The problem of recovering an image (a function of two variables) from experimentally available integrals of its grayness over thin strips is of great importance in a large number of scientific areas. An important version of the problem in medicine is that of obtaining the exact density distribution within the human body from X-ray projections. One approach that has been taken to solve this problem consists of translating the available information into a system of linear inequalities. The size and the sparsity of the resulting system (typically, 25,000 inequalities with fewer than 1 percent of the coefficients nonzero) makes methods using successive relaxations computationally attractive, as compared to other ways of solving systems of inequalities. In this paper, it is shown that, for a consistent system of linear inequalities, any sequence of relaxation parameters lying strictly between 0 and 2 generates a sequence of vectors which converges to a solution. Under the same assumptions, for a system of linear equations, the relaxation method converges to the minimum norm solution. Previously proposed techniques are shown to be special cases of our procedure with different choices of relaxation parameters. The practical consequences for image reconstruction of the choice of the relaxation parameters are discussed.
The goal of this covert channel is to prove the feasibility of using encrypted HTTPS traffic to carry a covert channel. The encryption key is not needed because the original HTTPS payload is not decrypted. The covert message will be appended to the HTTPS data field. The receiver will extract the covert channel and restore the original HTTPS traffic for forwarding. Only legitimate HTTPS connections will be used as the overt channel. A Man-in-the-Middle (MITM) attack at the sending and receiving ends will give access to modify the traffic streams. The HTTPS return traffic from the server can carry a covert channel. Without the original HTTPS traffic for comparison or the original encryption keys, this covert channel is undetectable.
Many covert channels take advantages of weaknesses, flaws, or unused data fields in network protocols. In this paper, a behavior-based covert channel, that takes advantages of behavior of an application, is presented along with a formal definition in the framework of finite state machines. The behavior-based covert channel is application specific and lies at the application layer of the network OSI model, which makes the detection of this type of covert channel much more difficult. A detailed sample implementation demonstrates an example of this type of covert channel in the form of a simple online two-person game.The potential of this type of covert channel is also discussed.
The ICMP protocol has been widely used and accepted as a covert channel. While the ICMP protocol is very simple to use, modern security approaches such as firewalls, deep-packet inspection and intrusion detection systems threaten the use of ICMP for a reliable means for a covert channel. This study explores the modern usefulness of ICMP with typical security measures in place. Existing ICMP covert channel solutions are examined for compliance with standard RFCs and resiliency with modern security approaches.
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