• The security in UMTS is built upon the security of GSM and GPRS.
• UMTS uses the robust security features from GSM and new security features are added as necessary for new services offered by UMTS and the changes in network architecture.
• In UMTS the SIM is called UMTS SIM (USIM). UMTS uses public keys. It has increased key lengths and provides end-to-end security.

• The other security features of UMTS are listed below:

  (i) Subscriber individual key K;

  (ii) Authentication center and USIM share

  • User specific secret key K,

  • Message authentication functions f1, f2

  • Key generating functions f3, f4, f5

  (iii) The authentication center has a random number generator and has a scheme to generate fresh sequence numbers(SEQ).

  (iv) USIM has a scheme to verify freshness of received sequence numbers.

  (v) Authentication functions f1, f2 are:

  • MAC (XMAC)

  • RES: Response for the network

  • Expected Response (XRES).

  (vi) Key generating functions f3, f4, f5 are:

  • f3: ciphering / Confidentiality key CK (128 bits);

  • f4: integrity key IK (128 bits) and

  • f5: anonymity key AK (128 bits).

  (vii) Key management is independent of equipment. Subscribers can change handsets without compromising security.

  (viii) Assure user and network that CK / IK have not been used before.

  (ix) For operator specific functions, UMTS provides an example called Milenage based on the Rijndael block cipher.

  (x) Integrity function f9 and ciphering function f8 are based on the Kasumi block cipher.

The security mechanism is called Authentication and Key Agreement (AKA). The mechanism is based on a mutual authentication between the MS and BS by a challenge/response authentication protocol. The key concept is that each MS must prove that it knows the pass word key without revealing or transmitting such a password. The authentication and key agreement procedure is as follows:

Step 1: Requests of Authentication Vectors (AVs): Visited network’s VLR or SGSN requests a set of (AVs) from the HLR/AuC from the MS’s home network.

Step 2: Computation of Authentication Vectors (AVs): HLR/ AuC computes an array of AVs by means of authentication algorithm (using functions f1, f2) and MS’s private secret key K ( using f3, f4, f5). This key is stored in HLR/AuC and the user Identity module(USIM).

Step 3: Transmission of Authentication Vectors (AVs): HLR /AuC responds by sending n authentication vectors back to to the visited network’s VLR/SGSN.

Step 4: Challenge to MS: Visited network’s VLR/SGSN chooses one AV and challenges the MS’s USIM by sending the RAND and AUTN fields in the vector to it.

Step 5: Verification of AVs & Generation of RES: The MS’s USIM processes the AUTN. With the help of private secret key K, the MS is able to verify that the received challenged data could only have been constructed by some one who had access to the same secret key. USIM also verifies that the AV has not been expired by checking the sequence number (SEQ) field. If the AV is still valid and network is authenticated, the USIM proceeds to generate a confidentiality Key (CK), Integrity key (IK) and response for the network (RES).

Step 6: Reply by MS via RES: The MS responds with RES to the visited network.

Step 7: Verification of RES: Visited network’s VLR/ SGSN verifies that the response is correct by comparing with the Expected Response (XRES) from the current AV with the response received from the mobile subscriber’s USIM.