As an append-only distributed database, blockchain is utilized in a vast variety of applications including the cryptocurrency and Internet-of-Things (IoT). The existing blockchain solutions have downsides in communication and storage efficiency, convergence to centralization, and consistency problems. In this paper, we propose LightChain, which is the first blockchain architecture that operates over a Distributed Hash Table (DHT) of participating peers. LightChain is a permissionless blockchain that provides addressable blocks and transactions within the network, which makes them efficiently accessible by all the peers. Each block and transaction is replicated within the DHT of peers and is retrieved in an ondemand manner. Hence, peers in LightChain are not required to retrieve or keep the entire blockchain. LightChain is fair as all of the participating peers have a uniform chance of being involved in the consensus regardless of their influence such as hashing power or stake. LightChain provides a deterministic fork-resolving strategy as well as a blacklisting mechanism, and it is secure against colluding adversarial peers attacking the availability and integrity of the system. We provide mathematical analysis and experimental results on scenarios involving 10K nodes to demonstrate the security and fairness of LightChain.

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As a Distributed Hash Table (DHT), Skip Graph routing overlays are exploited in several peer-to-peer (P2P) services, including P2P cloud storage. The fully decentralized replication algorithms that are applicable to the Skip Graph-based P2P cloud storage fail on improving the performance of the system with respect to both the availability of replicas as well as their response time. Additionally, they presume the system as homogeneous with respect to the nodes' latency distribution, availability behavior, bandwidth, or storage. In this paper, we propose Pyramid , which is the first fully decentralized utility-and locality-aware replication approach for Skip Graph-based P2P cloud storage systems. Pyramid considers the nodes as heterogeneous with respect to their latency distribution, availability behavior, bandwidth, and storage. Pyramid is utility-aware as it maximizes the average available bandwidth of replicas per time slot (e.g., per hour). Additionally, Pyramid is locality-aware as it minimizes the average latency between nodes and their closest replica. Our simulation results show that compared to the state-of-the-art solutions that either perform good in utility-awareness, or in locality-awareness, our proposed Pyramid improves both the utility-and locality-awareness of replicas with a gain of about 1.2 and 1.1 times at the same time, respectively.

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Skip Graph, a type of DHT, plays an important role in P2P cloud storage applications, where nodes publicly or privately store, share, and access data. Nowadays P2P storage systems are widely using replication to support data availability, reliability, and maintainability. With replication, the main consideration is determining peers to replicate the data. Traditional replication algorithms are partially randomized and employ rigid assumptions about nodes' distribution. This results in high access delay between nodes and their closest replicas, which degrades the system performance. We propose GLARAS, a dynamic and fully decentralized locality aware replication method for Skip Graph. In contrast to the traditional algorithms, which replicate based on strict assumptions about the distribution of nodes, GLARAS aims to approximate the underlying distribution by interacting with a very small subset of nodes and minimize the average access delay of replication accordingly. To ensure GLARAS performs at its best, we also propose a dynamic fully decentralized landmark-based locality aware name ID assignment namely LANS. This ensures that the nodes' distances in the overlay and the underlying network are consistent with each other. Our extensive experiments and analysis results demonstrate that compared to the best existing decentralized locality aware replication, GLARAS improves the average access delay of public and private replications by about 13% and 17%, respectively. Likewise, in comparison to the best existing decentralized locality aware name ID assignment, LANS improves the locality awareness of name IDs and the end-to-end latency of search queries in Skip Graph with the gains of about 19% and 8%, respectively. The average replication's access delay of a Skip Graph-based P2P storage system that employs GLARAS and LANS has an improvement gain of about 2.7 over the best state-of-the-art algorithms. Since Skip Graph is a DHT, any other DHT-based P2P storage service would benefit from our solution.

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As an append-only distributed database, blockchain is utilized in a vast variety of applications including the cryptocurrency and Internet-of-Things (IoT). The existing blockchain solutions show downsides in communication and storage scalability, as well as decentralization. In this paper, we propose LightChain, which is the first blockchain architecture that operates over a Distributed Hash Table (DHT) of participating peers. LightChain is a permissionless blockchain that provides addressable blocks and transactions within the network, which makes them efficiently accessible by all peers. Each block and transaction is replicated within the DHT of peers and is retrieved in an on-demand manner. Hence, peers in LightChain are not required to retrieve or keep the entire ledger. LightChain is fair as all of the participating peers have a uniform chance of being involved in the consensus regardless of their influence such as hashing power or stake. We provide formal mathematical analysis and experimental results (simulations and cloud deployment) to demonstrate the security, efficiency, and fairness of LightChain, and show that LightChain is the only existing blockchain that can provide integrity under the corrupted majority power of peers. As we experimentally demonstrate, compared to the mainstream blockchains such as Bitcoin and Ethereum, LightChain requires around 66 times smaller per node storage, and is around 380 times faster on bootstrapping a new node to the system, and each LightChain node is rewarded equally likely for participating in the protocol.