SUMMARY
Context. S-blocks are one of the main transformations of many ciphers, and the search for S-boxes with improved cryptographic indicesa great deal of attention in the literature of this direction is paid. Thus, it that there are permutations (S-blocks), which should be consideredbad is assumed, i.e. those that are not suitable for building reliable ciphers. On the other hand, one of the directions for improving the design of ciphers, which has been developing recently, with the construction of ciphers is connected, in which S-blocks of random type can be used. There is an important question. Which S-boxes are not suitable for building encryption transformations? In this connection, the study of the properties and features of the formation of S-blocks of degenerate structures, which are interpretations that degrade the cryptographic exponents of ciphers, is becoming increasingly important.Objective. A study of the properties and features of the formation of permutations of a degenerate type, an estimate of the probabilityof their generation with the aid of a random permutation generator. Determination of the characteristics by which degenerate substitutions can distinguished.Method. Construction of the piecemeal laws of the distribution of the maxima of tables of differential differences and tables of linearapproximations for reduced models of ciphers, using different (degenerate) S-block constructions in them. Determination of the law ofdistribution of maxima of XOR tables and shifts of tables of linear approximations of byte permutations.Results. The ensemble characteristics of the set of byte substitutions are studied. Based on the study of the differential and linearproperties of the reduced models of ciphers, the characteristics by which degenerate substitutions can identified are determined. The probabilityof random generation (by choice) of a byte substitution of a degenerate type is determined computationally and experimentally.Conclusions. The results of the work confirmed that obtaining degenerate byte S-blocks for their random generation is an unlikely event. This means that almost without restrictions in the ciphers, S-blocks can used, generating with the help of the generator of random substitutions. The scientific novelty of the presented results is that the influence of degenerate permutations on the efficiency of encryptiontransformations has studied. For the first time it established that, the use of S-blocks generated randomly in ciphers with a very high probability does not lead to a deterioration in the ciphers’ resistance to differential and linear cryptanalysis attacks.The practical significance of the results of the work seen in the receipt of specific data confirming the main position of the new techniquedeveloped to assess the stability of block symmetric ciphers to attacks of differential and linear cryptanalysis on the independence of cipherstrength indicators from the applied S-blocks, including S-blocks of random type.