Abstract : During the last decade, distributed ledger solutions such as blockchain have gained significant attention due to their decentralized, immutable, and verifiable features. However, the public availability of data stored on the blockchain and its link to users may raise privacy and security issues. In some cases, addressing these issues requires blockchain data to be secured with mechanisms that allow on-demand (as opposed to full) disclosure. In this paper, we give a comprehensive overview of blockchain privacy and security requirements, and detail how existing mechanisms answer them. We provide a taxonomy of current attacks together with related countermeasures. We present a thorough comparative analysis based on various parameters of state-of the-art privacy and security mechanisms, we provide recommendations to design secure and privacy-aware blockchain, and we suggest guidelines for future research.
 ? The main issue is that it is hard for any widely acceptable currency to exist without all three conditions met. ? We describe the basic (and inherent) security properties of blockchain based on its first implementation in Bitcoin, and present the set of important additional security and privacy properties of blockchain, which are either present in some existing blockchain systems or desired by many blockchain applications. ? The second property is signatures are existentially unforgeable. It means that an adversary who has your public key cannot forge signatures on some messages with an overwhelming probability.
 ? The double-spending attack in the context of Bitcoin blockchain refers to a specific problem unique to digital currency transactions. ? Blockchain is the pivotal guard in securing bitcoin transactions from many known and hard security, privacy and trust problems, such as double spending, unauthorized disclosure of private transactions, reliance of a trusted central authority, and the untrustworthiness of decentralized computing. ? The UTXO model, one no longer needs to worry about solving the hard problem of keeping track of transaction sequence numbers in a fully distributed system.
 • Blockchain-based decentralized identifiers have been proposed, which rely on blockchain and additional cryptographic techniques to prove identifiers’ ownership without having to rely on a trusted entity. • Eventual consistency is a consistency model proposed for distributed computing systems by seeking a tradeoff between availability and consistency. • In this paper, we only consider analyzing privacy attacks in the Bitcoin blockchain as we only address Bitcoin privacy attacks in BTCR.
 ? Our scheme’s mixing performance with other schemes to assess its stability and the reliability of the processing environment. ? Moreover, for a better performance without a mixing third party, nodes in a mixing group can conduct broadcast processes in a lightweight blockchain among themselves. ? For efficiency, a convenient social media platform is needed to ensure that all users can communicate with each other.

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