Question: During handover from E-UTRAN to GERAN, how does the target SGSN derive the GPRS cipher key (Kc) for a UE capable of handling UMTS security contexts?
Option 1: Directly from the existing E-UTRAN encryption keys without any conversion.
Option 2: Using a random number generator to create new keys.
Option 3: By applying a key conversion function to CK' and IK' to derive Kc (and Kc128 if required).
Option 4: From the UE's stored GPRS security context before the handover.
Explanation: When transitioning from E-UTRAN to GERAN, maintaining secure communication requires adapting encryption keys. The target SGSN converts existing UMTS security contexts (CK' and IK') into a GPRS-compatible format using a key conversion function, ensuring seamless security during handover by deriving the appropriate GPRS cipher keys (Kc or Kc128), thus maintaining continuity of secure data transfer.