# Cryptographic Key Management in Delay Tolerant Networks: A Survey
## 4 Discussion  
In the previous section we have classified the proposed solutons of cryptographic key management in DTNs into three major categories. However, as shown in the corresponding subsections, the majority of the examined approaches are hybrid in nature and may fall into more than one category. Characteristic examples of this situation are the works in [12; 24; 25; 31; 35], where the authors propose schemes that can be used in security initialisation and key establishment. Contributions such as [34; 65] have shown that establishing the initial source context at the deployment phase is still an open issue. From Table 1 we can observe that the majority (8 out of 12) of the security initialisation methods are based on PKC. A number of other methods for security initialisation have relied on IBC, such as HIBC and its variations.  
Also, there is an almost unanimous agreement, that traditional PKI is not always suitable for DTN. Specifically, in disconnected DTNs, without online access to the necessary certificate or the certificate revocation list posted by CAs, sending an encrypted message and authenticate senders' identity is infeasible. For this reason, apart from PKI, IBC has been examined as a viable solution for security within the DTN context. From Tables 2 and 3 it can be observed that few works propose the usage of pre-shared keys or pre-established trust between the nodes [41; 47]. However, it is obvious that for scalability reasons such schemes apply only in a small and fixed-size DTNs.  
By observing Table 4 it is clear that group key management for DTN is still in its infancy, with only four proposed works based on LKH or Chinese Remainder Theorem. Moreover, as given in Table 5, even fewer works proposed ways of handling key revocation in DTNs. It is worth mentioning that researches tried to address the key revocation issue almost eleven years (2016) after the first work related to key management in DTNs has been published [24]. To further exemplify this, Figure 2 provides a timeline of all the different methods introduced for key management in DTNs. It can be noticed that all the different methods proposed between 2005 and 2013 and the rest of the papers are based on the already proposed methods. Moreover, the bottom part of the same figure, includes the different kind of DTN network that is addressed by each work. For instance, the first chronologically proposed works on DTN key management focused on rural area DTNs, while the last ones on large scale DTNs.  
In addition, as it can be seen from Tables 1-5, when applicable all the solutions included in this survey have been either evaluated only through theoretical proofs and/or simulations. This means that hardware testbeds and real-life deployments in cryptographic key management are still largely missing from the DTN research area. On top of that, up to date, most of the works in the context of key management in DTNs concentrate on rural or space networks neglecting other DTN applications, including vehicular or undersea ones. Last but not least, the analysis of the various works showed that the most recent ones (after 2012) tend to focus on space DTNs and generally on large scale DTNs.