Abstracts This article is devoted to the implementation of some concepts of nonlinear dynamics of complex systems in cryptography. Some parallels of these disciplines are discussed: Such as mapping, ergodicity, stirring, etc. The concept of the Poincare trajectory of return well-known in the dynamics is introduced in cryptography, as much as graphical representations of mappings. These permits to solve a problem of selection elements of contemporary block cryptographic systems such as the tables of substitutions, called as S-blocks from the entire set of substitutions. A statistical analysis of real S-blocks was made on the basis of their decompositions on cycles.