# Problem 79: Passcode derivation
A common security method used for online banking is to ask the user for
three random characters from a passcode. For example, if the passcode
was 531278, they may ask for the 2nd, 3rd, and 5th characters; the
expected reply would be: 317. The text file, keylog.txt, contains fifty
successful login attempts. Given that the three characters are always
asked for in order, analyse the file so as to determine the shortest
possible secret passcode of unknown length.