The registration flag is set by the s cscf sending a

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registered at a particular S-CSCF or if the initial registration at a particular S-CSCF is pending. The registration flag is set by the S-CSCF sending a Cx-Put to the HSS. If the IMPU is currently registered, the S-CSCF shall leave the registration flag set to registered . At this stage the HSS has performed a check that the IMPI and the IMPU belong to the same user. Upon receiving the SIP REGISTER the S-CSCF CSCF shall use an Authentication Vector (AV) for authenticating and agreeing a key with the user. If the S-CSCF has no valid AV then the S-CSCF shall send a request for AV(s) to the HSS in CM1 together with the number m of AVs wanted where m is at least one. CM1: Cx-AV-Req(IMPI, m) Upon receipt of a request from the S-CSCF, the HSS sends an ordered array of n authentication vectors to the S-CSCF using CM2. The authentication vectors are ordered based on sequence number. Each authentication vector consists of the following components: a random number RAND, an expected response XRES, a cipher key CK, an integrity key IK and an authentication token AUTN. Each authentication vector is good for one authentication and key agreement between the S-CSCF and the IMS user. 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 27 Release 12
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CM2: Cx-AV-Req-Resp(IMPI, RAND1||AUTN1||XRES1||CK1||IK1,….,RANDn||AUTNn||XRESn||CKn||IKn) When the S-CSCF needs to send an authentication challenge to the user, it selects the next authentication vector from the ordered array, i.e. authentication vectors in a particular S-CSCF are used on a first-in / first-out basis. The S-CSCF sends a SIP 4xx Auth_Challenge i.e. an authentication challenge towards the UE including the challenge RAND, the authentication token AUTN in SM4. It also includes the integrity key IK and the cipher key CK for the P-CSCF. RFC 3310 [17] specifies how to populate the parameters of an authentication challenge. The S-CSCF also stores the RAND sent to the UE for use in case of a synchronization failure. The verification of the SQN by the USIM and ISIM will cause the UE to reject an attempt by the S-CSCF to re-use a AV. Therefore no AV shall be sent more than once. NOTE: This does not preclude the use of the normal SIP transaction layer re-transmission procedures. SM4: 4xx Auth_Challenge(IMPI, RAND, AUTN, IK, CK) When the P-CSCF receives SM5 it shall store the key(s) and remove that information and forward the rest of the message to the UE i.e. SM6: 4xx Auth_Challenge(IMPI, RAND, AUTN) Upon receiving the challenge, SM6, the UE takes the AUTN, which includes a MAC and the SQN. The UE calculates the XMAC and checks that XMAC=MAC and that the SQN is in the correct range as in TS 33.102 [1]. If both these checks are successful the UE uses RES and some other parameters to calculate an authentication response. This response is put into the Authorization header and sent back to the registrar in SM7.RFC 3310 [17] specifies how to populate the parameters of the response. It should be noted that the UE at this stage also computes the session keys CK and IK.
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