The private user identity is a unique global identity defined by the home network operator, which may be used within the home network to uniquely identify the user from a network perspective [3GPP TS 23.228]. It does not identify the user herself; on the contrary, it identifies the user's subscription. Therefore, it is mainly used for authentication purposes. It is possible to utilize private user identities for accounting and administration purposes as well. The IMS architecture imposes the following requirements for private user identity [3GPP TS 23.228, TS 23.003]:
The private user identity will take the form of a Network Access Identifier (NAI) defined in [RFC2486].
The private user identity will be contained in all registration requests passed from the UE to the home network.
The private user identity will be authenticated only during registration of the user (including re-registration and de-registration).
The S-CSCF will need to obtain and store the private user identity on registration and on unregistered termination.
The private user identity will not be used for routing of SIP messages.
The private user identity will be permanently allocated to a user and securely stored in an IMS Identity Module (ISIM) application. The private user identity will be valid for the duration of the user's subscription within the home network.
It will not be possible for the UE to modify the private user identity. ...