Deanonymizing Tor hidden service users through Bitcoin transactions analysis Husam Al Jawaheri Qatar University Mashael Al Sabah Qatar Computing Research Institute, HBKU Yazan Boshmaf Qatar Computing Research Institute, HBKU Aiman Erbad Qatar University ABSTRACT With the rapid increase of threats on the Internet, people are contin- uously seeking privacy and anonymity. Services such as Bitcoin and Tor were introduced to provide anonymity for online transactions and Web browsing. Due to its pseudonymity model, Bitcoin lacks retroactive operational security, which means historical pieces of information could be used to identify a certain user. We investigate the feasibility of deanonymizing users of Tor hidden services who rely on Bitcoin as a payment method by exploiting public informa- tion leaked from online social networks, the Blockchain, and onion websites. This, for example, allows an adversary to link a user with @alice Twitter address to a Tor hidden service with private.onion address by fnding at least one past transaction in the Blockchain that involves their publicly declared Bitcoin addresses. To demonstrate the feasibility of this deanonymization attack, we carried out a real-world experiment simulating a passive, lim- ited adversary. We crawled 1.5K hidden services and collected 88 unique Bitcoin addresses. We then crawled 5B tweets and 1M Bit- coinTalk forum pages and collected 4.2K and 41K unique Bitcoin addresses, respectively. Each user address was associated with an online identity along with its public profle information. By ana- lyzing the transactions in the Blockchain, we were able to link 125 unique users to 20 Tor hidden services, including sensitive ones, such as The Pirate Bay and Silk Road. We also analyzed two case studies in detail to demonstrate the implications of the resulting information leakage on user anonymity. In particular, we confrm that Bitcoin addresses should always be considered exploitable, as they can be used to deanonymize users retroactively. This is espe- cially important for Tor hidden service users who actively seek and expect privacy and anonymity. 1 INTRODUCTION Anonymity and privacy over the Internet are becoming more criti- cal than ever. For that, many solutions are being deployed to im- prove the anonymity of users while making online transaction or browsing the web. The most famous of these solutions are the de- centralized cryptocurrencies and Tor anonymity network. One of the early examples is the Bitcoin network [29], which provides users with the ability to perform online transactions "pseudonymously". Due to its popularity, more than 100K merchants worldwide accept Bitcoin payments [11]. One of the reasons of Bitcoin’s popular- ity is its presumed anonymity. Tor [14] is the most widely used anonymous communication network with millions of daily active users [31]. In addition to client-side privacy and anonymity, Tor also enables server-side anonymity through the design of hidden services. The goal of hidden services is to safely enable online free- dom, anticensorship, and end-to-end anonymity and security [12]. Indeed, for those reasons, hidden services are being operated by whistleblowing websites such as WikiLeaks, 1 search engines such as DuckDuckGo, 2 and online social networks such as Facebook. 3 Hidden services have also become a breeding ground for Dark Web vendors, such as Silk Road [9] and Agora [39], which ofer illicit merchandise and services [4, 27]. As discussed by Vincent and Johan [26], Tor and Bitcoin repre- sent the main components needed to achieve anonymous online purchases with exhaustive operational security. In this context, op- erational security is the process of protecting individual pieces of information that could be used to identify a user. Unfortunately, Bit- coin lacks retroactive operational security due to its pseudonymity model [29]. This model has an important limitation because of the linkability of Bitcoin transactions that are stored in the Blockchain and their public availability. Problem. Due to potential information leakage, using Bitcoin as a payment method is a serious threat to the anonymity of Tor hidden services and their users. Yet, Bitcoin is the most popular choice for these services for accepting donations or selling merchandise [4]. Moore and Rid [27] studied how hidden services are used in prac- tice, and noted that Bitcoin was the dominant choice for accepting payments for these services. Although multiple studies [15, 16, 25] demonstrated that Bitcoin transactions are not as anonymous as previously thought, Bitcoin remains the most popular digital cur- rency on the Dark Web [8], and many users still choose to use it despite its false sense of anonymity. Biryukov et al. [3] showed that even if users use Bitcoin over an anonymity network such as Tor, they are still vulnerable to deanonymization and man-in-the-middle attacks at the network level. While previous studies analyze the vulnerabilities that result from using Bitcoin over Tor [3], mostly at the network level, we provide the frst study that focuses on the application level, shedding light on an exploitable information leakage resulting from correlating public profles of online social network users with Bitcoin transactions and onion websites. Hidden service users are one class of Bitcoin users whose anonymity is particularly important. Hidden service operators and users are actively seeking to maintain their anonymity. However, those users are under the risk of deanonymization when they reveal their Bit- coin addresses. By studying the transactions associated with these addresses, a signifcant amount of information can be leaked and 1 https://wikileaks.com 2 https://duckduckgo.com 3 https://facebook.com 1 arXiv:1801.07501v3 [cs.CR] 10 Jul 2019