In this paper, we present the design of foundation PKI (fPKI), a PKI system constructed on an overlay network for an open source community. fPKI can provide an alternative PKI system to the current server-client PKI system. The Certificate Authority (CA) system of the fPKI is composed of the reputation of each committer, which has been assessed by the SecureEigenTrust[4] algorithm. fPKI inherits the scalability of SecureEigenTrust, and also inherits the common CA usage of the server-client PKI system.