Statement

Luca loves IT security, but he is a bit worried about the increasing number of attacks targeting smartphones. Recently, a number of smartphone companies have been discovered putting a sort of "keylogger" which secretly writes everything that is typed to a hidden file. Without much surprise, Luca found that even his smartphone does this thing and wants to understand the security implication of this absurd behaviour. He has gathered the log file from the phone in order to see whether he is able to recover the PIN (this would be a huge security issue!). Luca has stripped away all non-digit characters from the file, leaving a sequence of N digits. Under the assumption that the PIN consists of K digits and that the PIN is frequently typed, what is the sequence of four digits that is more likely to be Luca's PIN?