Question: What is the procedure initiated by the UE to establish cryptographic protection for radio bearers in 3GPP access, and what message does the AMF use to derive key KgNB/KeNB?
Option 1: The UE initiates the procedure with a NAS Service Request message or Registration Request message without "PDU session(s) to be re-activated", and the AMF derives key KgNB/KeNB using the downlink NAS COUNT.
Option 2: The UE initiates the procedure with a NAS Service Request message or Registration Request message with "PDU session(s) to be re-activated", and the AMF derives key KgNB/KeNB using the uplink NAS COUNT corresponding to the NAS message that initiated transition from CM-IDLE to CM-CONNECTED state.
Option 3: The UE initiates the procedure with a NAS Service Request message or Registration Request message without "PDU session(s) to be re-activated", and the AMF derives key KgNB/KeNB using the uplink NAS COUNT corresponding to the NAS message that initiated transition from CM-IDLE to CM-CONNECTED state.
Option 4: The UE initiates the procedure with a NAS Service Request message or Registration Request message with "PDU session(s) to be re-activated", and the AMF derives key KgNB/KeNB using the downlink NAS COUNT corresponding to the NAS message that initiated transition from CM-IDLE to CM-CONNECTED state.
Explanation: The correct answer is Option 2 because, according to the text, the UE initiates the procedure with a NAS Service Request message or Registration Request message with "PDU session(s) to be re-activated", and the AMF derives key KgNB/KeNB using the uplink NAS COUNT corresponding to the NAS message that initiated transition from CM-IDLE to CM-CONNECTED state. This is specified in the text as: "The AMF shall derive key KgNB/KeNB as specified in Annex A using the uplink NAS COUNT (see TS 24.501 [35]) corresponding to the NAS message that initiated transition from CM-IDLE to CM-CONNECTED state and the KAMF of the current 5G NAS security context."