Chapter 1 Main Notions 1. Introduction How can one transmit information without privacy violation! For sure, each reader tried to solve this problem, at one time or another, pursuing various goals and under various circumstances. (For conve- nience of further references we call this problem the "ST problem", that is, the Secure Transmission problem.) The odds are that the solution found by the reader repeated one of the ways of secure infor- mation transmission already discovered by the humankind. It is not difficult to conclude that there are three approaches to solving the problem: 1. Establish an absolutely secure communication line between the users. 2. Use a public communication link, but hide the very fact that information was transmitted. 3. Use a public communication link, but transform the informa- tion to be transmitted in such a way that only the authorized receiver would be able to reconstruct it. Let us make some comments concerning these possibilities. 1
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