Store the last file number created in a global counter. Increase it by any fixed or random amount and ... Eh Voila! I speak of very easy things. The things that work. If any Hacker discovers it, he will try to exploit it. They are reckless. They love to discover silly things. Create a file with an attractive name. Record all metadata. Never use it. If something changes, even if it is the last reading date, you are not alone.