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168 note that sending communication from X to Y requires raising the lower bound on the X ↔ C link and lowering the upper bound on the Y ↔ C link, which is equivalent to adjusting X’s and Y ’s user balance ranges in the same manner. Figure 8.5 (b) shows an example of this generalization for the specific set of user accounts in Figure 8.5 (a). More importantly, Ostra preserves the conservation of credit that was present in the strawman system. This can be derived from the fact that credit is associated with links instead of users. Any credit in Ostra is naturally paired with a corresponding X→Y debt: for example, if the state of a link is −1+3, then X owes Y one credit, but Y −2 is owed a credit by X. Thus, all outstanding credit is balanced by outstanding debt, implying that credit cannot be created or destroyed. The conservation of credit holds for each link independently, and is therefore in- dependent of the trust network topology (Figure 8.5 (c) shows an example of a trust network with a different topology). As a result, the analysis of the strawman sys- tem in Section 8.1.5 applies to the full version of Ostra. For example, malicious, colluding users cannot conspire to manufacture credit; the amount of unwanted com- munication that such users can produce together is the sum of what they can produce independently. 8.2.3 Security properties We now discuss the security properties of Ostra’s refined design in detail. Ostra’s threat model assumes that malicious users have two goals: sending large amountsPDF Image | Online Social Networks: Measurement, Analysis, and Applications to Distributed Information Systems
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