Output feedback encryption mode: periodic features of output blocks sequence

We investigate periodic characteristics of sequence of output blocks in the output feedback encryption mode. The model of random homogeneous substitution is used for an abstract description of this formation. This property is directly related to the periodic properties of output feedback encryption mode, since it characterizes the probabilistic distribution of output blocks with certain period appearance, provided that the assumption is made that the properties of the block symmetric cipher are consistent with certain properties of the random substitution. Also in the work specific practical tasks are solved, namely recommendations are being developed for the application of the outbound feedback on the encryption threshold, certain requirements and limitations are justified.