## 1 Introduction
Simply put, if the network activity is normal without suspicious CSI collection in public WiFi, the users could use their wireless network normally; otherwise, the users need to take some protective measures before inputting sensitive information. First, the user needs to detect which channel the target public AP is on, then switch the channel of a safe wireless transmitter (as described in Section 6.1) to a proper channel to interfere  
Figure 1: Attack scenarios. The target is inputting sensitive information in a public place, and the attacker uses NIC as the receiver in Scenario 1 and Scenario 4, while in Scenario 2 and Scenario 3, the attacker uses her/his laptop to receive CSI. with the target public AP. All this effort is to make the attacker's receiver lose a massive amount of key packets of CSI measurements, so that the attacker cannot recover the corresponding finger motions correctly. Our approach automatically detects when an attack is likely to happen by monitoring the network activities and runs the protection scheme if an attack is detected. Different from past works [12; 13], which consider channel interference as detrimental and seek reasonable channel assignment methods to avoid channel interference [14; 15; 16; 17], WiGuard exploits channel interference to prevent public WiFi from leaking users' privacy. To transform the above idea into a practical, feasible system, we need to solve the following challenges:  
(1) When users access the public WiFi, how do they know whether there exists an attack in the current public WiFi? From the CSI-based keystrokes recognition [5], we know that if there exists a CSI-based attack, the attacker's receiver needs to continuously ping the public AP at a high rate, such as 2500 packets per second, and the user can monitor the number of ICMP packets in the network to decide whether there exist attacks in public places. (2) How can one increase the packet loss rate of CSI channel so as to distort the CSI measurements of the corresponding finger motions obtained by attackers, while not affecting the normal network communication of public WiFi? After detecting the channel of the target public AP, the channel of a safe wireless transmitter is switched to interfere with the attacker's obtained CSI measurements. However, there are many adjacent channels that can be switched to interfere with the attacker. For example, when the target public AP works on Channel 6, the channel of the safe wireless transmitter can be switched to Channel 4 or Channel 5. Therefore, which channel should the safe wireless transmitter be switched to so that the channel interference between the safe wireless transmitter and the target public AP can achieve the maximum while ensuring that the users' normal network services on the target public AP are not affected by the channel interference? (3) How does the system choose a proper distance for the safe wireless transmitter, and from this distance, there will not exist adjacent channel interference between the safe wireless transmitter and other normal APs? There may exist many public APs in public places. If the distance between the safe wireless transmitter and the normal APs is short, then the channel switching of the safe wireless transmitter may interfere with the normal public APs. Thus, we propose a CSI-based localization method for public APs and based on the localization results, the system will give a proper position for the safe wireless transmitter if the user chooses to use the system to protect his/her privacy. Summary of the results: We build a prototype of WiGuard and evaluate it in different conditions. Our extensive experiments lead to the following findings:  
* In order to interfere with the attacker's received CSI measurements, the channel spacing between the safe wireless transmitter and the target public AP should be one, and it can interfere with the target public AP to the greatest extent.