Abstract: Classic cryptographic primitives of nonlinear cryptographic substitution perform a simple replacement of each character of the encrypted text with some character of the same alphabet, effectively implementing the transformation of a single-alphabet simple substitution cipher. As a result, the entropy of the text transformed by the nonlinear substitution operator is the same as the entropy of the original text. This paper examines one variant of constructing quasi-randomized nonlinear substitution algorithms using the so-called weak determinism scheme, which yields a significant increase in the entropy of the output text. We will refer to nonlinear substitution operators in which one or more transformation parameters are selected in a deterministic manner, but contain signs of almost random selection as quasi-randomized. With such transformations, the output text acquires properties that are pretty close to those of white noise.

Keywords: cryptographic transformations, criteria for cryptographic strength, symmetric block ciphers, cryptographic primitives, nonlinear cryptographic substitution, S-box, randomization, scatter diagrams

Pages: 139-145