In this dissertation, cryptographic mechanism-based data sharing schemes are presented that address the challenges of dependence on third trusted parties to facilitate sensitive data sharing and censorship. This work is driven by the lack of elevated security and privacy countermeasures necessary for use with sensitive data that many third parties exhibit. Our proposed schemes transition from centralized to distributed models, thus alleviating trust on third parties to realize data owner sharing preferences.We first propose a secure Privilege-based Multilevel Organizational Data sharing (P-MOD) scheme that uses the cloud as a storage medium. P-MOD integrates a privilege-based access structure into an attribute-based encryption mechanism to facilitate sensitive data sharing in hierarchical settings. This structure allows data owners to share their sensitive data selectively among all levels of the hierarchy in a fine-grained manner. It also reduces computational complexity by minimizing the overall cryptographic operations. Following the development of P-MOD, we wished to gain a better understanding of distributed systems in an effort to eliminate the need to trust third parties. Therefore, we conducted a comprehensive study of the first system to adopt blockchain, Bitcoin. In this study, we aimed to identify the security points of weakness of these distributed systems. We delved deeply into one of the major security threats, double-spending attacks, by performing two thorough probability analyses of its likelihood of success. Next, we conducted a probability of success versus profitability analysis of double-spending attacks to investigate the trade-offs between waiting time before accepting a transaction and the profitability of these attacks.Motivated by our study of blockchain and the underlying foundation of distributed peer-to-peer (P2P) networks, we developed a distributed Multilevel Attribute-based EMR management (d-MABE) scheme based on our groundwork of P-MOD. The d-MABE scheme incorporates smart contracts deployed and executed over the blockchain to ensure the data sharing preferences of the data owners are maintained. It also replaces the cloud storage with a distributed storage system that is managed by a P2P network to improve the reliability of retrieving data when requested. Using electronic medical records (EMR) as a use-case, our goal is to demonstrate the benefits of alleviating dependence on the electronic record-generating institutions and thus granting data owners (patients) control of their sensitive data in a distributed manner.To further expand our research and reflect its applicability to a wider domain space, we proposed a blockchain-based distributed Coercion-Resistant and Anonymous Mobile Electronic (d-CRAME) voting scheme. The proposed scheme is secure and preserves voter privacy through secure multi-party computations performed by parties of differing allegiances. It also leverages a blockchain running smart contracts as a publicly accessible and tamper-resistant bulletin board to permanently store votes and prevent double-voting. Using voting as an application, our goal is to demonstrate the potential and feasibility of designing a distributed and remote voting scheme for large-scale elections, thus increasing voter turnout and accuracy in the decision-making process.
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