The proliferation of network technologies (wired, cellular, ad-hoc, etc.) leads to many different network architectures. These different architectures cohabitate to provide services and contents to end customers. In order to secure services in such mixed networks, it is necessary to rely on a homogeneous trust model. The trust model must define trust relationships between the mixed architecture actors, provide elementary ingredients to secure top level services, and guarantee the security service availability. In this paper, we propose a trust infrastructure for mixed networks architectures. The model uses two particular certification authorities, which ensure X509v3 certificates management: the central certification authorities (CCA) are tied to the portions of the network having a pre-existent communication infrastructure (such as wired networks, cellular networks, etc.), and mobile certification authorities (MCA) which are on the ad-hoc portion of the network. The MCA servers emulate the certification authority role using a (k, n) threshold cryptography scheme, and the CCA servers delegate the role of certification to the MCA servers by using a (t,m) scheme of threshold cryptography. This solution is decentralized and partially distributed, supports the nodes mobility and the failure of, up to n−k, among n MCA servers. The simulation results and the performance evaluation prove the adequacy of this solution to mixed networks architectures.