The book is divided into three parts: the basic part (Chapter 1-4) focuses on the principles of cryptography, introducing the background knowledge and mathematical models related to information hiding; the watermark part (Chapter 5-9) centers around digital authentication and digital tracing, mainly introducing the author’s research results on digital copyright protection and the future development trend of this technology; the cryptic communication part (Chapter 10-12) mainly elaborates how to use information cryptography to address information leakage of communications content in a network environment. In addition, this book proposes new cryptography solutions for the protection of personal privacy in rapid-developing PC systems and mobile phone App systems in recent years, and presents new research results in a comprehensive manner.