# Cryptographic Key Management in Delay Tolerant Networks: A Survey
## 3 Key Management Taxonomy in DTNs
### Key Establishment  
#### 3.2.1 Two-Party Communication  
Identity-Based Cryptography (IBC)  
As already mentioned, IBC has been examined as a possible solution for key management in DTN. In this context, we can stand out the works in [12; 40; 41; 42]. In [12], the authors introduce an anonymous authentication scheme. They also propose a secure communication solution based on the non-interactive Sakai-Ohgishi-Kasahara (SOK) key agreement scheme. This scheme is based on Boneh-Franklin HIBC, for greater scalability and signature verification. Also, according to the authors, it is more efficient compared to [24], as it incurs no additional overhead for routing and can optionally be made non-interactive. Nevertheless, this scheme is very tightly tied to the DakNet model [43], and it assumes a strongly trusted central authority [44]. Instead, for DTN a more general approach is required, where a trusted central authority cannot be assumed. The work in [40] is based on IBC too. The authors present a method using IBC and pseudonyms to transfer securely medical data from rural areas to a hospital in a remote city. They also state that there is no need to check frequently the public parameter as suggested in [24]. More recently, the author in [41] presents a key distribution protocol for infrastructure-less networks, which is based on the BP [4] and more specifically on the BSP [39]. The BP is used to send application data across a DTN network, while BSP provides data integrity and confidentiality services for the BP [39]. It can be argued that with this non-interactive scheme, cryptographic keys can be established for all the BSP mechanisms. The derived keys will be used for the BSP supported algorithms. For instance, HMAC-SHA1 for authentication, RSA for signatures, and AES for encryption. However, this scheme assumes a pre-distributed key. The authors in [42] present an anonymous combined public key (CPK) based protocol. CPK techniques integrate public key cryptography with IBC. Their CPK cryptosystem is based on elliptic curves cryptography (ECC) and eliminates the need of PKC on-line retrieval compared to IBC. Instead, an offline repository is needed. In fact, this work is also based on IBC, which has proved impractical for DTNs. In addition, IBC solutions are undesirable due to intractability of some problems such as the PKG parameter distribution, private key revocation, identity name space management, key escrow, and so forth [35]. The PKG parameter distribution is the main problem in IBC. More specifically, a single PKG has to generate private keys for all the users and also establish secure channels to transmit them, which is a burdensome job in large networks. The use of hierarchy in HIBC alleviates the aforementioned problem making the process faster and more secure in case of key compromisation. Table 2 presents a comparison of IBC-based key establishment schemes proposed for DTNs using the same criteria as in Table 1.  
Public Key Infrastructure (PKI) & Pretty Good Privacy (PGP)  
An analysis related to the applicability of IBC in DTN systems by [25] resulted that for authentication IBC had no significant advantage over traditional cryptography. Works such as [45; 46; 47; 48; 49] are based on the classic PKI because it is well-examined and recognised. However, PKI schemes are associated with limitations such as server unavailability and cryptographic operations overhead. The authors in [45] proposed a DTN security architecture, which focuses on different key management parameters based on proxy certificates and PKI. Their method supports both hop-by-hop and end-to-end authentication with the aim of ensuring data correctness before forwarding using BAB. In their work, they identify that a single key management scheme does not suffice for DTNs because of the overlaid heterogeneous networks. The authors in [46] propose a one-pass key establishment protocol for space DTNs. Their protocol is based on an adoption of the Horsters-Michels-Petersen (HMP) protocol.