S-CSCF1 selects an authentication vector for use in the authentication challenge. The authentication vector may be of the form as in TS 33.203, §6 (if IMS AKA is the selected authentication scheme):
AV = RANDn||AUTNn||XRESn||CKn||IKn, where:

• RAND: random number used to generate the XRES, CK, IK, and part of the AUTN. It is also used to generate the RES at the UE.
• AUTN: Authentication token (including MAC and SQN).
• XRES: Expected (correct) result from the UE.
• CK: Cipher key (optional).
• IK: Integrity key.