I have a three figure ID code that users enter. (Less than one thousand) this integer number must be passed to the function and a check code returned.

This code must be vertually unguessable and very unlikely to be entered by accident.

This is part of a security system for signing up new users.

Any ideas for a good formula? It's got to stop the user from useing the softwear with out phoneing head office for their unique ID.