iderable increase of the sum of the infor- mation flowing through the network, but also a question of confidentiality and integrity. With the rapid evolution of wireless systems, the need for security is increasingly indispensable. This necessitates the advanced methods of cryp- tography and authentication, management and distribution of keys between © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security the different entities on the system, while respecting constraints imposed by wireless systems, such that the capacity of the radio interface and resources of the terminal that creates a bottleneck that strangles this type of system. This chapter describes the problem of implementing intelligent network services in VoIP applications with SIP (signaling protocol), TRIP (routing protocol over IP) and ENUM and proposes a method based on the integration of signaling protocol SIP on IN to realize a procedure of client authentication and data confidentiality. 21.2 Introduction The architectural model of Internet telephony is rather different than that of the traditional telephone network. The basic assumption is that all sig- naling and media flow over an IP-based network, either the public Internet or various intranets. IP-based networks present the appearance at the net- work level that any machine can communicate directly with any other, unless the network specifically restricts them from doing so, through such means as firewalls. This architectural change necessitates a dramatic transformation in the architectural assumptions of traditional telephone networks. In particu- lar, in a traditional network a large amount of administrative control, such as call-volume limitation, implicitly resides at every switch, and thus additional controls can easily be added there without much architectural change, whereas in an Internet environment an administrative point of control must be explic- itly engineered into a network, as in a firewall; otherwise end systems can simply bypass any device 