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Characterized by dynamically changing topology as well as frequent connectivity disruptions, delay tolerant networks (DTNs) represent a common type of self-organizing networks, such as pocket switched network, mobile social network and mobile sensor network (Hrabčák, Matis, Doboš, & Papaj, 2017). In these networks, numerous mobile intelligent devices exchange data through short-range communication interface (Pentland, Fletcher, & Hasson, 2004). Since end-to-end paths cannot be guaranteed, messages are transmitted through opportunistic encounters between nodes (Fall, K). That is, a message could be sent over an existing link, get buffered at the current node until the next link in the path comes up, until it reaches its destination (Spyropoulos, Psounis, & Raghavendra, 2005). As a result, the path that a message passed through is more difficult to be tracked than infrastructure-based communication and traditional wireless communication. This feature makes DTN an appropriate masking channel for information hiding systems. Generally, information hiding system includes content hiding and transmission hiding (Johnson, & Katzenbeisser, 2000; Chen, et al., 2017). The content hiding mainly focuses on technologies such as cryptography, steganography and digital watermarking (Lin, Yang, & Wang, 2016; Soderi, Mucchi & Hämäläinen, 2017; Hust, & Wuhan, 2005). Besides, the transmission hiding is concerned with the design of masking channel, which exploits some of the inherent characteristics to achieve communication hiding in physical layer (Yang, Kim, & Lo, 2016; Yang, Kim, & Lo, 2015, Huang, Liu, & Lv, 2015; Liu, 2015). In this paper, the DTN is used for the hidden transmission of messages in information hiding systems. After encryption or steganography of the content, the encrypted information such as images or videos are delivered by multi-hop under the intermittently connected environment. In this way, the transmission path becomes unpredictable, and the source senders can be protected from being tracked by the unknown third party. Therefore, both the transmission process and the sender can be “hided” in network layer.
Although DTN is an excellent masking channel, its intermittently connected property makes the delivery of message a challenge (Hui, Chaintreau, Scott, Gass, Crowcroft, & Diot, 2005). As a result, the communication quality can be affected seriously. Thus, to make sure that the DTN is capable to be used as masking channel, the delivery efficiency of message forwarding must be improved. In this paper, an efficient routing strategy which includes two critical schemes is proposed to solve this issue. Firstly, considering that message may stuck and expire in an inactive node, a multi-copy scheme is used to increase the probability of the delivery of message. To enhance the safety of message, a dynamic control of the number of message copies is designed. Secondly, in order to choose a more appropriate next hop for message, available encountering information of nodes in DTN should be exploited, and a utility-based relay node selection mechanism that can assess the delivery capability of the encountered node is developed.
On the other hand, a relatively long TTL (Time-To-Live) will be helpful to improving delivery probability of the message in DTN. However, in information hiding systems, long TTL of message will increase the risk of the message being exposed. To make a balance between delivery ratio and concealment, the lifetime of message should be designed very strictly. In this paper, the sensitivity of message TTL is analyzed, and then a suitable value of lifetime is given.