This book conveys the study of key authentication schemes in public key cryptography. In current scenario, public key cryptography plays an important role in online payment services. These online services are secure only when the authenticity of public key and signature of the user are secure. Key authentication is the major chunk in public key cryptography. It avoids more security problems of public key cryptography. In this book, we illustrated all the existing key authentication schemes advantages and disadvantages and also our developed secure key authentication schemes.