According to rapid development, popularity of internet and online procedures, the potential of internet attacks has increased substantially in recent years. Therefore, internet security is a significant issue to keep the confidential information secret from being accessed by unauthenticated users. Remote user authentication plays the most important service on the internet. It is a process of identifying an authorized user, machine or any other entity, which requesting access for a particular web service on the internet under security constraints. Also, cryptography plays a major role in helping to prevent eavesdropping of sensitive information. This book is devoted to focus on how to construct a high confidential authentication system.