3GPP TS 24.501 version 16.6.0 Release 16
4.4 NAS security
4.4.6 Protection of initial NAS signalling messages
1) the UE needs to send non-cleartext IEs in a REGISTRATION REQUEST
or SERVICE REQUEST message, the UE includes the entire REGISTRATION
REQUEST or SERVICE REQUEST message (i.e. containing both cleartext IEs
and non-cleartext IEs) in the NAS message container IE and shall cipher
the value part of the NAS message container IE. The UE shall then send
a REGISTRATION REQUEST or SERVICE REQUEST message containing
the cleartext IEs and the NAS message container IE;
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