# A Privacy-Preserving Routing Protocol Using Mix Networks in Opportunistic Networks
## 1 Introduction  
We live during a global deployment of computer networks and network devices. This technology has allowed for the creation of a wide range of different network-based applications that enable collaborative behavior among users to share their personal interests, such as their hobbies and professions. During the last few years, the research community has proposed different solutions to allow for mobile nodes to communicate with each other, even in extreme cases where no end-to-end connection is guaranteed. This network paradigm has been termed as opportunistic networks (OppNets) [1]. In these opportunistic social networks, users carry mobile devices to communicate with each other and share data as in traditional social networks. Nevertheless, when it comes for privacy in OppNets, not all that glitters is gold.  
There is a range of applications in OppNets, such as crisis management, battlefield coordination, wildlife monitoring, transportation engineering, and remote healthcare. They are typically used in environments that require networks to be tolerant of long delays, interruptions, and high error rates. Nodes in these networks communicate with each other in an opportunistic manner. It is common to use routing strategies on the basis of the so-called store-carry-forward strategies. That is, a node can store and carry a message until it finds another node to forward it.  
Because of the unstable connection property of OppNets, it is difficult to directly use traditional network privacy and anonymity mechanisms. Current proposals are mostly centered on providing extensions or strategies to adapt onion routing for OppNets. Although these could be interesting approaches, some OppNets scenarios need to address the problem of traffic analysis. Contrary to the use of onion routing on the Internet, for example, in some OppNets scenarios, having an attack model where the attacker has access to the whole communication medium is not rare. Typical OppNets usewireless connections where traffic analysis imposes a dangerous threat. Attacks can exploit message flow to detect the whole onion routing path, which leaks the communication pattern between sender and receiver, assuming that all network traffic is not extremely high. A common approach to deal with this problem is to generate noisy network traffic to mask actual traffic. Another alternative that has not been investigated in depth is the use of mix networks (mixnets). In this paper, we propose the use of a mixnet mechanism to provide privacy in opportunistic networks.  
Mixnets can be used to provide anonymous communication in OppNets without high impact on the normal operation of the network. Some nodes from the network act as mix nodes enabling the implementation of different strategies. To the best of our knowledge, our anonymous schema is the first work to propose the use of mixnets in OppNets. Our main contributions are the following: First, we designed a mixnet-based communication approach for OppNets. Second, we validated and showed the particularities of our proposed schema in the ONE simulator (a common network simulator especially focused towards OppNets). Third, we analyzed the performance and privacy property of our proposed mechanism.  
This paper is organized as follows. In Section 2, we introduce the current related work. We then present our anonymous-routing mechanism using mixnets in Section 3. Section 4 describes our experiment simulation on the ONE simulator. Lastly, in Section 5, we conclude our proposed work.