Strategy of selection of S-blocks for nonlinear transformation of cipher Strumok

Abstract

The peculiarities of the construction of the S-transform of the Stream cipher “Strumok” are discussed. In particular, attention is focused on the choice of S-blocks for this transformation. The rates of its randomness are estimated, in particular, the minimum number of cycles of reuse of this transformation is determined, after which the result of encryption comes to the indicators of the stationary state of random substitution. Consideration of the possibility of using in the S transformation of random S-blocks. It is concluded that random S-blocks do not worsen the indicators of its randomness. It is proposed to improve this S-conversion design, which allows to improve its randomness. It is built on using instead of a parallel set of S-blocks of a managed substitution chain with a positive blended linear transformation of segments of data blocks at the input of a layer of controlled substitutions. It is shown that this design allows, at a single pass, to activate at least four S-blocks, instead of one S-block in the original design. The proposed enhanced S-conversion design appears to be more efficient and allows for at least 20 cycles of S-blocks to be activated in three cycles in the original design, which further improves the chance of improvement.