Providing secure communications is a crucial task for the success of future ubiquitous mobile communication systems. Using public key infrastructure (PKI) is considered as a good solution to fulfill the task. However, as mobile ad hoc networks (MANET) inherit unique characteristics such as dynamic topology, non-infrastructure architecture, centralized PKI architectures are not suitable for dynamic MANET. The use of distributed PKI models is more appropriate but requires additional modifications to adapt with network changes. In this paper, we introduce a novel key management scheme for MANET, which exploits advantages of threshold cryptography. The major innovative aspect of this scheme is the use of temporal substitute certificate authorities (SCA), which form a PKI model of multi SCA groups. Performance results obtained by computer simulation show that the proposed key management scheme can reduce the latency of authentication, certificate update delay and the signaling load.