UMTS Security: Entity Authentication

Entity authentication is the procedure of mutual authentication of UE or USIM and the network. There are few fundamental requirements for this procedure:

  1. First to permit the network to check whether the identity provided by the mobile station is acceptable or not;
  2. Second to provide parameters enabling the mobile station to calculate a new UMTS ciphering key;
  3. Third to provide parameters enabling the mobile station to calculate a new UMTS integrity key;
  4. Fourth to permit the mobile station to authenticate the network.

Authentication Vectors

The Authentication Vectors are used in Entity Authentication and Security procedures.

clip image0011 UMTS Security: Entity Authentication

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.

clip image003 UMTS Security: Entity Authentication

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)

Authentication Procedure

The authentication procedure is as follows:

clip image005 UMTS Security: Entity Authentication

The steps are as follows:

Step#1

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.

Step#2

Network starts the authentication procedure by sending the User Authentication Request with the parameter RAND and AUTN.

Step#3

After UE receives RAND and AUTN, the USIM first computes the anonymity key AK = f5K (RAND) and retrieves the SQN = (SQN clip image007 UMTS Security: Entity Authentication AK) clip image007 UMTS Security: Entity Authentication 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.

Step#4

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

References

3G Security: Security architecture: 3GPP TS 33.102

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About Prashant Panigrahi

I am Prashant Panigrahi, live in Sweden. Now working as wireless professional and this website is just a reflection of what I learned. You can connect me on LinkedIn, Twitter
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