The need of a certification process for cloud-based services is emerging as a way to address some of the remaining obstacles facing the effective development and diffusion of the cloud-computing paradigm. In this paper we move the first steps towards a complete approach containing a conceptual framework where the specifications of basic, hybrid and incremental certification models for cloud-based services can be given. Specifically, we focus on the definition of a unifying meta-model to provide representational guidelines for (i) the definition of the security properties to be certified, (ii) the types of evidence underlying them, (iii) the phases of the certificate life cycle, as well as of all mechanisms for generating supporting evidence.