# Cryptographic Key Management in Delay Tolerant Networks: A Survey
## 3 Key Management Taxonomy in DTNs
### Key Establishment
#### 3.2.2 Group Communication  
Security in-group communications is a highly desirable feature in military and law enforcement DTN scenarios and the need of confidentiality in group communication grows at a day-by-day basis [51]. As presented in Table 4, nearly a handful of works [51; 52; 53; 54] cope with group key management and specifically with rekeying in DTNs. Before delving into the details of the aforementioned works, it is to be noted that group key management protocols should take into account various security requirements such as forward and backward secrecy. More specifically, the most important security requirements for a group key management protocol are [55]:* _Forward secrecy (FS)_--requires that users who left the group and know a contiguous subset of old group keys cannot discover subsequent group keys. This ensures that a member cannot decrypt data sent immediately after it leaves the group.
* _Backward secrecy (BS)_--mandates that a new user that joins the group and knows a contiguous subset of group keys cannot discover preceding group keys. This ensures that a member cannot decrypt data sent before it joins the group.
* _Collusion freedom (CF)_--requires that any set of fraudulent users who have much information about past keys should be incapable of deducing the current used group key.
* _Key independence (KI)_--requires that a passive adversary who knows any proper subset of group keys cannot compromise other past or future group keys. That is, the combination of backward and forward secrecy yields key independence.  
The first attempt for group key management in DTNs is presented in [52]. Specifically, the authors proposed a group-oriented security solution for DTNs that provides access control and secure group communications. They suggest a centralised group key management mechanism based on the Logical Key Hierarchy (LKH). Group key management in DTN has been studied in [53] as well. The proposed protocol capitalizes on the Chinese Remainder Theorem (CRT). The concept of key lifetime is also introduced to alleviate the forward security problem in many-to-many DTN communication scenarios. In addition, the authors suggest that group key management for DTNs should use stateless and not stateful schemes such as LKH because there is no need for users to possess any previous keys. On the downside, the drawback in LKH scheme is that whenever a user joins or leaves, the group structure of tree has to be rearranged and logical key at each ancestor node has to be recomputed. Specifically, the computation cost is analogous to the network scale [54]. Actually, this is the reason why LKH scheme is unsuitable for space DTN.  
More recently, another research work on group key management is given in [54]. More precisely, the authors propose an autonomic group key management (AGKM) scheme based on one-encryption-key multi-decryption-key (OMPK) key protocol for deep space DTNs. In this work, the authors also prove the forward, backward, passive security, and key independence qualities of their proposed protocol. In terms of efficiency, their scheme seems to produce a smaller penalty than other proposed (e.g., LKH), making it more suitable for deep space DTNs. Due to the lack of central key management center support, rekeying can be attained by a local leaving or joining user.  
Last but not least, the work in [51] proposed a scheme which is based on a modified version of Chinese remainder theorem. By shifting more computing load onto the key server, their scheme optimize the number of re-key broadcast message. For simulation results Opportunistic Networking Environment (ONE) Simulator is used and the results suggest that this scheme is better than LKH as well as Chinese remainder group key schemes. More precisely, this scheme does not broadcast any key update message in case of user join or leave, thus making it very efficient for secure communication in DTNs. Their work, reduced the complexity of user leave from \(O\left(n\right)\) to constant \(O\left(I\right)\).