Section: 8.6.2 Mapping of an EPS security context to a 5G security context
Content: The derivation of a mapped 5G security context from an EPS security is done as described below. - The KAMF' key, taken as the KAMF, shall be derived from the KASME using the EPS NAS Uplink COUNT of the TAU message included in the Registration Request message in idle mode mobility or the NH value in handovers as described in clause A.15. - The ngKSI for the newly derived KAMF key shall be defined such as the value field is taken from the eKSI and the type field is set to indicate a mapped security context. - The 5G NAS COUNT values in the mapped 5G security context shall be set to 0. NOTE: The selection of the 5G NAS algorithms is performed by the AMF and signalled to the UE either in the NAS Container during handovers as described in clause 8.4, or in a NAS SMC during idle mode mobility as described in clause 8.2.