The primary objective of this book is to let readers abreast of fundamentals of cyber security, ranging from IP address, Protocols to different techniques of cyber attacks & defenses. Special emphasis has been given on Public Key Infrastructure and related security aspects as digital signature & digital certificates have now become essential parts of e-tendering/procurements. Separate chapters have been provided for web applications & its security and mobile security. An innovative mathematical model based on AHP has also been provided to properly asses and mitigate organisation specific security threats. A detailed check list covering security audit aspects further adds towards organisation's information security measures. This book is intended to serve as the 1st book on cyber security, for budding information security students/professionals.