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Anonymous Group Communication for Internet Services based on Publish/Subscribe

Subject Area Security and Dependability, Operating-, Communication- and Distributed Systems
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 317688284
 
Final Report Year 2021

Final Report Abstract

The growing popularity of online social networks (OSN) and the increasing importance of the Internet of Things lead to an increasing prevalence of group communication. OSN operators are increasingly scrutinizing the metadata, i.e., communication relationships and frequencies of their users—meanwhile considered more valuable than message contents. Anonymous communication is on the rise, especially where metadata are considered particularly worth protecting, such as in autocratic surveillance states. While anonymous point-to-point communication has been widely researched, its approaches cannot be easily applied to group communication. Hence, this project focused on researching efficient and robust anonymous group communication, especially in the context of the publish/subscribe (pub/sub) model. At first, we investigated what attacks are possible on anonymous group communication and how the number of participants, network configuration, and user communication behavior affect anonymity. Then, a broadly applicable attacker model for anonymous group communication systems was developed. As important contributions, several novel approaches to anonymous pub/sub-group communication were also developed and evaluated. Notable in this regard are: A method for ensuring sender anonymity, a method for ensuring reliable anonymous group communication in the face of ongoing subscriber fluctuation ("churn"), a publish/subscribe approach with provable anonymity, and a method for grouping communication subscribers into anonymity sets that can mitigate so-called traffic analysis attacks considerably more efficiently than comparable known approaches. In addition, simulation studies identified several factors that influence the limits of anonymity. The relationship between anonymity and efficiency was also investigated, and solutions were proposed to satisfy these two competing requirements simultaneously as much as possible. Far-reaching new contributions were made in the area of formal modeling and analysis of anonymous communication. In addition to group communication, we also included point-to-point communication, which allowed us to highlight weaknesses in approaches known from the literature. In particular, we defined and compared privacy goals by formalizing them as privacy notions and identifying their building blocks. For each pair of notions, we were able to demonstrate whether one of the two notions was strictly stronger. Thereby, we established a complete hierarchy of privacy notions. We further bridged the worlds of the indistinguishability game and function-view-based notions with "Exists INDistinguishability" (E-IND), a weak notion corresponding to Plausible Deniability. In the context of onion routing systems, we analyzed and established the proof strategy. After proving the effectiveness of the ideal functionality, we showed the flaws of the original properties and proposed improved effective properties. Finally, we introduced the first framework and protocols for onion routing with protected responses.

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