Security within network architectures is mainly a question about trust. The security solutions must be well organized so all participating entities will have faith in the security solutions. It includes solutions with cryptology to establish secure network communications and file systems. The security issues are especially important in peer-to-peer systems. It is because these systems are decentralized and no central administrator is responsible for the security issues. Providing authentication mechanism and managing the task is not a simple task.This book is a contribution to the new authentication mechanism and heartful thanks to the peoples who have contributed their special attention in completing this work.