e MS MAC address (alternatively known as MS ID) and the STID through eavesdropping the information exchange over the air interface. In order to protect the mapping between the STID and the MS MAC address, two types of STIDs are assigned to an MS during network entry, i.e., a Temporary STID (TSTID) and a permanent STID. As shown in Figure 8-6, a TSTID is assigned during the initial ranging process and is used until the permanent STID is assigned to the MS. The STID is assigned during the registration process following successful authentication, and is encrypted during transmission. The TSTID is released after the STID is securely assigned. The STID is used for all information exchange with the MS until the session is active. The MS ID is protected by hashing the value of the real MS ID, which is denoted by MS ID* in Figure 8-6. The MS generates a new NONCE_MS and derives the MS ID* value, subsequently it sends an AAI_RNG-REQ message carrying the MS ID* to the BS. When the BS receives the AAI_RNG- REQ message, it returns an AAI_RNG-RSP message containing the temporary STID instead of the STID and the MS ID* value that the MS had sent. After the temporary STID is assigned, it is used for subsequent network entry procedures until the STID is allocated. The real MS ID is transmitted to the BS in an AAI_REG-REQ message in an encrypted manner. The STID is assigned after the authen- tication procedure is successfully completed. Note that the assignment message, i.e., the AAI_REG- RSP message, is encrypted. Once the MS receives the STID via the AAI_REG-RSP message, it releases the temporary STID [2]. 8.8 3GPP LTE SECURITY ASPECTS The E-UTRAN security has been designed based on the following principles. The keys used for Non- Access Stratum (NAS) and Access Stratum (AS) protection are dependent on the algorithm with which they are used. The eNB keys are cryptographically separated from the EPC keys that are used for NAS protection, making it impossible to use the eNB key to drive an EPC key. The AS and NAS keys are derived