Analysis of Synchronous Stream Cryptoconversions

Methods of construction of synchronous SSC are divided into two large groups: with uniform movement of shift registers and PRS generators with uneven movement (shift) of registers. In the first case, all LFSR used in the PRS generator are shifted by the same clock signals, i.e the movement of registers is uniform (uniform). This greatly simplifies the implementation of generators and usually increases their performance. Compared to this approach, in non-uniform motion generators, different LFSR can be shifted by different clock signals. This is achieved either through the use of the so-called control register—one of the used in the scheme of the generator LFSR performs the function of controlling the clock frequency of other LFSR. Another approach is to build generators in which the clock speed of the individual LFSR is determined by the logic of the feedback or certain functional transformations between the outputs or contents of the LFSR used in the generator. Such schemes are commonly called self-managed generators. Methods of construction of synchronous SSC with uneven movement of registers, as a rule, have a more complex structure, which reduces the performance of the respective generators. However, cryptographic analysis of such algorithms is usually much more complicated, in particular, the linear complexity of the formed SSC is usually higher than in the schemes with uniform movement of registers.