Question: What is the source of the KASME' key used for deriving a mapped EPS security context from a 5G security context?
Option 1: The KAMF using the 5G NAS Uplink COUNT value derived from the TAU Request message or Attach Request message in idle mode mobility.
Option 2: The ngKSI value field taken from the current 5G security context.
Option 3: The EPS uplink and downlink NAS COUNT values of the target MME.
Option 4: The UE's stored KASME key from a previous authentication procedure.
Explanation: According to the 3GPP TS 33.501 specification, the derivation of a mapped EPS security context from a 5G security context involves using the KAMF and the 5G NAS Uplink COUNT value derived from the TAU Request message or Attach Request message in idle mode mobility to derive the KASME' key. This ensures secure key exchange between the UE and the network during mobility procedures.