Abstract
Currently, an increasing number of third-party applications exploit the Bitcoin blockchain to store tamper-proof records of their executions, immutably. For this purpose, they leverage the few extra bytes available for encoding custom metadata in Bitcoin transactions. A sequence of records of the same application can thus be abstracted as a stand-alone subchain inside the Bitcoin blockchain. However, several existing approaches do not make any assumptions about the consistency of their subchains, either (i) neglecting the possibility that this sequence of messages can be altered, mainly due to unhandled concurrency, network malfunctions, application bugs, or malicious users, or (ii) giving weak guarantees about their security. To tackle this issue, in this paper, we propose an improved version of a consensus protocol formalized in our previous work, built on top of the Bitcoin protocol, to incentivize third-party nodes to consistently extend their subchains. Besides, we perform an extensive analysis of this protocol, both defining its properties and presenting some real-world attack scenarios, to show how its specific design choices and parameter configurations can be crucial to prevent malicious practices.
Highlights
In [9], the authors discussed the different types of attacks related to Bitcoin and other cryptocurrencies, whereas in [10,11], the authors introduced and discussed, respectively, the security and privacy issues related to Bitcoin and to the blockchain technology
The main peculiarity of Bitcoin lies in the fact that it does not require the control of any central authority, relying instead on a peer-to-peer network of nodes called miners that are responsible for maintaining a public ledger called the blockchain [1]
It consists of a protocol that essentially pushes miners to compete with each other for the right to add these blocks to the blockchain: to append a new block Bi to the blockchain, each miner has to solve a cryptographic challenge, which requires a high consumption of computational resources
Summary
The emergence of Bitcoin [1] and other cryptocurrencies has revolutionized many sectors of modern society, as reported in [2], which provides a systematic review of the literature on major topics related to the cryptocurrencies market: firstly, the financial one, as they represent innovative assets, characterized by the concept of scarcity [3], and elusive to traditional market logics; the IT sector, since these technologies introduced, for the first time on a large scale, the concept of the blockchain, a decentralized and secure data structure for the certification of information (initially, payment transactions), immutable, and that does not require the intervention of a central control authority such as a bank or a government; and last, but not least, the legal sphere, where this lack of central control makes these instruments difficult to regulate, but at the same time attractive, thanks to the collateral opportunities they offer. Bitcoin transactions may include some bytes of metadata, through the OP_RETURN instruction, as reported in the study performed in [12], where the authors identified and classified real-life blockchain transactions embedding metadata, with regard to several major protocols that operate over the Bitcoin blockchain This led to an increase in the number of third-party services that take advantage of this possibility, in order to permanently store messages, digests, or information generated by their execution. Since subchains often need to preserve the order of the messages they contain (mainly to guarantee an agreement, between the distributed participants, on the application execution), it is important to ensure that their contents are unambiguous and consistent This would require, e.g., a protocol for managing the simultaneous publication of (potentially conflicting) new messages, as well as the possibility of recognizing invalid or fraudulent sequences.
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