Question: In a Radio Access Network (RAN) scenario where a User Equipment (UE) performs Idle Mode mobility from E-UTRAN to GERAN, how does the target Serving GPRS Support Node (SGSN) derive the GPRS cipher key Kc for security context?
Option 1: Directly from the UE's stored UMTS security contexts without any conversion.
Option 2: By applying a defined key conversion function to CK' and IK', as per 3G Security Architecture standards.
Option 3: Through a random key generation process initiated by the SGSN upon detecting UE's idle mode mobility.
Option 4: By requesting the Kc directly from the UE via a dedicated signaling message.
Explanation: When transitioning from E-UTRAN to GERAN, to maintain seamless security, the target SGSN adapts existing UMTS security contexts (CK' and IK') for use in the GERAN environment by applying a key conversion function, ensuring compatibility and continued security across different radio access technologies.