Section: 5.4.2 Authentication procedure 5.4.2.1 General
Content: The purpose of the EPS authentication and key agreement (AKA) procedure is to provide mutual authentication between the user and the network and to agree on a key KASME (see 3GPP TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [19]). The cases when the EPS AKA procedure should be used are defined in 3GPP TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [19]. The EPS AKA procedure is always initiated and controlled by the network. However, the UE can reject the EPS authentication challenge sent by the network. The UE shall proceed with an EPS authentication challenge only if a USIM is present. A partial native EPS security context is established in the UE and the network when an EPS authentication is successfully performed. During a successful EPS authentication procedure, the CK and IK are computed by the USIM. CK and IK are then used by the ME as key material to compute a new key, KASME. KASME is stored in the EPS security contexts (see 3GPP TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [19]) of both the network and in the volatile memory of the ME while attached to the network, and is the root for the EPS integrity protection and ciphering key hierarchy.