In recent years, secret key generation method based on the physical layer of wireless channel has been widely studied. Since Channel State Information (CSI) is a fine-grained characterization of channel information, there are currently many studies using CSI for key generation. However, existing CSI extraction tools have the problem that the time interval between uplink and downlink interaction is uncertain, which may result in insufficient reciprocity of CSI. In this paper, we propose a channel measurement method based on the response mechanism of the IEEE 802.11 protocol, and develop a low-cost and universal CSI extraction tool using ESP32. We further design and implement a wireless key generation system based on the tool. To evaluate the performance of the proposed method, we have conducted extensive experiments in an indoor environment. Results show that our method can obtain highly reciprocal CSI in both static and mobile scenarios and can achieve a high key generation rate at around 1788.23 bits/s.