MANET is a temporary network without any existing infrastructure. Security of mobile ad hoc networks is built upon a reliable key management system to generate and distribute symmetric encryption/decryption keys for communicating parties. While central servers generate and distribute the keys in traditional wired networks, distributive key management systems are used in mobile ad hoc networks where central approaches will fail in such dynamic, high mobility networks. Also, Key management schemes based on public key cryptography are not suitable for MANET because of its computation inefficiency and node's resources constraints. In this paper, a novel key management scheme is presented that emphasizes the secure and efficient key updates by using a node Authenticated Symmetric Key Sharing approach.