terface to the packet data network (PDN) and provides mobile attachment and bearer establishment. E-UTRAN Figure I.1. 4G mobile network architecture The evolved node B station (eNB) compresses and encrypts traffic data on the radio interface, as well as encrypts and checks the integrity of signaling data exchanged with the mobile. Introduction The mobility management entity (MME) allows mobile access to the EPS network and controls the establishment of bearers for the transmission of traffic data. The SGW (Serving Gateway) entity is the anchor point for intra-system handover (mobility within the 4G network) and inter-system handover in packet-switched (PS) mode, requiring transfer of mobile traffic to a second- or third-generation mobile network. The PGW (PDN Gateway) entity is the gateway router that connects the EPS network to the PDN. It provides the mobile with its configuration (IP address) and traffic information to the online charging system (OCS) for the prepaid and offline charging system (OFCS) for the postpaid. The home subscriber server (HSS) is a database that stores data specific to each subscriber. The main stored data include subscriber identities, authentication parameters and service profile. The policy charging and rules function (PCRF) provides the PGW entity with the rules to apply for the traffic (rate, quality of service, charging mode) when establishing the bearer. This information is stored in the subscription profile repository (SPR) when the subscription is created. I.1.2. Security architecture The mutual authentication between the mobile and the MME entity is based on the EPS-AKA (Authentication and Key Agreement) mechanism: - the HSS entity provides the MME entity with the authentication vector (RAND, AUTN, RES, KASME) from the secret key Ki created during the subscription of the mobile; - the MME entity provides the mobile with the random number (RAND) and the seal (AUTN) of the network; - the mobile calculates the seals (AUTN, RES) and the key KASME from its key Ki stored in the uni