In this article are described the historical evolution of cryptography stages, from antiquity, the Middle Ages and then modern and classic devices and are analyzed encryption of data. It is also described the objectives safeguards in a computer system, the definition of cryptographic system, means of data protection, data authentication methods. Currently the public key cryptosystems are widely used because they allow users to communicate only knowing each other's public key. In these systems protection and authentication are performed by distinct changes. The same, in this article it is introduced the notion of digital signature, which solves the problem of authentication transmitter and on the authentication data. The system of public key authentication enables simple implementation of digital signatures. At the end of the article it is analyzed cryptographic security as a method of data transmission in computer networks.