Zhu LH, Zheng BK, Shen M et al. Data security and privacy in bitcoin system: A survey. JOURNAL OF COMPUTER SCIENCE AND TECHNOLOGY 35(4): 843–862 July 2020. DOI 10.1007/s11390-020-9638-7 Data Security and Privacy in Bitcoin System: A Survey Lie-Huang Zhu1, Member, CCF, IEEE, Bao-Kun Zheng1,2, Meng Shen1,3,∗, Member, CCF, IEEE, Feng Gao1 Hong-Yu Li1, and Ke-Xin Shi1 1School of Computer Science and Technology, Beijing Institute of Technology, Beijing 100081, China 2School of Information Management for Law, China University of Political Science and Law, Beijing 102249, China 3Key Laboratory of Information Network Security, Ministry of Public Security, Shanghai 201204, China E-mail: [email protected]; [email protected]; [email protected]; [email protected] E-mail: [email protected]; [email protected] Received April 16, 2019; revised April 8, 2020. Abstract To date, bitcoin has been the most successful application of blockchain technology and has received considerable attention from both industry and academia. Bitcoin is an electronic payment system based on cryptography rather than on credit. Regardless of whether people are in the same city or country, bitcoin can be sent by any one person to any other person when they reach an agreement. The market value of bitcoin has been rising since its advent in 2009, and its current market value is US160 billion. Since its development, bitcoin itself has exposed many problems and is facing challenges from all the sectors of society; therefore, adversaries may use bitcoin’s weakness to make considerable profits. This survey presents an overview and detailed investigation of data security and privacy in bitcoin system. We examine the studies in the literature/Web in two categories: 1) analyses of the attacks to the privacy, availability, and consistency of bitcoin data and 2) summaries of the countermeasures for bitcoin data security. Based on the literature/Web, we list and describe the research methods and results for the two categories. We compare the performance of these methods and illustrate the relationship between the performance and the methods. Moreover, we present several important open research directions to identify the follow-up studies in this area. Keywords security, privacy, bitcoin, availability, consistency 1 Introduction The underlying technology of bitcoin is blockchain. The blockchain technology combines multiple computer Bitcoin○1 is a distributed electronic payment sys- technologies such as encryption, distributed storage, tem developed by a scholar named Satoshi Nakamoto. consensus, and peer-to-peer (P2P) network [1]. These Since its invention, many merchants have increasingly key technologies make blockchain open, secure, and expressed their willingness to accept bitcoin as a pay- ment method. Currently, 14 355 merchants across the trustworthy. Moreover, these techniques allow trans- globe are already using bitcoin○2 , and the market value actions to be continuously linked to blockchain, which of bitcoin continues to rise with the current market records all transactions and historical data by estab- value at US160 billion○3 . Consequently, bitcoin has had lishing a jointly maintained and untampered database. a major economic and technological impact worldwide. Internet users who are unaware of one another can reach Survey This work was supported by the Key-Area Research and Development Program of Guangdong Province of China under Grant No. 2019B010137003, the National Natural Science Foundation of China under Grant Nos. U1836212, 61972039, 61872041, 61602039 and 61871037, the Beijing Natural Science Foundation of China under Grant No. 4192050, the Key Laboratory of Information Network Security, Ministry of Public Security, and the Pre-Study Foundation of Weapons and Equipment under Grant No. 31511020401. ∗Corresponding Author ○1 Nakamoto S. Bitcoin: A peer-to-peer electronic cash system. https://bitcoin.org/bitcoin.pdf, Apr. 2019. ○2 NewsBTC. Coinmap’s heat map shows places that accepts Bitcoin. https://www.investopedia.com, Jun. 2019. ○3 Golden finance. Market value. https://www.jinse.com/coin/bitcoin, Jun. 2019. ©Institute of Computing Technology, Chinese Academy of Sciences 2020 844 J. Comput. Sci. & Technol., July 2020, Vol.35, No.4 a credit agreement through a point-to-point ledger or consistency. Data privacy attacks include the threats digital encryption without any central trust [2]. There- of transaction and identity privacy; data availability fore, blockchain has attracted considerable research at- attacks include the threats brought by network trace- tention from various industries [3–11]○4 ○5 . ability and eclipse attacks; data consistency attacks in- The security of bitcoin data, which is the funda- clude the threats caused by double