The textbook is intended for studying courses "Methods and means of information protection," "Basics of cryptography," other related disciplines of the main educational programs of higher education, for studying elective disciplines devoted to the basics of cryptography and applied issues of number theory. The manual includes theoretical facts, exercises and tasks of various levels of complexity in all major sections of cryptography and the corresponding sections of applied number theory. In addition to an extensive list of exercises and tasks, the manual presents individual tasks for creative and laboratory work, test questions and typical tasks of the mandatory minimum for each topic.