To understand the choice of an intermediary, let us focus on the organization ofexchange of products (goods and services) between sellers and buyers throughthe two extreme forms of intermediary exchanges, ignoring the other roles thatintermediaries often play. In one оf thе forms of intermediary exchanges, the intermediary acts as a dealer (retailer) in a sense that it buys products from sellersand resells them to buyers: pricing is centralized by the intermediary. In the second form of intermediary exchange, the intermediary does not take control of theseller’s products, but simply offers access to a platform (or marketplace) wherebuyers and sellers can interact as they see fit: pricing is decentralized to marketparticipants, and the platform- taxes trade. The first form corresponds to a business model in which the dealer sets the wholesale price for sellers and sets theretail price for buyers, and sellers and buyers are price-takers. The second formcorresponds to a business model in which the platform operator collects a platform usage charge from each seller and collects a platform usage fee from each buyer, and sellers set retail prices for buyers. Market intermediaries coordinatethe actions of buyers and sellers. Firms carry out transactions, servicing thepayment system, inventory control, and record keeping, which are important forthe functioning of markets. In addition, firms provide a central place of exchange, thus reducing the search costs for buyers and sellers. By comparing thecosts of intermediation with the costs of non-intermediated exchange in the markets, matching buyers and sellers, or the costs of search, it can be shown that anintermediated exchange happens to be more beneficial. Indirect network effectson both sides of the market lead to the concept of so called two-sided platforms.In such a platform, the primary role of intermediary is to control access to theplatform that at least two groups of economic agents use to their interaction. Asimilar platform should be valued more by users of each group when the platform is used more by another group. Individual decisions to join a particularplatform then generate indirect network effects on agents on the other side ofplatform. Due to the centralized operation of the platform, the intermediary canadd value and capture rents by facilitating the internalization of the externalitiesrelated with network effects. The characteristics of intermediaries must meetcertain expectations of market participants.

Critical infrastructure of interdependent modern sectors is increasingly relying on cyber systems and cyber infrastructures, which are characterized by growing risks of their cyber components, including cyberphysical subsystems. Therefore, cybersecurity is important for the protection of critical infrastructure. The search for cost-effective ways to increase or improve the security of cyber infrastructure is based on optimization models and methods of cyber infrastructure stability, safety, and reliability. These models and methods have different fields of application and different directions, not necessarily focused on the cyber infrastructure resilience. The growing role of information and communication technologies has influenced the concept of security and the nature of war. Many critical infrastructures (airports, hospitals, oil pipelines) have become potentially vulnerable to organized cyber attacks. Today, the implementation of the major state function of defense and security largely depends on the successful use of information and communication technologies as modern competitive (final and intermediate) dual-use products used by different people for different purposes. Game theory is increasingly used to assess strategic interactions between attackers and defenders in cyberspace. Game research and modeling combinations are combined to study the security of cyberspace. In cyberspace, the arsenal of weapons is built by finding more vulnerabilities in the defense of the target. Vulnerability is a weakness in the security procedures of the system, the design of the system or its implementation, as well as in the organization of internal control, which may be used by the source of the threat. The dynamic nature of vulnerabilities means that they are constantly changing over time. Detecting a vulnerability by a defender reduces the effectiveness of the attacker’s cyber weapon, which exploits the vulnerability, and increases the target protection. Game theory has been applied to many issues, including resource allocation, network security, and human cooperation. In cyberspace, there is often a placement game where the attacker and the defender decide where to allocate their respective resources. Defender’s resources can be security infrastructure (firewalls), finance, training. For example, a network administrator might look for a resource allocation that minimizes the risk of (cyber) attacks and at the same time protects against cyberattacks. The attacker has limited resources and is at risk of being tracked down and punished. The problem of resource allocation in cyberspace can be formulated as a game-theoretic problem, taking into account the concept of common knowledge and the problem of uncertain observability.