This paper has proof of perfectly security (against passive adversaries) for all ideal stegosystems, if two trivial assumptions are obtained. "Model of tree channels" of stegosystem, using Error Correction Code steganography is described. It is explaned that "Model of tree channels" is a special situation of Cachin "Information-Theoretic model". Perfect security stegosystem and ideal stegosystem are described. As an example of perfectly secure stegosystem, a mathematical model of optical disk Error Correction Code (ECC) is elaborated. Using this model perfectly secure stegosystem can be simply constructed. Perfect security steganography algorithm of inserting stegomessage to ECC code words is described in detail.