In his bestseller, William Stallings offers an overview of the basics of cryptography and the practice of using it to protect networks. Carefully reworked so that the material presented in the book is organized in an optimal way both in terms of its use by teachers, as well as from the point of view of self-study of the subject, the second edition of the book includes consideration of safety issues at the system level, in particular, the principles of developing block ciphers, encryption algorithms using public keys, description of basic network and data protection in the Internet, issues of protection against viruses and unauthorized access to data, protection against network attacks, using firewalls and trusted systems, as well as related applications, including Kerberos, X.509vZ, PGP, SMIME, IP security, SSLTLS and SET.