Stream and finite automaton ciphersystems

The following statements proved by the author are presented in the paper: 1) the classes of the stream and of the finite automaton ciphersystems are functionally equivalent (Theorem 1); 2) every selfsynchronizing with a delay $\tau$ finite automaton ciphersystem with the strongly connected projections of the decryption automaton is not differed from a ciphersystem built on the base of a shiftregister of the length $\tau$ (Theorem 3). Besides, a descriptive and a constructive definitions of the selfsynchronizing stream ciphersystem are introduced, and the equivalence between them are stated (Theorem 2).