This book will guide researchers who had keen interest to work on user authentication. In this book we had discussed click-based captcha and graphical password authentication schemes. Implemented system inside book, provides choice of various authentication schemes to user at time of login. Along with these schemes session based authentication is also provided which will protect system from unauthorized access. We extend the use of captcha as human present recognition as well as graphical password hence it provides all benefits of captcha and make system more powerful from security point of...