Section: 8.6.1 Mapping of a 5G security context to an EPS security context
Content: The derivation of a mapped EPS security context from a 5G security context is done as described below: - The KASME' key, taken as the KASME, shall be derived from the KAMF using the 5G NAS Uplink COUNT value derived from the TAU Request message or Attach Request message in idle mode mobility or the 5G NAS Downlink COUNT value in handovers as described in Annex A.14. - The eKSI for the newly derived KASME key shall be defined such as the value field is taken from the ngKSI and the type field is set to indicate a mapped security context. - The EPS uplink and downlink NAS COUNT values in the mapped context shall be set to the uplink and downlink NAS COUNT values of the current 5G security context respectively. - The selected EPS NAS algorithms shall be set to the EPS algorithms signalled to the UE by the AMF during an early authentication procedure followed by a NAS SMC as described in clause 6.7.2. NOTE: Whenever an algorithm change is required, the target MME initiates an NAS SMC to select other algorithms as described in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [10].