## 5 Discussion
### Interplay between the Various Techniques Used in the Proposed Approach  
The proposed approach in this paper combines the benefits of multiple techniques: cipher block chaining, physical layer security, and symmetric and asymmetric encryption techniques. The purpose is not to perform a dull combination of these techniques. Quite the opposite; they operate in tandem in order to secure the transactions' data in the absence of internet connectivity. In fact:  
* DTN is a precondition for the proposed approach to be applicable: If internet connectivity is available, then the drone can communicate directly with the Control Center, encrypt the transaction using any traditional technique, and send it immediately (in encrypted form) to the center without any need for storing it on the drone or having to carry this information through multiple destination/stops. The proposed approach is unnecessary in the presence of connectivity. * Asymmetric cryptography (e.g., RSA) is needed to encrypt the keys so that they can be securely carried by the drone until its return to the Control Center. Since the private key is only stored at this Control Center, then an attacker accessing the memory location of the encrypted keys on the drone will not be able to decrypt them. * Symmetric encryption following the diagram of Figure 3 is used with different keys, although traditional CBC might use the same key. This is necessary for the scenario considered in this paper, where the drone must make multiple stops at different destinations. In fact, if the same key is used, once the drone lands at a given destination, the data of the previous destinations can be recovered and decoded, which violates the confidentiality of the data and the privacy of the previous users. Using a different key per destination, such that this key is unknown to the other destinations, provides protection against such a scenario. * In the proposed approach, each one of these keys is used once, immediately after being generated, then stored after being encrypted using the Control Center's public key. Thus, after using a key \(K_{i}\) for one of the blocks shown in Figure 3, the drone itself cannot recover its value after storing it in encrypted form (decryption can be performed only with the private key of the Control Center). To be able to use these symmetric keys without carrying them in an accessible form, the framework proposed in this paper suggests generating them using physical layer security. In this case, the key for a given transaction can be generated immediately at the corresponding location, used for encrypting the corresponding information block, and then stored securely in the drone without having to be exchanged/transmitted over the wireless channel. * The use of CBC adds an extra layer of security to this symmetric encryption approach. This is achieved by linking the transactions at the multiple stops of the drone in a blockchain-like approach, showing the order of delivery along the followed trajectory, not just the separate encoding of the data as standalone delivery operations. Furthermore, it is necessary in case the modified version of the proposed approach, discussed in Section 4 while analyzing Equations (1) and (2), is adopted. Finally, it should be noted that the proposed approach is a framework that does not depend on a specific algorithm. It is a suggested approach for protecting transaction data logs in autonomous multiple-delivery scenarios in the absence of connectivity. The proposed approach can be used with any symmetric encryption algorithm that can be implemented in the \"Encryption\" blocks of Figure 3: DES, 3 DES, AES, AES-GCM, etc. It can also be used with any asymmetric encryption approach to encrypt the keys \(K_{i}\) and store them in the drone's memory. In practice, the selected encryption algorithm could depend on the capabilities of the drone, such as the computational power available, amount of energy that can be spared for cryptographic operations, etc.