that not all required USSs (identified by GSIDs) are present in the subscriber's GUSS, it sends an error message to the NAF. This is a critical security measure to ensure that access is only granted when all policy requirements are met, thus protecting both the network and the user. The correct action in this case is to deny authorization due to non-compliance with the local policy, which aligns with sending an "not authorized" error message.