The book opens a new series “Mathematical Foundations of Cryptology”. It was written by employees of the Moscow State University laboratory on mathematical problems of cryptography as a popular introduction to cryptography. For the first time in Russian, the book explains the basic concepts of cryptography in a strict but accessible form. The necessary information from the mathematical apparatus of cryptography is provided. In addition, the latest ideas of modern cryptography are presented. Ciphers that are well known from history and detective literature are used as examples. The book can also be used as a popular reference book for the basic concepts of cryptography. For a wide range of readers.