Secret writing was apparently more advanced and the administrators are used secret codes to communicate with a network of spies spread out through the country. This book focuses on the authentication, confidentiality and the limitations existing in the present methods. Information security refers to protecting information and information systems from unauthorized access. The main objective of this book is to develop a more secured and authenticated system using a Modified Message Digest Algorithm (MMD). In this process a concept called cheating text is introduced for providing confidentiality and authentication. The concept of cheating text together with MMD is used in applications of Zero knowledge protocol, a schnorr¿s scheme of digital signature scheme. A methodology for using the MMD along with cheating text is used for authentication in mobile devices and the performance of the results are compared with the existing ones. This system will be tested with different kinds of attacks that are possible with steganographic messages and can easily overcome with stego only, known cover, chosen stego, known stego attacks.