Cryptography is an indispensable tool for protecting information in computer systems. Cryptography provides important tools for ensuring the privacy, authenticity, and integrity of the increasingly sensitive information involved in modern digital systems. Nowadays, core cryptographic tools, including encryption, message authentication codes, digital signature, key agreement protocols, etc., are used behind millions of daily on-line transactions. In this course, we will unveil some of the "magic" of cryptography. In this course you will learn the inner workings of cryptographic systems and how to correctly use them in real-world applications. The objective of the course is to provide a basic understanding of the various issues related to information systems security (esecurity). The course will present an overview of the risks encountered in information systems security, and the tools used for resolving these risks.