Problem 2-4. 3G/4G
While GSM is ubiquitously deployed, 3rd and 4th generation technologies such as UMTS and LTS are wide spread today. This question explores some of the security aspects of 3G/4G systems, especially as they relate to GSM. Universal Mobile Telecommunications System (UMTS) is a 3G system based on the GSM standard. The authentication and key agreement procedure in UMTS is similar to GSM, with some notable differences:
•The security algorithms and protocols used are negotiated between the operator and the mobile station during authentication.
•Both a cipher key and an integrity key are derived during authentication.
•In addition to the RAND challenge, the mobile network sends an AUTN message that contains a Message Authentication Code (MAC).
Research the UMTS authentication and key agreement (AKA) to answer the following questions:
(a) Briefly explain the purpose of the AUTN message.
(b) Assume Eve observes a < RAND, AUTN > pair for a particular IMSI during the device’s authentication. What prevents an attacker from replaying this same < RAND, AUTN > pair? Explain your answer; you may need to examine the 3GPP UMTS spec and AUTN messages in more detail to answer.
(c) Xavier Breath believes that both the AUTN and the integrity-protected protocol negotiation messages are required to ensure mutual authentication and prevent Manin-the-Middle attacks. Explain why, or argue why not.
(d) Xavier further argues that the UMTS authentication protocol prevents downgrade attacks (for instance where the mobile station is instructed to use no encryption). Explain why Xavier is correct or incorrect.
Long Term Evolution (LTE) is a popular 4G system enjoying increased deployment.
(e) Briefly describe at least two differences between EPS-AKA (LTE’s authentication and key agreement) and UMTS’s AKA.