The textbook summarizes the main provisions of cryptography, which according to the mathematical apparatus used can be considered as a section of discrete mathematics. The first section of the manual practically does not require special mathematical training and is available to high school students. The second and subsequent sections use some concepts and theorems of number theory. To get acquainted with them may be useful tutorial ''Discrete mathematics . Number theory' of the same author or any literature on number theory. The manual is aimed at students of technical universities, graduate students and teachers of the discipline ''Discrete mathematics'' of technical universities