## 5 Discussion
### Physical Layer Security (PLS) Key Generation  
The PLS key generation methods could vary. For example, in orthogonal frequency division multiple access (OFDMA) communications, channel state information (CSI) of multiple subcarriers can be exchanged to select the best group of subcarriers for communication. This CSI exchange could be used to generate a secret key: If the CSI level of a certain subcarrier is above a pre-defined threshold, this corresponds to a bit value of \"1\" in the key and to a value of \"0\" otherwise. Concatenating these 1/0 values for a large number of subcarriers leads to a long key that can be used for symmetric encryption.  
Another approach is to use some indicative feature about the state of the UAV hardware at a given instant and use it as a seed for a pseudorandom number generator, which could lead to the generation of a pseudorandom key for encrypting a specific transaction.  
These PLS key generation methods rely on randomness, depend on the particular state of the UAV at the time of the transaction, and they are used at the UAV only, without being exchanged or transmitted over any wireless link. Consequently, they are extremely hard to guess by a malicious attacker. In fact, an eavesdropper attempting to guess the key obtained through CSI thresholding cannot measure the signal level between the destination device and the drone. It can only measure the signal strength it receives at its current location. Due to fading fluctuations over the OFDMA subcarriers, even assuming it knows the threshold agreed upon for deciding the 1/0 values of the key bits, the device-eavesdropper and drone-eavesdropper links will have significant differences compared to the device-drone link. To be able to correctly capture the key bits, the eavesdropper must come too close to the device so that its channel characteristics become similar to the device-drone link, which would lead to its discovery and detection.  
For example, considering PLS key generation using the CSI values for the subcarriers in an OFDMA system, we define \(\mathrm{d_{D}}\) as the distance between a legitimate device and the drone, and \(\mathrm{d_{E}}\) as the distance between the device and a malicious eavesdropper. It should be noted that, although the drone delivers goods at the exact location of the device, communication to generate the encryption key using PLS can start when the drone is still at a distance \(\mathrm{d_{D}}\).  
We assume the eavesdropper attempts to guess the key values using the signal strength it receives at its own location, and we plot the percentage of correctly guessed \"1/0\" values of the key bits for various values of \(\mathrm{d_{D}}\) and \(\mathrm{d_{E}}\). The results are shown in Figure 5.  
Figure 5 shows that the correct guesses are around 50%, which leads to maximum confusion for the eavesdropper. In fact, if the approach was known to lead to a much lower percentage, e.g., 10%, the eavesdropper would simply have to invert its decisions (if it guesses a \"0\" bit, flip it to \"1\" and vice versa), in order to reach a guess-rate of 90%! However, a 50% guess rate leads to maximum confusion [24; 25] as the eavesdropper would not know the positions in the key in which bits are correctly guess  
Figure 5: Percentage of eavesdropper’s correct “guesses” of the bits of a given key.  
Inverting the decisions for all of them will still leave the eavesdropper with a 50% guess rate, thus maximizing uncertainty. Since the key is used only once for encrypting a single transaction (as per the proposed approach of Figure 3), the eavesdropper will not get a second chance to try to enhance its estimates. Encryption at a new location is performed with a completely new key.