Secure eGovernment aims at supporting public administration in delivering enhanced public services. In eGovernment electronic signatures and certification services are used to invoke trust and security in services and applications. Certification services, however, are often on offer in an apparent geographical or contextual isolation threatening to create new fault lines across eGovernment services that rely on them. As public administration often operates at multiple levels and in a compartmental manner, the risk is that limitations in the interoperability of certification services might hamper trust and security in the whole value chain of eGovernment. Drawing from the case of small public administrations this paper proposes a certification service architecture and approach to support interoperability in secure eGovernment services.