How to perform authentication without central server in P2P?
For P2P authentication, you can go for web of trust concept. Simply this means, if someone is trusted by people you can trust, you can also trust that unknown person. In OpenPGP, a certificate can be signed by other users who trust the association of that public key with the person or entity listed in the certificate. So trust relationships can be propagated.
In addition This presentation discusses about P2P authentication, including authentication methods without central server. It proposes web of trust and few other concepts. Also I found a survey about this topic. You can choose a method according to your requirements. Hope this helps.