The paper is devoted to the development of the main theoretical provisions of the method of hidden data embedding based on three-dimensional subband analysis. The problem of hidden data embedding in separate blocks of video stream frames is considered. The description of the spatial frequencies subdomain of the three-dimensional cosine transform is given. The concepts of the part and the fraction of the square of the norm of a frames block with a cosine transform corresponding to a given spatial frequencies subdomain, as well as formulas for their calculation based on the corresponding subband matrices elements values, are proposed. The formulas that are the basis for constructing the frames block projections on the eigenvectors of subband matrices forming a three-dimensional orthonormal basis are given, as well as the formulas that are the basis for conducting sub-band synthesis of a frames block based on the frames block projections modified in accordance with the embedding data. The main stages of the method of hidden subband data embedding in video stream frames blocks are formulated. The considered method of hidden subband data embedding into a given frames block based on three-dimensional subband analysis can be used to increase the secrecy of embedded data.