# Tor Hidden Services: A Systematic Literature Review
## 1 Introduction  
The Onion Router (Tor) [1] is currently the broadest low-latency anonymous communication network and is used every day by many people to protect their privacy and overcome censorship [2, 3]. The common belief in the Tor project is that \"[I]Internet users should private access to an uncensored web\" [4]. The main function that provides Tor to any citizen is, through a set of nodes in the Tor network, to protect his/her anonymity by hiding his/her network IP address [1]. The security and anonymity of Tor have been deeply studied in many works [2, 5], and although different types of attacks are possible [3, 6], the research community is working toward providing solutions to the different attacks [5, 7, 8]. However, Tor goes beyond protecting a citizen's network identity. Since 2004, it has offered the possibility of protecting the locations of services through a mechanism that formerly was named Tor hidden services (HS) and that is currently named onion services (OS). Thus, anyone can provide services (_hidden services_ or _onion services_), and the server's IP address cannot be learned by a client of the service. These services can be clearly identified because their addresses are finished in.onion. According to Tor metrics, there are more than 150,000 of these hidden services [9]. Any hidden service requires that any citizen executes the rendezvous protocol to contact the service. This protocol guarantees that the citizen accesses the service without having knowledge of server's IP address. While the citizen is using Tor, the service does not know citizen's IP address. Thus, using this protocol both the citizens and server IP addresses are hidden from each other, and other entities cannot see that they are connected. Since their introduction, the number of HS offered has exponentially increased [9]. Users use them to access services related to various topics, such as pornography, drugs, politics, security, and anonymity. Cybercriminals make use of HS to perform illegalactivities [10]. These kinds of activities are related to counterfeit credit cards, drugs, pornography, and weapons. However, although it is used for these activities, which are not desired, as [PERSON] said, \"Tor is a critical technology, not just in terms of privacy protection, but in defense of our publication right\" [11]. Tor is a key tool to circumvent censorship. Indeed, it is the most widely system used for this purpose. The whole set of services that are accessed in this way with other services from other anonymous networks that follow the same approach forms what it is called the dark web [12]. This dark web has become famous in mass media because many illicit services have been offered through darknet markets [13] or cryptomarkets [14]. However, as mentioned, Tor aims to help circumvent censorship and support freedom of speech. Thus, it is used by journalists and dissidents; and to provide freedom of speech it is being used by human rights and whistle-blowing organisations such as Wikileaks and Goldballeaks [15]. It is also fundamental to a secure political freedom [16]. Furthermore, there is an important number of HS providing links to services of the surface Web--namely, to social networks, web content management, news, and adult content [12]. Thus, Tor is a neutral tool that, depending of the purposes we use it for, can be either good or bad [16]. As mentioned previously, due to its popularity and the services it offers, Tor has been broadly studied and many issues regarding it have been analysed. It has been analysed both from the user's point of view [17] and a technical point of view [5, 12, 15]. Technical studies are the most common analyses. In a taxonomy presented by [PERSON] et al. [5], they classified Tor research topics into de-anonymisation, performance analysis and architectural improvements, and path selection. Within these topics, HS were included. But, in this analysis, they only covered four papers. These hidden services are key within the Tor network and we believe they deserve to be studied in detail.