  Wireless devices resorting to event-triggered communications have been proved
to suffer critical privacy issues, due to the intrinsic leakage associated with
radio-frequency (RF) emissions. In this paper, we move the attack frontier
forward by proposing BrokenStrokes: an inexpensive, easy to implement,
efficient, and effective attack able to detect the typing of a pre-defined
keyword by only eavesdropping the communication channel used by the wireless
keyboard. BrokenStrokes proves itself to be a particularly dreadful attack: it
achieves its goal when the eavesdropping antenna is up to 15 meters from the
target keyboard, regardless of the encryption scheme, the communication
protocol, the presence of radio noise, and the presence of physical obstacles.
While we detail the attack in three current scenarios and discuss its striking
performance--its success probability exceeds 90% in normal operating
conditions--, we also provide some suggestions on how to mitigate it. The data
utilized in this paper have been released as open-source to allow
practitioners, industries, and academia to verify our claims and use them as a
basis for further developments.
