ИСТИНА

The topic of my report is covert channels. The expression «covert channel» encompasses all communications that are hidden and communicate stealthily between endpoints. The goal of such a channel is not necessarily to obscure the data flowing through the channel, but to obscure the very fact that a channel exists. The notion of covert channels has been introduced by Lampson in 1973 in the paper «A note on the confinement problem» devoted to a mechanism by which a process at a high security level leaks information to a process at a low security level that would otherwise not have access to it. Nowadays, the importance of covert channels has increased dramatically because of growing use of Internet, especially network ones. Any communication channels can be noisy, the covert channels are not the exception because most of covert channels are embedded in noisy overt ones. The questions about estimation of noisy covert channel capacity and reliability are important. For example, see the «Trusted Computer Systems Evaluations Criteria» (so-called «Orange Book») containing requirements to constrain covert channels“ throughtputs to secure some computer system. Particularly, we investigate covert channels that can be constructed in online shooter games. In such games there is a plane field and there are some players moving on the field to transmit information. There exists a number of examples of such channels; however, to the best of our knowledge, the problems of reliability and throughput in the presence of errors remain uncovered. Futher, we investigate possible movements of the players assuming that game time is discrete and that number of directions of movement is finite.The main presented result of the investigation is that player“s locations can be identified by k-tuples and that directions of movements corresponds to remainders after division 1, z,...z^(k-1) monomials by so-called cyclotomic polynomial. We also assume that there exits noise generated by periods of invisibility of the transmitting player (due to network failures, game mechanics, etc.); thus there arises the problem of reliable transmission of information over a noisy channel. To take into account this information loss, we introduce a formal model called a structure of partial erasure. A structure of partial erasure is a triple consisting of an alphabet A, a finite set of partitions on A and probabilities endowed to partitions. The natural example of such structure related to walks on the plane is the set of trajectories of fixed length N endowed by 2^N partitions (with probability odds) corresponding to possible choices of moments of visibility. Next we consider construction of a protocol for transmitting information via a channel specified by such structure. The protocol builds a structure of full erasure over a structure of partial erasure. The structure of full erasure is just an alphabet endowed with the probability odds. We also build a protocol of cyclic symbol one-way-at-a-time transmission over the channel of full erasure and formulate its mathematical model. The average transmission time grows logarithmically with transmitted message“s size. That is good for practical purposes because logarithm is slow-growing function. Finally, we give an explicit example of implementation and estimate throughput of this example.