*4.5. Limitations of Fixed Dummy Value Model*

The proposed fixed dummy value model could detect the stealth FDI attacks but, at the same time, there was a limitation of the model. As the dummy value does not change, by looking at the measurements continuously for some time, the attacker will come to know which one is the dummy value and the attacker will not change that value while doing an attack. In this way, the attack may be done such that it is unable to be detected. Figure 7 shows the results where the fixed dummy value model was bypassed by the stealth attack. It can be seen from the graph that the value of the residue in the case of attacks came out to be zero, as the attacker did not change the dummy values while launching the attack. Therefore, this limitation of the fixed dummy value was evaluated using the results.

**Figure 7.** Limitations of the fixed dummy value model for simple and stealth attacks.
