From GSM to LTE: An Introduction to Mobile Networks and Mobile Broadband

4.8 LTE Security Architecture

The LTE security architecture is similar to the mechanisms already used in UMTS and discussed in Section 3.9. The architecture is based on a secret key which is stored on the SIM card of the subscriber and in the HSS in the network. The same key is used for GSM, UMTS and LTE. It is therefore possible to efficiently move the security context between network nodes when the user roams between different RATs.

During the initial contact with the LTE network, that is, during the attach procedure described earlier, security procedures are invoked between the UE, the MME and the HSS. During this process, the UE authenticates to the network and the network authenticates to the UE. This prevents man-in-the-middle attacks. The authentication algorithms required for the process are stored and executed in the SIM card and in the HSS. This way, the secret key remains in a protected environment and cannot be read by potential attackers eavesdropping on the message exchange on an interface between the SIM and the mobile device or the HSS and the MME. SIM cards must be capable of performing UMTS authentication. Consequently, old GSM-only SIM cards cannot be used for authentication in LTE and the attach procedure is rejected with such SIM cards.

Once authentication has been performed, a set of session keys are generated as described in more detail in [23]. Afterward, ciphering and integrity protection can be activated for all NAS messages between the UE and the MME. ...