| CPC H04W 28/06 (2013.01) [H04W 4/14 (2013.01); H04W 12/10 (2013.01); H04W 74/0833 (2013.01)] | 12 Claims |

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1. An operation method of user equipment (UE), comprising:
if the UE is using an Evolved Packet System (EPS) service with control plane Cellular Internet of Things (CIoT) EPS optimization with overhead reduction, generating an EPS Mobility Management (EMM) transport message,
wherein the EMM transport message includes a protocol discriminator, a security header type, a message authentication code and a sequence number,
wherein if the EMM transport message is sent in an EMM data transport procedure or if the EMM transport message is sent in a service request procedure for UE using the EPS service with control plane CIOT EPS optimization with overhead reduction to send a user data, a short message service (SMS) message or a location service (LCS) message, the EMM transport message further includes a data container,
wherein if the data container is included in the EMM transport message, the EMM transport message is transmitted partially ciphered by using a ciphering algorithm,
wherein input parameters to the ciphering algorithm includes a length of a key stream to be generated by an encryption algorithm, and
wherein for the EMM transport message, the length of the key stream is set to a length of a portion of the EMM transport message starting with octet 7 until an end of the EMM transport message.
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