# A Privacy-Preserving Routing Protocol Using Mix Networks in Opportunistic Networks
## 5 Conclusions  
In this paper, we proposed the use of a mix network to provide anonymous communications in OppNets. Our approach is to rely on a free routing mixnet with opportunistic routing between mix nodes. We showed that the introduced performance penalty by using such a system in common OppNet scenarios is tolerable, making it an interesting solution to consider.  
As we pointed out in Section 2, using an onion-routing mechanism cannot ensure anonymous communication well in some situations, as it is often easier to be compromised with entry and exit nodes. Our mixnet-based proposal has interesting advantages as compared to existing approaches that are mainly based on some form of onion routing. One of the main advantages is making the system more secure against traffic analysis. Some typical OppNet scenarios involve wireless communications that can be easily monitored. Even monitoring the whole network is not unrealistic. The use of mix nodes provides more resistance to such attacks at the expense of introducing a larger penalty to the performance of the network, both in terms of message-delivery ratio and delay. Our goal here was to show that such a penalty can be tolerable in most cases.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Attack Range (\%)** & **Broken Circuits (\%)** & **Scheme** \\ \hline
0 & 0 & OR \\
0 & 0 & Mix networks \\
20 & 16.7 & OR \\
20 & 0 & Mix networks \\
40 & 46.7 & OR \\
40 & 0 & Mix networks \\
60 & 63.3 & OR \\
60 & 0 & Mix networks \\
100 & 100 & OR \\
100 & 0 & Mix networks \\ \hline \hline \end{tabular}
\end{table}
Table 6: Attack range with broken-circuit percentage.  
There are some potential extensions to our presented work. As network traffic in OppNets significantly affects the attacker's judgement of the flow of each message, using dummy traffic can be a useful tool to enhance the difficulty of linking the relationship between sender and receiver. Dummy traffic can be exploited to increase the complexity of message flow and hide end-to-end communication intention [19]. It is also essential to investigate the trade-off between dummy traffic and network performance. Another future work will be the investigation of more complex mix networks. In this paper, we exploited a simple mix network mechanism to provide anonymous communication in OppNets. However, there are more other mix networks methods [20; 21] that can be potential techniques to improve anonymous communications in OppNets.  
Conceptualization, D.C. and G.N.-A.; methodology, D.C. and C.B.; software, D.C. and C.B.; validation, D.C., C.B. and G.N.-A; formal analysis, D.C., C.B.; data curation, D.C.; writing--original draft preparation, D.C.; writing--review and editing, G.N.-A, D.C. and C.B.; visualization, D.C.; supervision, G.N.-A and C.B. All authors have read and agreed to the published version of the manuscript.  
This work was partly supported by the Catalan AGAUR 2017 SGR-463 project, and the Spanish Ministry of Science and Innovation TIN2017-87211-R project. [PERSON] acknowledges the support from a China Scholarship Council grant.  
The authors declare no conflict of interest.