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Mini-DCS containing process stations, operator stations, and engineering stations are considered in the paper. Process stations provide the control of the process. Operator stations are designed to process visualization, as well as to the operator interaction. Configuration of the system and supervision of the communication process are carried out at the engineering stations. Between the stations of the system, the process data are transmitted. The fitness of the control process depends on correct transmission of the data with process values. Anti-destructive processes are responsible for the correctness of the received data. Fitness of the communication system determines the fitness of the entire control system. The article contains considerations relating to dependency of integrity and confidentiality of the transmitted data on the level of fitness of communication system connecting the stations. Keywords: fitness, integrity, confidentiality

The theme of this paper is to present the data transmission security system between the stations of the industrial control system. The possible options for secure communications between process stations, as well as between process and operator station are described. Transmission security mechanism is based on algorithms for symmetric and asymmetric encryption. The authentication process uses a software token algorithm and a one-way hash function. The algorithm for establishing a secured connection between the stations, including the authentication process and encryption of data transmission is given. The process of securing the transmission consists of 4 sub-processes: (I) authentication; (II) asymmetric, public keys transmission; (III) symmetric key transmission; (IV) data transmission. The presented process of securing the transmission was realized in the industrial controller and emulator. For this purpose, programming languages in accordance with EN 61131 were used. The functions were implemented as user function blocks. This allows us to include a mixed code in the structure of the block (both: ST and FBD). Available function categories: support of the asymmetric encryption; asymmetric encryption utility functions; support of the symmetric encryption; symmetric encryption utility functions; support of the hash value calculations; utility functions of conversion.[b]Keywords[/b]: transmission security, encryption, authentication, industrial control system