Question: In establishing cryptographic protection for radio bearers in an EPS network, what is the primary basis for a UE to derive the key KeNB when receiving an AS SMC without a prior NAS Security Mode Command?
Option 1: The KASME of the current EPS NAS security context and the NAS uplink COUNT of the triggering NAS message.
Option 2: The NH parameter derived from the initial KeNB and the KASME, along with the current NCC value.
Option 3: The latest downlink NAS COUNT value stored in the UE's security context.
Option 4: A random number generated by the UE's security module to ensure uniqueness.
Explanation: According to the described security architecture, when a UE receives an AS SMC without having first received a NAS Security Mode Command, it uses the KASME of the current EPS NAS security context and the NAS uplink COUNT of the triggering NAS message as a freshness parameter to derive the KeNB, ensuring secure key derivation based on both the existing security context (KASME) and a measure of freshness (NAS uplink COUNT).