# Tor Hidden Services: A Systematic Literature Review
## 6 Conclusions  
Tor is currently the largest anonymous network, and the Tor protocol lends itself to in-depth research, because all its development is carried out publicly and the source code and specifications of each version are openly available.  
In this article, we have presented a systematic review of the literature on Tor's hidden services. Our review showed that the dark web formed of THS is very volatile, since the HS tend to be ephemeral. Furthermore, many of the services published are not available. In this network, the HS are highly interconnected; an important percentage of these nodes have no outbound links; most of the content is in English; there is content imported from the surface web; and there is an important amount of illegal or unethical content (ranging from 38% to 45%, depending on the study). Furthermore, access to HS is slow due to the six relays that form a circuit to a HS from a client.  
In this SLR, we also found that most of the papers are focused on security, either presenting attacks or proposing protection methods. Furthermore, researchers have focused on discovering at the directory level as many.onion addresses as possible, and with these addresses, classifying the content hosted on these anonymised services. To a lesser extent, research formulates changes in the protocol design to achieve faster connections to hidden services or make circuits more secure.  
We also found out that many studies exposing attacks have not been tested in realistic Tor environments. Therefore, they have wide margins of failure. Another problem encountered was the inconsistency in the number of services reached or discovered using different techniques. The hidden service descriptors are stored distributively. Thus, there is no central entity storing the complete list of.onion addressed. The number of addresses found may be far from exhaustive and realistic. Therefore, it is not possible to give a size to Tor's hidden server network beyond the metrics supplied.  
We have already seen that the establishment of a connection between a client a hidden service is a complex and slow process. Although research is being done on this issue, we have seen several proposals that introduced improvements regarding several issues that can improve it. We consider that this is an issue that should see more research to address it in a global and consistent way, as for other issues such as security and anonymity.  
Finally, emphasis was placed on the lack of research on the new generation of hidden services. Although the new version is not the one used by most of the services deployed, according to the Tor Project, the support for version 2 of HS will stop, and it will only be possible to use version 3 of HS to give way to a much more secure ecosystem.  
As future work, a deeper evaluation of the evolution of the attacks presented and their degrees of success or failure could be performed, especially since the Tor project has fixed the vulnerabilities that many of these attacks were based on.  
In the medium term, this SLR should be taken up again in search of new research that has included the new generation of onion services, and we will make a comparison with the research presented here.  
Conceptualization, A.R.-M. and D.L.H.T.; Investigation, D.L.H.T. and A.R.-M.; Methodology, A.R.-M. and D.L.H.T.; Writing-original draft, A.R.-M. and D.L.H.T.; Writing-review and editing, A.R.-M. and D.L.H.T. All authors have read and agreed to the published version of the manuscript.  
This work was funded by the European Commission's H2020 Programme under grant agreement number 830929; and the Spanish Ministry of Science, Innovation and Universities, FEDER funds, under grant numbers RTI2018-095855-B-I00 and TIN2017-86885-R.  
Not applicable.  
The authors declare no conflict of interest.