Entity authentication is the procedure of mutual authentication of UE or USIM and the network. There are few fundamental requirements for this procedure:
- First to permit the network to check whether the identity provided by the mobile station is acceptable or not;
- Second to provide parameters enabling the mobile station to calculate a new UMTS ciphering key;
- Third to provide parameters enabling the mobile station to calculate a new UMTS integrity key;
- Fourth to permit the mobile station to authenticate the network.
The Authentication Vectors are used in Entity Authentication and Security procedures.
The VLR/SGSN starts this procedure by requesting authentication vector to the HE/AuC.
When HE/AuC receives the request from the VLR/SGSN it retrieves the calculated authentication vectors AV (1…n) from the HLR database and sends it to the VLR/SGSN in Authentication data response.
The Authentication Vectors generation procedure in HE/AuC is as follows:
HE/AuC first generates a fresh sequence number SQN and an unpredictable challenge RAND.
After that the following will be calculated:
Message Authentication Code MAC = f1K (SQN || RAND || AMF), f1 is the message authentication function.
Expected response XRES = f2K (RAND)
Cipher key CK = f3K (RAND), f3 is a key generating function
Integrity key IK = f4K (RAND), f4 is a key generating function.
Anonymity key AK = f5K (RAND)
The authentication procedure is as follows:
The steps are as follows:
In the beginning both the USIM and the Network are not authenticated. That means USIM does not know whether the network is a real network and network does not know whether the USIM is a valid Subscriber.
Network starts the authentication procedure by sending the User Authentication Request with the parameter RAND and AUTN.
After UE receives RAND and AUTN, the USIM first computes the anonymity key AK = f5K (RAND) and retrieves the SQN = (SQN AK) AK
After that UE computes XMAC = f1K (SQN || RAND || AMF) and compares with MAC.
If both are different UE send user authentication reject back to the VLR/SGSN.
If the USIM finds the SQN is not in the correct range, it sends synchronization failure.
UE sends expected response RES to the VLR/SGSN.
If RES = XRES, then the authentication procedure completes
Message sequence from UE point of view
[UE <-- NW] DOWNLINK DIRECT TRANSFER (IDENTITY REQUEST)
[UE --> NW] UPLINK DIRECT TRANSFER (IDENTITY RESPONSE)
[UE <-- NW] DOWNLINK DIRECT TRANSFER (AUTHENTICATION REQUEST)
[UE --> NW] UPLINK DIRECT TRANSFER (AUTHENTICATION RESPONSE)
[UE <-- NW] SECURITY MODE COMMAND
[UE --> NW] SECURITY MODE COMPLETE
3G Security: Security architecture: 3GPP TS 33.102