The IP Multimedia Subsystem (IMS) is an access-independent, IP based service control architecture. Although, the security and data privacy aspects are very important in the IMS global objectives, they attract little attention so far. Secure access to multimedia services is based on SIP and HTTP digest on top of IMS architecture. Authentication is only based on SIP and is directly coupled to the SIP registration procedure. The third Generation Partnership Project (3GPP) provided Generic Bootstrapping Architecture (GBA) to authenticate the subscriber before accessing multimedia services over HTTP however through a heavy process. In this paper, we propose a new IMS Service Authentication scheme using Identity Based cryptography (IBC). Our solution enhances the security of the IMS authentication process and is promising in enabling new social services since it is based on the user's identity (which could be mapped to his presence). We analyzed the security of our new protocol and we began the evaluation of its performance. We obtained a better security as well as a performance gain compared to the classical service authentication approach in IMS