E-valimiste jaoks mõeldud turvalistest teadetetahvlisüsteemidest
Date
2017
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Abstract
Peaaegu iga elektroonilise hääletamise protokolli esimeseks etapiks on häälte kogumine ning nende talletamine. Seda teenust pakub teadetetahvlisüsteem (bulletin board). Paljud teadusartiklid eeldavad turvalise teadetetahvlisüsteemi olemasolu, kuid konkreetseid süsteeme on välja pakutud vähe. Tihti eeldatakse, et teadetetahvlisüsteem on tsentraalne usaldatav osapool, kuid hiljutistes töödes on tähelepanu juhitud tõrkekindla hajustalletuse olulisusele. Käesolevas töös pakume välja formaalse mudeli teadetetahvlisüsteemi funktsionaalsuse ning turvalisuse analüüsimisseks. Meie mudeli aluseks on Culnane ja Schneideri poolt konverentsil Computer Security Foundations Symposium 2014 väljapakutud teadetetahvlisüsteemi omadused. Me käsitleme turvalist teadetetahvlisüsteemi kui Garay ja teiste poolt konverentsil Eurocrypt 2015 tutvustatud avalikku tehingute pearaamatut, mis õnnestunud hääle talletamise korral väljastab kviitungi. Täpsemalt, me defineerime omadused (tõendatav) püsivus ning tõendatav elusus.Me analüüsime Culnane ja Schneideri väljapakutud teadetetahvlisüsteemi turvalisust ning näitame, et nende protokolli korral ei ole elususe omadus täidetud, kui mõni kogumisneel (item collection peer) on ebaaus. Nende süsteem saavutab tõendatava püsivuse kasutades triviaalset lävisignatuuri juhul, kui ebaausaid kogumisneele on <N/3, vastasel korral on tõke <N/4. Culnane ja Schneideri teadetetahvlisüsteemist motiveeritult pakume välja uue süsteemi, mille korral on tagatud nii tõendatav püsivus kui ka tõendatav elusus, kui ebaausaid kogumisneele on vastavalt <N/3 ning <N/2. Lisaks on meie protokoll lihtne suhtluskeerukuselt. Antud töö põhineb konverentsile esitatud artiklil „A Cryptographic Approach to Bulletin Boards“ („Krüptograafiline lähenemine teadetetahvlisüsteemidele“), mille kaasautoriteks on Aggelos Kiayas, Helger Lipmaa, Janno Siim ja Thomas Zacharias.
Vote collection together with storage of collected votes is the first phase of practically any electronic voting (e-voting) protocol. This functionality is provided by a bulletin board system. Many research papers in e-voting require the existence of a secure bulletin board, but there are only a few concrete systems. In the literature it is common to assume that bulletin board is a centralized trusted party, but in recent works the importance of a distributed fault-tolerant bulletin board has been raised. In this thesis, we propose a formal model for analysis of security and functionality of a bulletin board system motivated by the security requirements Culnane and Schneider introduced in Computer Security Foundations Symposium 2014. We consider a secure bulletin board as a robust public transaction ledger presented by Garay et al. in Eurocrypt 2015 that additionally provides receipts for successful postings. More precisely, we introduce two properties: (Confirmable) Persistence and Confirmable Liveness. We study a bulletin board system proposed by Culnane and Schneider in our model, and show that their protocol does not achieve Confirmable Liveness if there exist corrupted item collection peers, but achieves Confirmable Persistence for <N/3 corrupted item collection peers using only our trivial threshold signature scheme, otherwise the bound is <N/4. Motivated by the security analysis of Culnane-Schneider bulletin board system, we propose a fully secure bulletin board system and prove that it tolerates <N/3 corrupted item collection peers for Confirmable Persistence and <N/2 corrupted item collection peers for Confirmable Liveness. This thesis is based on a submitted paper "A Cryptographic Approach to Bulletin Boards" with co-authors Aggelos Kiayas, Helger Lipmaa, Janno Siim and Thomas Zacharias.
Vote collection together with storage of collected votes is the first phase of practically any electronic voting (e-voting) protocol. This functionality is provided by a bulletin board system. Many research papers in e-voting require the existence of a secure bulletin board, but there are only a few concrete systems. In the literature it is common to assume that bulletin board is a centralized trusted party, but in recent works the importance of a distributed fault-tolerant bulletin board has been raised. In this thesis, we propose a formal model for analysis of security and functionality of a bulletin board system motivated by the security requirements Culnane and Schneider introduced in Computer Security Foundations Symposium 2014. We consider a secure bulletin board as a robust public transaction ledger presented by Garay et al. in Eurocrypt 2015 that additionally provides receipts for successful postings. More precisely, we introduce two properties: (Confirmable) Persistence and Confirmable Liveness. We study a bulletin board system proposed by Culnane and Schneider in our model, and show that their protocol does not achieve Confirmable Liveness if there exist corrupted item collection peers, but achieves Confirmable Persistence for <N/3 corrupted item collection peers using only our trivial threshold signature scheme, otherwise the bound is <N/4. Motivated by the security analysis of Culnane-Schneider bulletin board system, we propose a fully secure bulletin board system and prove that it tolerates <N/3 corrupted item collection peers for Confirmable Persistence and <N/2 corrupted item collection peers for Confirmable Liveness. This thesis is based on a submitted paper "A Cryptographic Approach to Bulletin Boards" with co-authors Aggelos Kiayas, Helger Lipmaa, Janno Siim and Thomas Zacharias.