Section: 8 Security of interworking 8.1 General
Content: As described in TS 23.501[ System architecture for the 5G System (5GS) ]  [2], in order to interwork with EPC, the UE can operate in Single Registration or Dual Registration mode. When operating in Dual Registration mode, the UE shall independently maintain and use two different security contexts, an EPS security context to interact with the Evolved Packet System and a 5G security context to interact with the 5G System. Therefore, during inter-system mobility, when the target system is EPS, the UE shall take into use the EPS security context and hence all the security mechanisms described in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [10] are applicable. In the other direction, i.e. when the target system is the 5GC, the UE shall take into use the 5G security context and hence all the security mechanisms described in the present document are applicable. When operating in Single Registration mode, there are two cases depending on the support of the N26 interface between the AMF and the MME. In both cases the security mechanisms described in all the subsequent sub-clauses are applicable. Upon registration during mobility from EPS to 5GS, the UDM may decide to trigger the procedure defined in clause 6.1.5 based on the local operator authentication policy.