Security is a major requirement on every telecommunications system. Everyday brings new technologies, techniques and standards; in order to guarantee these telecommunication systems a complete solution to secure networks. Since IP Multimedia Subsystem (IMS) is an evolving standard that intends to support Next Generation Networks (NGN) and Next Generation Mobile Networks (NGMN), its security techniques and objectives should be the result of methodologies that identity security needs. In this paper, the authors present a new authentication schema on IMS, based on a model that provides security in a transversal way, using public key techniques.