Question: In a direct cross-certification scenario for Network Domain Security (NDS) as per Authentication Framework (AF), where are certificates from other operators **most likely** to be stored by each operator, considering typical telecom infrastructure?
Option 1: Exclusively on User Equipment (UE) devices
Option 2: On the Serving and Packet Data Network Gateway (S/PGW) nodes
Option 3: Either on Security Edge Protection (SEG) devices or a common repository
Option 4: Only on the Home Subscriber Server (HSS) database
Explanation: In direct cross-certification for NDS, storing certificates from other operators requires secure, centralized, and possibly distributed solutions. Option 3 is correct because SEG devices are part of an operator’s security infrastructure, making them a suitable storage location, while a common repository also offers a centralized management approach, aligning with the need for efficient certificate management across different operators. Options 1, 2, and 4 are incorrect as they either refer to end-user devices (UE), core network nodes not primarily focused on security (S/PGW), or a database mainly used for subscriber information (HSS), respectively.