HW-Spy: Handwriting Inference by Tracing Pen-Tail Movements

While keyboard typing has been the most common way of inputting texts, handwriting still plays an important role in generating, inputting, or recording information like filling out essential/private forms. Considerable research has been done to identify and demonstrate the risk of keystroke-inference attacks. However, little has been done on handwriting inference despite its high risk of leaking sensitive information. To assess this under-explored risk of information leakage, we present a novel handwriting-inference attack, called HW-Spy, by tracing the victim's pen-tail movements when both the pen tip and the writing surface are outside the view of the attacker's camera, which usually happens when the victim is multitasking during an online meeting, when the victim's writing scene (in a public space) is recorded by a remote camera, or when the victim's writing behaviors are captured by the surveillance camera in a bank/dealership/realty office.