Chemical Combinatorial Attacks on Keyboards

This paper presents a new attack on keyboards. The attack consists in depositing on each keyboard key a small ionic salt quantity (e.g. some NaCl on key 0, some KCl on key 1, LiCl on key 2, SrCl2 on key 3, BaCl2 on key 4, CaCl2 on key 5...). As the user enters his PIN, salts get mixed and leave the keyboard in a state that leaks secret information. Nicely enough, evaluating the entropy loss due to the chemical trace turns out to be a very interesting combinatorial exercise. Under the assumption that mass spectroscopic analysis can reveal with accuracy the mixture of chemical compounds generated by the user, we show that, for moderate-size decimal PINs, the attack would generally disclose the PIN. The attack may apply to door PIN codes, phone numbers dialed from a hotel rooms, computer keyboards or even ATMs. While we did not implement the chemical part of the attack, a number of mass spectrometry specialists confirmed to the authors its feasibility.