# Tor Hidden Services: A Systematic Literature Review
## 3 Related Work  
In this section we present some related work on Tor. Namely, we will focus on Tor hidden services and previous surveys. In the literature, to the best of our knowledge, there is no a comprehensive survey focused directly on HS. Despite this fact, we can mention that there are several reviews of anonymous communication systems (ACS): two of them are focused on Tor, and there is anoter focused on the de-anonymisation of hidden services. We can start the review of the different surveys on ACS with the work of [PERSON] and [PERSON] [23]. They reviewed the main techniques in the field of ACS, including Tor, and also included a quick review of hidden services. In their work, the main topics covered addressed security issues, and they concluded by exposing that the main vulnerability for a HS in Tor is the selection of the first and last node in the communication path. Next, we can consider [PERSON] and [PERSON]'s survey research on Tor [24]. They examined its design, identified weaknesses and deficiencies, and presented a classification system for the directions that the research is taking in these areas. As for HS, their greatest findings were again in security matters. In this work, they concluded by stating that improvements are needed in the design of THS. They mainly exposed the problems with malicious services directories and problems with services that accept \"anonymous\" payments with Bitcoin. They also provided a walk-through of documents that mention the ease with which a command and control (C&C) server may be protected behind an HS. [PERSON] and [PERSON] [2] also published, in 2017, a systematic review of the literature on anonymous communication systems. They conducted their review based on the collection of information in seven academic databases and selected 203 papers for analysis. Of these papers, nine covered issues related to hidden services: aspects related to security, main attacks, proposals to strengthen security, analysis, and measurements of anonymity were addressed. The main topic was the de-anonymisation of hidden services. Finally, from this SLR, from all the papers analysed, the paper with the most references was a paper related to HS [25], which focused on trawling for HS. This paper showed the research interest in services of this kind. More recently, [PERSON] et al. [5] published a review focused on Tor. This review classified the collected articles into three main groups: de-anonymisation, path selection, and improvement and performance analysis. Of all the articles that analysed Tor, only 9% were related to hidden services, and most of these focused on de-anonymisation, indicating that relays and traffic are the most susceptible factors. As a last general review on Tor, we can mention [PERSON] et al. 's [3] review. They analysed traffic analysis attacks on Tor and evaluated how practically these attacks can be performed in real-time in the Tor network. They pointed out that many of the de-anonymisation attacks on HS, based on Tor's threat model, assume that there are one or multiple malicious Tor relays. This model is also considered in flow watermarking attacks, where the malicious client injects the watermark into the flow communication, and then it can be detected by the corrupted entry guards of the hidden service and break anonymity. The analysis provided a classification of the nine attacks analysed, but they did not cover HS in detail. Finally, [PERSON] et al. 's survey [26] is the only paper that presents a review on a particular issue of HS: attacks schemes for revealing hidden servers' identities. This paper is from 2015, and they analysed three methods of attack: the manipulating Tor cells method, the cell counting-based method, and the padding cell method. However, the paper is not focused on reviewing the literature. As we have seen throughout this section, Tor has significant attention from the research community. Within Tor, hidden services is an important issue that has generated attention. However, so far no survey has analysed this issue.