In this paper, we present O3, a social link based private storage cloud for decentralized collaboration. O3allows users to share and collaborate on collections of files (shared folders) with others, in a decentralized manner, without the need for intermediaries (such as public cloud storage servers) to intervene. Thus, users can keep their working relationships (who they work with) and what they work on private from third parties. Using benchmarks and traces from real workloads, we experimentally evaluate O3and demonstrate that the system scales linearly when synchronizing increasing numbers of concurrent users, while performing on-par with the ext4 non-version-tracking filesystem.

In this paper, we present O3REAL, a privacy-preserving distributed middleware for real-time collaborative editing of documents. O3REAL introduces a novel approach for building peer-to-peer real-time collaborative applications, using a reliable broadcast channel mechanism for network communication, but at the same time provides for persistent storage management of collaborative documents using the filesystem interface of a POSIX compliant filesystem. This approach enables real-time, completely decentralized collaboration among users, without the need for a third party to intervene, and significantly simplifies the creation of peer-to-peer collaborative applications. We demonstrate that O3REAL scales well for real-time collaboration use-cases. For example, with 33 users simultaneously collaborating on a document in real time over a WAN with a 50 ms link delay, the average perceived latency is approximately 54 ms, which is very close to the optimal baseline. In comparison, Etherpad exhibits nearly twice the perceived latency.

Online publishing of news and information enables important content to reach a much larger audience than traditional paper publishing, but there is no guarantee of long-term, reliable, and persistent access to the content. Over time, links "decay" because they are not reliably updated when content changes location, or even worse, content is deliberately altered from its original published form or deleted altogether. We present the design of Watchdog, a permissioned distributed ledger that securely and reliably monitors and preserves dynamic web content such as government and news sites. Unlike prior preservation systems, our system design allows nodes to be dispersed across multiple administrative domains, thus eliminating single points of trust and at the same time, is the first to monitor how content changes both over time and across geographical location. Watchdog achieves Byzantine fault-tolerance via a novel interactive consistency algorithm that offers a twist on the traditional definition and use of interactive consistency; the algorithm enables a set of mutually suspicious nodes, with arbitrary-sized, potentially overlapping data collections, to efficiently agree on the exact data collection in possession by each node. We demonstrate how our algorithm enables Watchdog nodes to exchange and agree upon the content each has observed individually, in a fault-tolerant, tamper-proof manner.

We present the “It's My Data file system” (IMDfs), a personal, version-tracking, distributed file system that allows users to take custody of their cross-device data management needs, without depending on cloud storage providers. Such a system may be desirable to privacy-conscious users, who worry about the increasing number of cloud storage incidents involving data leakage and corruption, or simply want to synchronize their devices without depending on third parties. IMDfs provides a global, unified view of a user's distributed data collection from any device, user-controlled replication of data across devices, transparent remote access, and support for non-restrictive conflict management. IMDfs achieves all this without sacrificing fault-tolerance or performance and with modest storage overhead. We present the design, implementation, and detailed performance evaluation of IMDfs using benchmarks and traces from real workloads, and we demonstrate that: 1) IMDfs, as a local file system, has reasonable overhead compared with a FUSE loopback file system and Wayback, a local versioning file system, and 2) IMDfs, as a network file system, outperforms NFS and Ori, a state-of-the-art file system for cross-device data management.

Summary Digital repositories must periodically check the integrity of stored objects to assure users of their correctness. Prior solutions calculate integrity metadata and require the repository to store it alongside the actual data objects. To safeguard and detect damage to this metadata, prior solutions rely on widely visible media (unaffiliated third parties) to store and provide back digests of the metadata to verify it is intact. However, they do not address recovery of the integrity metadata in case of damage or adversarial attack. We introduce IntegrityCatalog, a novel software system that can be integrated into any digital repository. It collects all integrity‐related metadata in a single component and treats them as first class objects, managing both their integrity and their preservation. We introduce a treap‐based persistent authenticated dictionary managing arbitrary length key/value pairs, which we use to store all integrity metadata, accessible simply by object name. Additionally, IntegrityCatalog is a distributed system that includes a network protocol that manages both corruption detection and preservation of this metadata, using administrator‐selected network peers with 2 possible roles. Verifiers store and offer attestations on digests and have minimal storage requirements, while preservers efficiently synchronize a complete copy of the catalog to assist in recovery in case of a detected catalog compromise on the local system. We present our approach in developing the prototype implementation, measure its performance experimentally, and demonstrate its effectiveness in real‐world situations. We believe the implementation techniques of our open‐source IntegrityCatalog will be useful in the construction of next‐generation digital repositories.

E-voting systems have emerged as a powerful technology for improving democracy by reducing election cost, increasing voter participation, and even allowing voters to directly verify the entire election procedure. Prior internet voting systems have single points of failure, which may result in the compromise of availability, voter secrecy, or integrity of the election results. In this paper, we present the design, implementation, security analysis, and evaluation of D-DEMOS, a complete e-voting system that is distributed, privacy-preserving and end-to-end verifiable. Our system includes a fully asynchronous vote collection subsystem that provides immediate assurance to the voter her vote was recorded as cast, without requiring cryptographic operations on behalf of the voter. We also include a distributed, replicated and fault-tolerant Bulletin Board component, that stores all necessary election-related information, and allows any party to read and verify the complete election process. Finally, we also incorporate trustees, i.e., individuals who control election result production while guaranteeing privacy and end-to-end-verifiability as long as their strong majority is honest. Our system is the first e-voting system whose voting operation is human verifiable, i.e., a voter can vote over the web, even when her web client stack is potentially unsafe, without sacrificing her privacy, and still be assured her vote was recorded as cast. Additionally, a voter can outsource election auditing to third parties, still without sacrificing privacy. Finally, as the number of auditors increases, the probability of election fraud going undetected is diminished exponentially. We provide a model and security analysis of the system. We implement a prototype of the complete system, we measure its performance experimentally, and we demonstrate its ability to handle large-scale elections.

Interactive consistency is the problem in which n nodes, where up to t may be byzantine, each with its own private value, run an algorithm that allows all non-faulty nodes to infer the values of each other node. This problem is relevant to critical applications that rely on the combination of the opinions of multiple peers to provide a service. Examples include monitoring a content source to prevent equivocation or to track variability in the content provided, and resolving divergent state amongst the nodes of a distributed system. Previous works assume a fully synchronous system, where one can make strong assumptions such as negligible message delivery delays and/or detection of absent messages. However, practical, real-world systems are mostly asynchronous, i.e., they exhibit only some periods of synchrony during which message delivery is timely, thus requiring a different approach. In this paper, we present a thorough study on practical interactive consistency. We leverage the vast prior work on broadcast and byzantine consensus algorithms to design, implement and evaluate a set of algorithms, with varying timing assumptions and message complexity, that can be used to achieve interactive consistency in real-world distributed systems. We provide a complete, open-source implementation of each proposed interactive consistency algorithm by building a multi-layered stack of protocols that include several broadcast protocols, as well as a binary and a multi-valued consensus protocol. Most of these protocols have never been implemented and evaluated in a real system before. We analyze the performance of our suite of algorithms experimentally by engaging in both single instance and multiple parallel instances of each alternative.