4.4.3.1 General

Each 5G NAS security context shall be associated with two separate counters NAS COUNT per access type in the same PLMN: one related to uplink NAS messages and one related to downlink NAS messages. If the 5G NAS security context is used for access via both 3GPP and non-3GPP access in the same PLMN, there are two NAS COUNT counter pairs associated with the 5G NAS security context. The NAS COUNT counters use 24-bit internal representation and are independently maintained by UE and AMF. The NAS COUNT shall be constructed as a NAS sequence number (8 least significant bits) concatenated with a NAS overflow counter (16 most significant bits).

When NAS COUNT is input to NAS ciphering or NAS integrity algorithms it shall be considered to be a 32-bit entity which shall be constructed by padding the 24-bit internal representation with 8 zeros in the most significant bits.

The value of the uplink NAS COUNT that is stored or read out of the USIM or non-volatile memory as described in annex C, is the value that shall be used in the next NAS message.

The value of the downlink NAS COUNT that is stored or read out of the USIM or non-volatile memory as described in annex C, is the largest downlink NAS COUNT used in a successfully integrity checked NAS message.

The value of the uplink NAS COUNT stored in the AMF is the largest uplink NAS COUNT used in a successfully integrity checked NAS message.

The value of the downlink NAS COUNT stored in the AMF is the value that shall be used in the next NAS message.

The NAS sequence number part of the NAS COUNT shall be exchanged between the UE and the AMF as part of the NAS signalling. After each new or retransmitted outbound SECURITY PROTECTED 5GS NAS MESSAGE message, the sender shall increase the NAS COUNT number by one, except for the initial NAS messages if the lower layers indicated the failure to establish the RRC connection (see 3GPP TS 38.331 [30]). Specifically, on the sender side, the NAS sequence number shall be increased by one, and if the result is zero (due to wrap around), the stored NAS overflow counter shall also be incremented by one (see clause 4.4.3.5). If, through implementation-dependent means, the receiver determines that the NAS message is a replay of an earlier NAS message, then the receiver handles the received NAS message as described in clause 4.4.3.2. Otherwise, in order to determine the estimated NAS COUNT value to be used for integrity verification of a received NAS message:

• The sequence number part of the estimated NAS COUNT value shall be equal to the sequence number in the received NAS message; and
• If the receiver can guarantee that this NAS message was not previously accepted, then the receiver may select the estimated NAS overflow counter so that the estimated NAS COUNT value is lower than the stored NAS COUNT value; otherwise, the receiver selects the estimated NAS overflow counter so that the estimated NAS COUNT value is higher than the stored NAS COUNT value.

During the inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, when a mapped 5G NAS security context is derived and taken into use, the AMF shall set both the uplink and downlink NAS COUNT counters of this 5G NAS security context to zero. The UE shall set both the uplink and downlink NAS COUNT counters of this 5G NAS security context to zero.

During the inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, the AMF shall increment the downlink NAS COUNT by one after it has created an S1 mode to N1 mode NAS transparent container (see clause 9.11.2.9).

During the inter-system change from N1 mode to S1 mode in 5GMM-CONNECTED mode, the AMF shall increment the downlink NAS COUNT by one after it has created an N1 mode to S1 mode NAS transparent container (see clause 9.11.2.7).

During N1 mode to N1 mode handover:

• if the new 5G NAS security context is created with the same KAMF, the AMF shall signal the 8 least significant bits of the current downlink NAS COUNT value in an Intra N1 mode NAS transparent container (see clause 9.11.2.6). The AMF shall then increment the downlink NAS COUNT by one; or
• if the new 5G NAS security context is created with a new KAMF, the AMF shall signal the 8 least significant bits of the current downlink NAS COUNT value in an Intra N1 mode NAS transparent container (see clause 9.11.2.6) and shall then set both the uplink and downlink NAS COUNT counters of this 5G NAS security context to zero. The AMF shall then increment the downlink NAS COUNT by one. The UE shall also set both the uplink and downlink NAS COUNT counters to zero.

NOTE: During the inter-system change from S1 mode to N1 mode in 5GMM-CONNECTED mode, the S1 mode to N1 mode NAS transparent container (see clause 9.11.2.9) is treated as an implicit SECURITY MODE COMMAND message for the UE and the AMF, and therefore the AMF regards the sending of the S1 mode to N1 mode NAS transparent container as the sending of an initial SECURITY MODE COMMAND message in order to derive and take into use a mapped 5G NAS security context for the purpose of the NAS COUNT handling.