The IMS (IP Multimedia Subsystem) architecture is the key control for next generation networks (NGN). IMS gives network operators the opportunity to extend their services, including voice and multimedia communications and deliver them in new environments with new goals. Its security is paramount, especially authentication. In IMS, authentication is divided into two phases a PS (Packet-Switch) domain-level with the 3GPP-AKA protocol, and a second at IMS level using the IMS-AKA protocol. The latter is based on 3GPP-AKA, which leads to a large duplication of steps between the two phases. Some Works have tried to reduce this duplication and increase the IMS-AKA efficiency, but they add new vulnerabilities to IMS-AKA. The aim of this paper is to solve the security problems of IMS-AKA while maintaining good efficiency.