Authentication Procedure authenticates both UE and network to each other. Authentication information of for an UE is stored at UE SIM and HSS. During the registration procedure, MME and UE authenticates to each other. Authentication for UE is necessary for network to validate whether UE trying to register to the network is the genuine one or not . And authenticity for network is necessary for UE to make sure the network is valid .
To achieve this authenticity between UE and Network, MME at the network initiates authentication procedure.
1. When MME receives attach request, it sends "Authentication-Information-Request" to HSS and expects "Authentication-Information-Answer" as response message. MME includes IMSI, Network Type and Serving PLMN information AVPs within the "Authentication-Information-Request" message.
2. MME receives multiple authentication vectors for each of requested network type within the "Authentication-Information-Answer" message.
3. MME challenges to UE by sending the "Authentication Request" Message . Authentication request mainly consists of RAND, AUTN and KSI.
4. UE responds to MME by using the "Authentication Response" Message. Authentication response message mainly consists of RES value. MME cross checks RES value with the XRES value. If both the value match then authentication procedure considered as successful.