Wi-Fi has imposed itself as one of the key radio technology in portable devices such as smartphones, tablets, and other wearable devices. Because they periodically scan for nearby access points, Wi-Fi devices act as portable radio beacons emitting short messages, called probe requests. The latter contain a unique identifier: the MAC address of the Wi-Fi interface. It can be used to passively track individuals. Owners of such devices are thus exposed to passive tracking in the physical world.
MAC address randomization has been proposed as a way to prevent passive tracking, and is being progressively adopted by the industry. However, the MAC address is not the only piece of information that can be used for tracking. For instance, it has been recently shown that exploiting the content of frames as well as their timing could still lead to tracking despite MAC address randomization.