Universal Mobile Telecommunications System (UMTS) is the third generation mobile communications system. It was built on the concepts of GSM and it is the starting point in the development of the fourth generation network. This paper proposes enhancements for the initial identification and for the Authentication and Key Agreement (AKA) protocols that solve two known vulnerabilities of UMTS security: identity catching and secret key exposure. It protects the messages exchanged during the authentication and key agreement protocol.