The aim of this work is a modification of the classical Vigenere cipher, in order to improve the statistical properties of the ciphertext obtained after operation. The work deals with the (third) modification of the classic Vigenere cipher. Classic code is modified to a state in which it can already be implemented and analyzed. The necessary information from the theory of algebraic systems (fields, near-fields, groups, quasigroups, Latin squares, orthogonal tables etc.) is provided. Using the properties of these algebraic systems the modification of the cipher is constructed and studied.