4.4.5 Ciphering of NAS signalling messages

The use of ciphering in a network is an operator option subject to AMF configuration. When operation of the network without ciphering is configured, the AMF shall indicate the use of "null ciphering algorithm" 5G-EA0 (see clause 9.11.3.34) in the current 5G NAS security context for all UEs. For setting the security header type in outbound NAS messages, the UE and the AMF shall apply the same rules irrespective of whether the "null ciphering algorithm" or any other ciphering algorithm is indicated in the 5G NAS security context.

When the UE establishes a new N1 NAS signalling connection, it shall apply security protection to the initial NAS message as described in clause 4.4.6.

The UE shall start the ciphering and deciphering of NAS messages when the secure exchange of NAS messages has been established for an N1 NAS signalling connection. From this time onward, unless explicitly defined, the UE shall send all NAS messages ciphered until the N1 NAS signalling connection is released, or the UE performs inter-system change to S1 mode.

The AMF shall start ciphering and deciphering of NAS messages as described in clause 4.4.2.5. From this time onward, except for the SECURITY MODE COMMAND message, the AMF shall send all NAS messages ciphered until the N1 NAS signalling connection is released, or the UE performs inter-system change to S1 mode.

Ciphering is never applied directly to 5GSM messages, but to the 5GMM message in which the 5GSM message is included.

If a NAS message is not sent as initial NAS message and needs to be sent ciphered, the sender shall cipher the plain 5GS NAS message portion of the security protected 5GS NAS message (see figure 9.1.1.2), i.e. octet 8 and all subsequent octets, using the ciphering algorithm of the current 5G NAS security context.

Once the encryption of NAS messages has been started between the AMF and the UE, the receiver shall discard the unciphered NAS messages which shall have been ciphered according to the rules described in this specification.

If the "null ciphering algorithm" 5G-EA0 has been selected as a ciphering algorithm, the NAS messages with the security header indicating ciphering are regarded as ciphered.

Details of ciphering and deciphering of NAS signalling messages are specified in 3GPP TS 33.501 [24].