The goal of a Public-Key-Infrastructure (PKI) is to prove whether a cryptographic public key is authentic for a certain user. This information is crucial for the reliability of asymmetric cryptographic methods. A widespread PKI has to handle an enormous number of queries for cryptographic certificates which attest the authenticity of public keys. For certain use cases a decentralized organization of the PKI is advantageously. Therefore the author developed a specialized Peer-toPeer-PKI realizing efficient search and transfer of certificates and trust-recommendations. It is based on a combination of a logic calculus model for PKIs and a scalable Peer-to-Peer lookup protocol. This distributed approach offers advantages in comparison to hierarchical or centralized systems in terms of fault resistance, load distribution, self administration and independence of an operating organization.