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Efficient Anonymous Group Communication Efficient Anonymous Group Communication Vom Fachbereich Informatik der Technischen Universität Darmstadt genehmigte Dissertation zur Erlangung des akademischen Grades Doctor rerum naturalium (Dr. rer. nat.) Eingereicht von: Tim Grube geboren in Seeheim-Jugenheim Tag der Einreichung: 15. Mai 2018 Tag der Disputation: 10. Juli 2018 Erstreferent: Prof. Dr. Max Mühlhäuser Korreferent: Prof. Dr. Mathias Fischer Fachgebiet Telekooperation Fachbereich Informatik Technische Universität Darmstadt Hochschulkennziffer D 17 Darmstadt 2018 Tim Grube: Efficient Anonymous Group Communication, Darmstadt, Technische Universität Darmstadt Jahr der Veröffentlichung der Dissertation auf TUprints: 2018 Veröffentlicht unter CC BY-NC-ND 4.0 International https://creativecommons.org/licenses/ Things are only impossible until they’re not. — Captain Jean-Luc Picard, Star Trek: The Next Generation, “When The Bough Breaks” (1988) Dedicated to the loving memory of my father Volker Grube 1958 – 2007 ABSTRACT This dissertation addresses the important challenge of efficiency in anonymous communication. Solving this challenge is essen- tial to provide anonymity in group communication. Every exchanged message leaks metadata: this information de- scribes the communication itself with, among others, sender, re- cipients, frequency of the communication. While the law pro- tects this information, it is often published and misused with consequences for the participants of the communication—often consequences particular for the senders of information. Anonymous communication systems like Tor break the link be- tween senders and recipients of messages and diminish emerg- ing metadata. However, their design requires duplicating mes- sages for all recipients early, mostly at the sender itself. With that, the system has to handle an unnecessary burden of pro- cessing identical messages. This dissertation contributes a novel mechanism that establishes communication groups such that the message duplication is pushed as close to the recipients as pos- sible. This dissertation also shows that this efficiency improve- ment does not come at costs of anonymity. Moreover, the group establishment mechanism increases the robustness of the com- munication against users that leave and join the communication system. To encounter the additional information leakage, dif- ferent mechanisms to share routing information are introduced and discussed under the angle of efficiency and anonymity. To also protect senders of messages, this dissertation adapts Dining- Cryptographer networks to enable sender anonymity with an adjustable trade-off between efficiency and anonymity. contributions The scientific contributions of this disserta- tion fall into the following categories: 1. Efficient Communication Overlays: A novel overlay establish- ment mechanism is presented. This mechanism adapts Ant Colony Optimization (ACO) to connect senders and recip- ients with a reduced number of connections while the anonymity remains stable. 2. Reliable Communication under Churn: Churn often disrupts communication overlays, this thesis proposes a mechanism to counter this disruptions and increase the robustness of the communication. For that, the ACO-based mechanism utilizes residual pheromones to reconnect subjects to the communication overlay. 3. Routing Information Exchange considering Efficiency and Anonymity: Four methods to share routing information, namely successor lists, successor lists with multi-layer en- cryption, Bloom filters, and distributed lookup tables, are introduced into the anonymous communication setting, discussed and evaluated with respect to their properties concerning efficiency and anonymity. 4. Efficient and Effective Sender Protection: A novel mechanism based on asymmetric dining-cryptographer networks (ADC- nets) is proposed to improve the efficiency of sender pro- tection without degrading anonymity over time. Moreover, the trade-off between efficiency and anonymity can be con- trolled. evaluation The developed mechanisms have been exten- sively evaluated using a combination of simulations and formal arguments. For this evaluation, a graph-based simulation model has been developed enabling to analyze the improvement of the ACO-based communication overlays over conventional overlays. An extensive simulation identified valuable parameter combina- tions for the ACO mechanism, leading to communication over- lays with an efficiency improvement of up to 40%. This efficiency improvement increases the communication delay only by up to 2 extra hops. The calculation of the achieved anonymity degree in- dicates no loss of anonymity in comparison to conventional over- lays; even more, the anonymity degree does even improve. Un- der churn, the robustness of the communication also