IMS Registration signalling flow —
User not registered
R1-2a. SIP 401 Unauthorized response: from S-CSCF1 to I-CSCF1(9 of 22)
S-CSCF1 challenges the user. The nonce includes the quoted string, base64 encoded
value of the concatenation of the AKA RAND, AKA AUTN and server specific data. S-CSCF1
appends also the Integrity Key (IK) and the Cyphering key (CK).
NOTE: The actual nonce value in the WWW-Authenticate header field is encoded in base64, and it may
look like: nonce="A34Cm+Fva37UYWpGNB34JP"