These keys are passed to each of the entities involved
in the registration (the S-CSCF and the subscriber device specifically).
When the device receives the response 401 Unauthorized from the S-CSCF, it will
then send another REGISTER message containing its credentials based on the
Establishing a Session in the IMS 113
authentication and cipher keys stored in the ISIM application. These keys cannot be
changed by the user and will be used each and every time the subscriber accesses the
network. They are only known to the service provider that assigns them and the device
itself. The new REGISTER message will use the same path as the first message, per
IMS SIP routing rules.
The REGISTER message will contain the P-CSCF address and network identifier
(usually in the form of a domain name), the public user identity to be registered, the
private user identity assigned to the subscriber, and the device??™s IP address. The CALLID
value will be the same as the initial REGISTER message so that the S-CSCF knows
that this is a response to a previous challenge.
When the S-CSCF receives the REGISTER message, it identifies the subscriber
by examining the TO header populated with the public user identity and the
AUTHORIZATION header populated with the private user identity. If the S-CSCF
verifies the authentication keys are correct, it will send a response 200 OK to the subscriber.
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