# Conservative Interference Injection to Minimize Wi-Fi Sensing Privacy Risks and Bandwidth Loss
## 6 Application of Defence Model Against Keystroke Attacks
### Results  
As the laptop begins to transmit ping packets, the captured CSI waveforms degrade, as demonstrated by Figures 13-17. We observe firstly from Figure 13 that the keypress events are clearly visible in the sampled CSI as time-varying ripples, in line with our expectations as discussed in Section 3 and in line with our earlier results in Section 4. In each Figure, a single keystroke is outlined by the dotted box, and the arrows point towards the other observed keystrokes. Then, in Figures 14-17, there is an incremental degradation in the quality of the CSI and the  
Figure 14: CSI time series with 50 Hz interference. Figure 12: Setup for interference experiment. Figure 13: CSI time series with no interference. clarity of the keystroke events. In Figures 14 and 15, we are still able to discern three clear keypress events; however, in Figures 16 and 17, this is no longer possible. This gradual degradation of the CSI time series further supports our idea that maximum interference rates are not necessary, and lower interference can be applied to sufficiently degrade sensing. To understand the impact of the degraded CSI on the sensing accuracy, we proceed with evaluating the sensing output of the keystroke recognition threat model using this dataset. In Figure 18, we present the sensing accuracy against the interference packet rate. The x-axis represents the packet rate from the interfering laptop. The green left y-axis then represents the accuracy of the keystroke recognition system as it operates in the presence of said interference, and the blue right y-axis represents the CSI data rate under each level of interference. For each interference rate, the blue point thus denotes the achieved CSI data rate (Hz) and the green diamond denotes the achieved sensing accuracy. There is a clear negative correlation between the interference frequency and the sensing accuracy. With an initial interference rate of 50 Hz, the sensing accuracy drops from 85% to 70%. Beyond this, the sensing accuracy crosses below 50% at an interference rate of 500 Hz, corresponding to a CSI data rate of 700 Hz. This corroborates our observations from  
Figure 16: CSI time series with 500 Hz interference. Figure 17: CSI time series with 1 kHz interference. Figure 15: CSI time series with 250 Hz interference. Figures 13-17, where the keystrokes could no longer be clearly identified with interference rates of 500 Hz. Beyond this, the accuracy appears to decay in a decreasing manner asymptotically. The poor performance of the sensing system with a CSI Data Rate of 700 Hz due to the interference injected at 500 Hz is an interesting result. We recall the earlier result in Figure 6, where a CSI sensing rate of 700 Hz was able to yield a keystroke detection rate of 100%. Once more we reiterate that detection refers to the ability to pick up the keystroke event in the CSI time series, _not_ the classification accuracy. Despite being able to pick up all the keystrokes, the sensing classification accuracy is below 50%. This result corroborates the ideas proposed in Section 3, where we postulated that the dependence of Wi-Fi sensing systems on frequency-based feature engineering techniques creates a susceptibility to changes in the CSI data rate. Even though all the keystrokes are picked up within CSI, the change in CSI sampling rate distorts the DWT features and hence reduces the accuracy of the sensing system. Overall, this experiment clearly shows that minimal interference can still be effectively used to defend against Wi-Fi sensing. To further visualise the improvement in privacy, we define the _Privacy Preserving Probability (PPP)_ metric as the percentage probability of safeguarding a key from being detected by Wi-Fi sensing. This is calculated as the complement of the true positive detection rate for each key. We plot the PPP under several rates of defensive interference in Figure 19. The x-axis represents the five keys that the Wi-Fi sensing system is attempting to detect.