  This paper presents a new radiometric fingerprint that is revealed by
micro-signals in the channel state information (CSI) curves extracted from
commodity Wi-Fi devices. We refer to this new fingerprint as "micro-CSI". Our
experiments show that micro-CSI is likely to be caused by imperfections in the
radio-frequency circuitry and is present in Wi-Fi 4/5/6 network interface cards
(NICs). We conducted further experiments to determine the most effective CSI
collection configuration to stabilize micro-CSI. To extract micro-CSI from
varying CSI curves, we developed a signal space-based extraction algorithm that
effectively separates distortions caused by wireless channels and hardware
imperfections under line-of-sight (LoS) scenarios. Finally, we implemented a
micro-CSI-based device authentication algorithm that uses the k-Nearest
Neighbors (KNN) method to identify 11 COTS Wi-Fi NICs from the same
manufacturer in typical indoor environments. Our experimental results
demonstrate that the micro-CSI-based authentication algorithm can achieve an
average attack detection rate of over 99% with a false alarm rate of 0%.
