Welcome to the first edition of the book ¿Network Security and Information Security¿. A course in Cryptography along with Networks and System Security is rapidly becoming an important course in Computer science/Application, Engineering & Information Technology curriculum. This book is written for the benefit of the students who are pursuing their Ph.D (Comp), MCA, MSc (Comp), BTech of various universities across Indian as a course curriculum. This book is not just an introduction to simple cryptography algorithms or basic security aspects, it contains in depth coverage of subject which especially gives you the tools that are needed to protect your own privacy. We have taken at most care to avoid errors, but there could be few minor errors. We will be very much grateful those who point out the errors and bring to our notice with relevant suggestions for improvement of the book in all aspects.