## 8 Key Performance Evaluation
### Diffusion of the Encryption System  
As explained in Section 4.1, diffusion is one of the main property of a good encryption system. It helps to increase the complexity of the statistical tests that an eavesdropper can perform on intercepted messages, prohibiting him/her from deciphering the format of the communicated packets and its potential contents and values. An eavesdropper cannot modify an intercepted packet and replay it to the system if he/she does not know what are the different parts of the packet and the role of each set of its bits. Figure 13 shows the relation between the number of the different bits of the original message and how they affect the encrypted message. The graph shows the mean value of changed bits in the encrypted message for a given number of bit changes in the original messages. The mean is computed such that the changes in the original messages occur in different bit locations. As we can see, for a slight change in the original message, the encrypted one has more variations in the bit values, even for a single-round algorithm. In fact, using one round in the encryption system is only dedicated to basic IMD that have a basic architecture and very limited resources and where the security aspect is not very demanded. However, this protocol still offers secure communication to the device. Adding an additional round to the encryption system will increase linearly the complexity of the encryption algorithm by a simple factor of two, however, the confusion and diffusion aspect of the system will be very strong. This can be seen from Figure 13. Also, the avalanche factor [37] of the algorithm will reach the value \(0.56\geq 0.5\). The value \(0.5\) is the least requested value for a robust encryption algorithm, this ensures that more than half of the encrypted output will change on a single change of an input message bit. This will ensure that if there is a recognizable pattern in the input messages, the encryption system will scramble the bits in a way that this pattern is lost.  
As the problem with any symmetric cryptosystem, the encryption system appears to be too simple to be robust. This a key limitaion for securing the communication. However, this fact is balanced with the robustness of the key and its sharing method. Yet, this is still a general case to define a good balance between the key generation, its expiraton time and the number of rounds of the encryption system. Therefore, a specefic IMD is to be studied individually to ensure such balance. In addition, that is one of our future goals based on this work's results.  
A main concern for the users is the pattern recognition of the messages, as explained in the work of [PERSON] et al. [32]. A typical message in IMDs with simple architecture is as shown in Figure 14. Several attempts to eavesdrop an unsecured message can lead to the identification of the flags of the different  
Figure 13: Encryption of Different Messages of 128 bit Length.  
packets. Thus, the attacker can gain control over the communication. Figure 15 shows that even listening to a communication where messages have a recognizable pattern, this pattern is eliminated after encryption. In fact, Figure 15 demonstrates the change rate of bits after the encryption process of the same message when only one flag changes. Even though the same encryption parameters are used, the eavesdropper cannot pinpoint the patterns of the message, as the bits appear to randomly change all along the fixed-length packet.