# Cryptographic Key Management in Delay Tolerant Networks: A Survey
###### Abstract  
Since their appearance at the dawn of the second millennium, Delay or Disruption Tolerant Networks (DTNs) have gradually evolved, spurring the development of a variety of methods and protocols for making them more secure and resilient. In this context, perhaps, the most challenging problem to deal with is that of cryptographic key management. To the best of our knowledge, the work at hand is the first to survey the relevant literature and classify the various so far proposed key management approaches in such a restricted and harsh environment. Towards this goal, we have grouped the surveyed key management methods into three major categories depending on whether the particular method copes with (a) security initialization, (b) key establishment, and (c) key revocation. We have attempted to provide a concise but fairly complete evaluation of the proposed up-to-date methods in a generalized way with the aim of offering a central reference point for future research.  
c 2017  
and the application layers. In addition, DTN architecture is based on the well-known store and forward model, an old mechanism used in postal systems since ancient times [5]. The main dissimilarities between the assumptions of traditional Internet-like networks and DTNs are the intermittent connectivity, implying the lack of a continuous end-to-end path between the source and destination and the long propagation delays. Conventional mechanisms for routing and key management do not work in a DTN mainly because of these assumptions. In fact, the literature has a relatively long domain of routing in DTNs, but very few consider the security parameter.  
The unique DTN characteristics, including long round-trip delay, frequent dis-connectivity, fragmentation, etc. [3], make the existing security protocols designed for the conventional networks unfit for DTN ecosystems. Cryptographic key management and secure routing are important issues in DTNs, but the solutions proposed until now tend to consider them separately. Several approaches have been adopted to achieve cryptographic key management in such challenged networks. The main bulk of research has been focused on two main approaches: the traditional Public Key Infrastructure (PKI) [6] and Identity Based Cryptography (IBC) [7]. Each of them has its own benefits and drawbacks and is suitable in certain domains of DTN [3].  
To our knowledge, so far no work in the literature has attempted to provide a comprehensive survey of the various works addressing cryptographic key management specifically for the DTN domain. Motivated by this fact, the survey at hand offers an extensive study of the relevant literature in the last 13 years, spanning a period from 2005 to 2017. The surveyed protocols are categorized based on four distinct factors, namely the communication type, method used, type of the challenged network, and evaluation method.  
The rest of the paper is organized as follows: The next section briefly reviews the background of research on preliminaries regarding key management and DTN characteristics. Section 3 reviews and classifies all major contributions in the field of key management in DTNs. A discussion on the surveyed schemes is provided in Section 4. Section 5 presents alternative key management taxonomies for DTNs, while Section 6 lists the main security callenges in this type of networks. The last section summarizes and concludes the survey by posing open questions and future directions of applying key management to secure DTNs.