4.4.3.3 Integrity protection and verification

The sender shall use its locally stored NAS COUNT as input to the integrity protection algorithm.

The receiver shall use the NAS sequence number included in the received message and an estimate for the NAS overflow counter as defined in clause 4.4.3.1 to form the NAS COUNT input to the integrity verification algorithm.

The algorithm to calculate the integrity protection information is specified in 3GPP TS 33.501 [24], and in case of the:

• SECURITY PROTECTED 5GS NAS MESSAGE message, the integrity protection shall include octet 7 to n, i.e. the Sequence number IE and the NAS message IE.
• Intra N1 mode NAS transparent container IE and S1 mode to N1 mode NAS transparent container IE, the integrity protection shall include all octets of the value part of the IE starting from octet 7.

NOTE: To ensure backward compatibility, the UE uses all octets starting from octet 7 in the received NAS transparent container for the purpose of integrity check of the NAS transparent container irrespective of the release/version it supports. After a successful integrity check, the UE can ignore the octets which are not specified in the release/version which the UE supports.

In addition to the data that is to be integrity protected, the BEARER ID, DIRECTION bit, NAS COUNT and 5G NAS integrity key are input to the integrity protection algorithm. These parameters are described in 3GPP TS 33.501 [24].

After successful integrity protection validation, the receiver shall update its corresponding locally stored NAS COUNT with the value of the estimated NAS COUNT for this NAS message.

Integrity verification is not applicable when 5G-IA0 is used.