Question: In a 3GPP System Architecture Evolution (SAE) security context, when a Master eNodeB (MeNB) updates the S-KgNB, what action must the User Equipment (UE) take upon receiving the new SCG Counter value via an integrity-protected RRC procedure?
Option 1: The UE shall discard all existing security contexts and establish a new connection with the Serving Gateway Node B (SgNB).
Option 2: The UE shall derive a new S-KgNB but only update KSgNB-RRC-int and KSgNB-RRC-enc.
Option 3: The UE shall derive the new S-KgNB based on the provided SCG Counter value and then re-calculate all of KSgNB-UP-int, KSgNB-UP-enc, KSgNB-RRC-int, and KSgNB-RRC-enc from the fresh S-KgNB.
Option 4: The UE shall ignore the new SCG Counter if it has an existing valid security context with the MeNB.
Explanation: When the UE receives a new SCG Counter value as part of the S-KgNB update procedure, it must derive the fresh S-KgNB using this counter. To maintain secure communication, the UE needs to recalculate all relevant security keys (KSgNB-UP-int, KSgNB-UP-enc, KSgNB-RRC-int, and KSgNB-RRC-enc) based on the newly derived S-KgNB, ensuring both the UE and network use the same security parameters for subsequent communications.