Sensing key strokes remotely

Side channel attacks involve using indirect methods to extract data.  In this case, the researchers used perturbations of Wi-Fi signals from hand movement to accurately detect keys being typed -- allowing one to remotely detect typing, e.g. to steal passwords.  Similar attacks exist such as the rhythm of key strokes from key-clicking sounds or screen refreshes from key strokes. A summary can be found here: https://www.schneier.com/