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27.06.2014 г.

3.1.4 Integrated information importance

       There is a set of IT systems, where it is not required to protect confidentiality of the information, but integrity and availability of the information play a significant role. The example of such system is the automated system of a dispatching service in airport. However we shall consider a common case, when in system it is necessary to protect availability, and integrity, and confidentiality of the information.
       It is obvious, that the most expedient common criterion can be probability that the information value loss on the system exit when at least one event in system happens: or the information confidentiality infringement, or the information integrity infringement, or denial in access service to the information.
       In this case by a rule of addition of probabilities we receive probability determining integrated importance:
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3.2 Risks assessment 

       One of main question of IT system security evaluation is the question of the analysis of possible losses of material and non-material assets of organization, when organization use the given IT system with influence on it of real threats.
 
       For formal risk assessment task determination we shall assume, that the IT system is being evaluated consists of some objects and communications between them. Let's call objects and communications as the system elements. In section 2 of work the common posedness of a task is submitted, where the system is submitted as the graph G(D, B). The graph elements as set E = {D, B} = {ei}L are determined as well. In subsection 3.1 the importance of the information in system A(td) is determined. In order to in developed system the established risk level will not outdone, we should take into consideration the following:
  • rationally distribution of threats implementation means by a thread agent on the system elements; 
  • if we know what the specific threats will influence on system elements, we can to calculate probability pE(td) that when the system is functioning in conditions of threats influence, the information security in system for the given time td isn’t broken.
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       For development of system for processing in it the information with the given level of its confidentiality are necessary in addition to taken into consideration the following. If for increase of probability of the information availability and integrity in system it is expedient to create some redundancy of its elements and communications between them, but for increase of the information confidentiality preservation probability it is expedient to reduce quantity of intermediary elements and ways to send of the information. However and in one and other case it is necessary to raise reliability and survivability of elements.
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       It is possible to develop set of systems with a risk level, necessary for us. Therefore by following task of risk assessment is the definition from set of such systems of system is minimal at cost.
       At development of system functioning model we consider functioning as process of the information transfer from an entrance to an system exit at access of the one object to another. If in model to present system as bipolar graph G(D, B), the threats agent does not have sense to carry out attacks on all elements of system, it is enough to determine in the graph all possible simple sections (minimal sets of elements of system), separating poles the graph, and then the threats agent should determine those sections, which require the minimal expenses for him. Knowing the cost values Cv(ei) of v-th threat implementation in i-th element of system and knowing as can rationally be distributed threats on system elements we can to determine the minimal expenses of the threats agent for rational distribution of threats in system. 
       Thus, we have described algorithm of the risk assessment in our statement of a task, which will allow us to develop IT system with a necessary risk level of the information value loss in system, will allow us to evaluate expenses for creation of such system and to estimate expenses of the threats agent for infringement of the information security in system. 
 

3.3 Threats models

3.3.1 Rational distribution of threats on elements of systems

       We know, that the set E = {D, B} = {ei}L is set of the graph G elements, describing in mathematical model our system. Everyone i-th element of system is characterized by A(td), integrated importance of the information processable in system. The task is, that we need to distribute on the graph’s L elements {ei}L  the N known threats implementation means by rational way the threats implementation means can be diverse. 
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