Certificate life-cycle management operations, especially the initial registration of users, are important but expensive tasks in Public Key Infrastructures. In this paper we propose a mechanism that integrates these tasks with established processes, technology and data. Our approach is based on directory services that are usually already employed by companies, public administration and similar organizations for non-PKI purposes. It spares additional personell and is less error-prone, since the utilized processes are already set up and known to administrative personell and users. This reduces the costs to bootstrap and operate a Public Key Infrastructure. We show a case study about the proposed mechanism that was conducted at the Technische Universität Darmstadt in Germany in order to supply 20,000 students with certificates and keys.