Authentication procedure in LTE is known as mutual authentication since both UE and MME authenticate to each other.
I would like to highlight few points which will help you to understand things better.
- MME retrieves Authentication vectors from HSS. Each authentication vector consists of (RAND, AUTN, K, XRES).
- AUTN is 16 octet long string which contains elements in following order:
a. SQN XOR AK
- MME prepares NAS Authentication Request by including RAND and AUTN.
- After receiving RAND and AUTN values, UE generates XMAC and compares with received MAC (part of AUTN). If MAC comparison is successful, it means UE has authenticated network.
- UE generates RES value and send it to MME using NAS Authentication Response message. MME compares the RES received in Authentication Response against the one it has. If both the same then it means network authenticated UE successfully.
This is the way both UE and Network authenticate to each other and known as mutual authentication.