Section: 6.16.4 Security handling in NAS based redirection from 5GS to EPS
Content: When a UE initiates registration procedure with the AMF, the AMF may redirect the UE from 5GC to EPC by including a EMM cause indicating to the UE that it shall not use 5GC, as described in clause 5.31.3 in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The following requirements apply to Registration Reject message with an EMM cause which indicates to the UE that the UE shall not use 5GC: - the AMF shall only send such a Registration Reject message once NAS security has been established between the AMF and the UE; and - the UE shall only act upon such Registration Reject message if received integrity protected and if UE has verified the integrity of the Registration Reject message successfully. NOTE: This solution does not apply to unauthenticated emergency calls.