Starting with the historical facts behind the concept of information, which led to the creation of computer networks, Internet of things and cryptocurrencies, the book then arrives at the main definitions of cryptography and network security, the protocols that keep the systems running and the cybercrimes that could disrupt the systems. The basics of information theory, how to measure information, and the information associated with a source are discussed. Source codes are presented, along with the concepts of information transmission, joint information, conditional entropy, mutual information and channel capacity. Computer networks are discussed, including the main protocols and network architectures, and the important TCP/IP protocol. Network security, a topic intrinsically connected to computer networks and the Internet, is presented, along with information about basic hacker attacks, alternatives to prevent attacks, data protection and secure protocols. The information theoretical aspects of cryptography are described including the hash function. Appendices include a review of probability theory, a discussion of cryptoalgorithms and cryptosystems, and a glossary of information security terms. Illustrations and graphics help the reader understand the theory. This book provides a comprehensive introduction to the foundations of cryptography and network security. As well as a thorough presentation of the evolution of the topic the book includes information about basic hacker attacks, alternatives to prevent attacks, ways to increase data protection and how to set up and implement secure protocols.