In which Hax promotes the use of combinatorics to justify sloth, and the reader learns about the dark side of forensics.
Fingerprints get everywhere, and they are usually not your friend unless you're doing something illegal or you have a warrant. They're no good as a means of electronic authentication, since the legitimate user's fingerprints inevitably end up elsewhere on and around the very device which uses them as a password1 (this means phones, laptops, etc). Even if the authentication isn't biometric, fingerprints will still betray a password or code if the input device being used to enter the code has that task as its only purpose, and this will be the focus of today's exercise.
It's the stuff of mystery/crime novels and Phoenix Wright; dusting the keypad for fingerprints to find the digits of the code2, as well as possibly gaining information about whether the killer was wearing gloves. This problem is fresh in my mind at the moment, having personally witnessed an electronic keypad which had not had its code changed for so long that the buttons themselves were worn away. Worse still, the code itself only used two unique digits, clearly discernable from a quick glance at the keypad. At this point, the only possible redemption would have been if the code was actually twenty digits long, but alas, it used the standard, meager four.