Cryptography is a technique of communication and information security that allows only the sender and the intended recipient of a message to view its contents. The key features of cryptography include confidentiality, integrity, non-repudiation, and authentication. The techniques used to safeguard information are derived from mathematical or rule-based concepts known as algorithms to convert messages in a manner that makes it difficult to decode it. These algorithms are applied for cryptographic key generation, digital signing, verification to protecting data privacy and web browsing on the Internet. They are also used to protect confidential transactions such as credit card and debit card transactions. Cryptography can be broadly categorized into three types, namely, symmetric key cryptography, hash functions, and asymmetric key cryptography. The most popular symmetric key cryptography system is data encryption system (DES). This book is a valuable compilation of topics, ranging from the basic to the most complex advancements in cryptography and computer security. It will serve as a valuable source of reference for graduate and post graduate students.