spending, selfish mental enabling factor of bitcoin, is particularly im- mining, and block withholding attacks. Accordingly, portant along with its future development. Adver- we present the corresponding countermeasures because saries currently use blockchain’s characteristics to con- of the threats for each type of attacks. duct various attacks on bitcoin data. First, the open- Our primary contributions are listed as below. ness of bitcoin data exposes users’ privacy. Adversaries • We present an overview and detailed investigation can define the relationship among addresses through of data security and privacy in the bitcoin system. transactions [12]. Second, adversaries initiate abnor- • We examine the studies in the literature/Web in mal or incorrect access to bitcoin data via the bitcoin two categories: 1) analyses of the attacks to the pri- network, which undermines the availability of bitcoin vacy, availability, and consistency of bitcoin data and data. A bitcoin address can be associated with an 2) summaries of the countermeasures for bitcoin data Internet protocol (IP) address; therefore, adversaries security. can track the correspondence among addresses, users, • Based on the literature/Web, we list and describe and real identities [13, 14]. Third, bitcoin data will be the research methods and results for the two categories. inconsistent if an adversary passes an attack on the We compare the performance of these methods and il- blockchain consensus mechanism or discards confirmed lustrate the relationship between the performance and blocks from the blockchain to make bitcoin data incon- the methods. sistent. Furthermore, bitcoin is vulnerable to selfish • We discuss research hotspots and present future mining attacks [15–17], which undermine the consistency research directions. of bitcoin data. In addition to these problems, many The rest of this study is presented as follows. In other security threats have been associated with bitcoin Section 2, we introduce an overview of bitcoin. In Sec- such as mining pool attacks [15] and miner attacks [18]. tion 3, we introduce the attack classification. In Sec- These threats significantly affect the security of bitcoin tion 4 and Section 5, we list and describe research meth- data, thereby threatening the related blockchain appli- ods and results for the attacks and the corresponding cations. countermeasures respectively. We compare the perfor- Recently, some surveys about bitcoin or blockchain mance of these methods and illustrate the relationship security have been conducted. Saad et al. [19] intro- between the performance and the methods. In Sec- duced the attacks of the public blockchain from the tion 6, we present the research directions for the fu- perspective of encryption, distribution, and applica- ture. Finally, in Section 7, we provide a summary of tion. Conti et al. [20] presented a survey on the se- our survey. curity and privacy of bitcoin. Li et al. [21] summa- rized some cases of attacks against blockchain 1.0 and 2 Overview of Bitcoin 2.0. Gervais et al. [22] surveyed the security and ad- Bitcoin implements transactions through addresses. versarial strategies of proof of work (PoW); however, The address is not associated with the user’s identity. these studies lack systematic description and catego- Each user may have multiple addresses, which can en- rization of threats and countermeasures. Because of sure better anonymity if different bitcoin addresses are the rapid development of bitcoin and blockchain, many used for each receiving transaction. new threats and countermeasures have emerged; there- fore, up-to-date research is required to meet the require- 2.1 Underlying Technology for Bitcoins ments of blockchain development. Our study summarizes and analyzes the security Blockchain is the underlying technology for imple- and privacy of bitcoin data. We present the attacks and menting bitcoins; it was originally a unique method for defenses of bitcoin in terms of privacy, availability, and storing data in cryptocurrencies. Moreover, blockchain ○4 Burniske C, White A. Bitcoin: Ringing the bell for a new asset class. https://research.ark-invest.com/hubfs/1 Download Fil- es ARK-Invest/White Papers/Bitcoin-Ringing-The-Bell-For-A-New-Asset-Class.pdf, Apr. 2019. ○5 Gartner. Top 10 strategic technology trends for 2017. http://www.gartner.com/technology/topics/trends.jsp, Apr. 2019. Lie-Huang Zhu et al.: Data Security and Privacy in Bitcoin System: A Survey 845 is a self-referencing data structure that stores a large • The incentive layer sets up incentives to reward amount of transaction information. Each record is users, who participate in the consensus, to encourage linked from the back to the front; thus, the blockchain is more users to participate