increases by 30% in comparison to conventional overlays. The four approaches to routing information exchange are discussed and compared using formal arguments. The evalua- tion enables to select the appropriate mechanism based on the requirements for memory and communication efficiency and anonymity for the desired application scenario. The novel adapted ADCnets enable sender anonymity with configurable efficiency and anonymity. As such, they are supe- rior to current approaches implementing cover traffic. A simu- lation study shows that cover traffic randomization improves ef- ficiency at the cost of anonymity. The proposed ADCnets have been evaluated using formal arguments that demonstrate the efficiency improvement and the preservation of anonymity. As both efficiency and anonymity are conflicting, they cannot be achieved at the same time; however, the proposed ADCnets en- able to balance efficiency and anonymity to the requirements of the application scenario. ZUSAMMENFASSUNG Diese Dissertation adressiert die Steigerung der Effizienz anony- mer Kommunikation um diese massentauglich zu machen. Bei jeglicher Kommunikation entstehen Metadaten, welche den Prozess des Nachrichtenaustausches beschreiben, beispiels- weise mit Informationen zu Sender, Empfängern, sowie der In- tensität der Kommunikation. Metadaten sind durch Gesetze ge- schützt, trotzdem werden sie immer wieder veröffentlicht und unbefugt genutzt mit oft unerwünschten Konsequenzen – insbe- sondere für den Sender der Informationen. Anonyme Kommunikationssysteme wie zum Beispiel Tor re- duzieren Metadaten: durch die Weiter- und Umleitung der Nachrichten wird die „sichtbare“ Verbindung zwischen Sen- der und den Empfängern unterbrochen. Aufbau und Funktions- weise anonymer Kommunikationssysteme erfordert zumeist die frühzeitige Duplikation versendeter Nachrichten für jeden ein- zelnen Empfänger; in der Regel erfolgt die Duplikation durch den Sender selbst. Hierdurch entsteht eine unnötige Menge iden- tischer Nachrichten im Kommunikationssystem. Es wird ein neuer Mechanismus zur Erstellung effizienterer Kommunikati- onsstrukturen eingeführt; dieser verschiebt die Duplikation der Nachrichten so nah wie möglich an die Empfänger. Es wird gezeigt, dass die daraus resultierende Effizienzsteigerung nicht zu Lasten der Anonymität geht. Neben der Effizienzsteigerung wird auch die Robustheit gegenüber Nutzern gesteigert, die das System dynamisch verlassen und es wieder betreten. Hierdurch müssen Routing-Informationen häufiger ausgetauscht werden; um dem zu begegnen werden verschiedene Verfahren aus den Blickwinkeln Effizienz und Anonymität betrachtet. Zuletzt führt diese Dissertation ein neues Verfahren zum Schutz der Sendera- nonymität ein; hierfür werden Dining-Cryptographer Netzwer- ke (DC-Netze) angepasst und deren Effizienz gesteigert. Das Ver- fahren ermöglicht die dynamische Abwägung von Effizienz und Anonymität. beiträge Die wissenschaftlichen Beiträge dieser Dissertation sind in die folgenden Kategorien eingeordnet: 1. Effiziente Kommunikationsstrukturen: Basierend auf Ameisenkolonie-Optimierung (ant colony optimization, ACO) werden effizientere Kommunikationsstrukturen erstellt, welche die Anzahl involvierter Verbindungen minimieren; hierdurch wird die Anonymität nicht beein- trächtigt. 2. Robuste Kommunikation: Dynamische Teilnehmer (churn) unterbrechen Kommunikationsstrukturen regelmäßig. Um diesen Unterbrechungen entgegenzutreten wird das eta- blierte Wissen (d.h. die Pheromone der Ameisen) genutzt um Teilnehmer schneller wieder mit der Kommunikations- gruppe zu verbinden. 3. Austausch von Routing-Informationen: Es werden vier Me- thoden zum Austausch von Routing-Informationen vorge- stellt: Pfadlisten mit und ohne mehrschichtige Verschlüs- selung, Bloom-Filter und verteilte Suchtabellen. Diese wer- den diskutiert und bezüglich ihrer Effizienz- und Anony- mitätseigenschaften beurteilt. 4. Effizienter und effektiver Schutz von Sendern: Basierend auf dem Konzept asymmetrischer DC-Netze (ADC-Netze) wird ein Verfahren zum effizienteren Schutz der Anonymität der Sender eingeführt. Dieses Verfahren verhindert die Be- einträchtigung der Anonymität im Verlauf der Zeit. Zu- dem ermöglicht es eine dynamische Abwägung von Effi- zienz und Anonymität. auswertung Die entwickelten Mechanismen wurden mit Hilfe einer Kombination von Simulationen und formaler Dis- kussion ausgewertet. Für diese Evaluation wurde ein Graph- basiertes Simulationsmodell entwickelt und implementiert. Da- bei wurden die ACO-basierten Kommunikationsstrukturen mit konventionellen Kommunikationsstrukturen verglichen. Eine ausführliche Simulation bewertete relevante Konfigurationspa- rameter des ACO-Verfahrens; dies führte zu Kommunikationss- trukturen die bis zu 40% effizienter sind. Zudem konnte gezeigt werden, dass die Verzögerung,
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