# Cryptographic Key Management in Delay Tolerant Networks: A Survey
## 3 Key Management Taxonomy in DTNs
### Security Initialization  
The first attempt of researchers to solve the problem of security initialization and key management in DTN was based on IBC rather PKI. This was mainly due to the frequently disconnected nodes and hostile nature of such networks. Works [24; 25; 26] are characteristic examples of this situation. More specifically, the authors in [24] proposed the first work based on IBC for key management in DTN. They state that the traditional PKI-based approach is unsuitable for DTNs due to their disconnected nature. Their work examines the practical aspects related to deployment of DTN in remote rural and/or disconnected areas. This includes practices for both initial key establishment and roaming among different service providers. They propose a forward-secure Hierarchical Identity Based Cryptography (HIBC) scheme that according to them can can be proved efficient and practical toward secure channel establishment, mutual authentication of parties, and revocation in DTNs. On the downside, as the authors admit, it is well-known that HIBC suffers from the problem of PKG compromisation, where all the generated private keys for lower level PKGs and users can be yield. To bypass this problem their work is founded on the assumption that PKG is trusted and uncompromisable. Moreover, their work is based on time-based keys (keys that rely on the high synchronized clocks between all entities), which can be a problem of practicality with respect to actual deployment [27]. Another work that evaluates IBC cryptography in the context of DTN and discusses the trade-offs between PKI and IBC is that in [25]. Specifically, the authors investigate how security in DTNs can be bootstrapped from existing cellular large-scale security infrastructure. They describe how a PKG can verify whether a new principal have the right to public identifier or not as compared to [24]. Moreover, in their work, they analised the applicability of IBC in DTNs and they found that there is no significant advantage over traditional cryptography in terms of authentication. In [26] the authors propose an architecture based on HIBC that according to them provides end-to-end security services as well as the ability to have fine-grained revocation and access control. In addition, their scheme is alleged to offer efficient key distribution across DTN regions. One drawback of IBC-based works is that there is a need to check IBC public parameter [28]. This is the same problem researchers tried to overcome in PKI with CA certificate verification. However, in [25] the authors argue that such a comparison is unfair. Another drawback of IBC is the difficulty of key revocation. The work in [29] focuses on the problem of initial secure context establishment in DTNs and proposes a method that allows users to leverage social contacts to exchange confidential and authentic messages. More specifically, if a node does not posses its peer's public key, then it can encrypt the message with the public keys of several nodes near the destination, in terms of either physical proximity or contact frequency. However, this algorithm has the problem of having to constantly maintain contact information for several nodes in the network, and therefore it does not scale well or it may lead to deadlock if the destination has currently no neighboring nodes. The authors in [30] propose a local and self-organised key management scheme in opportunistic networks (OppNets). They use pseudonym certificates and encapsulated signatures to enable the bootstrapping of local, topology-dependent security associations between a node and its neighbours along with the discovery of the neighbourhood topology. The authors identify that for content-based communication IBC-based solutions are inapt and self-organised solutions suit better. Their scheme consists of two phases the setup/initialization phase and the key agreement one. In [31] the authors describe a Distributed Key Establishment (DKE) protocol in location-based social wireless sensor and actor DTNs.