I sell an Excel worksheet and need to protect it with a unique serial number that is provided separately to each customer upon purchase (i.e., each customer gets a unique number).
How can a procedure be implemented in VBA to validate serial numbers and to block randomly generated numbers by non-subscribers?
For customer convenience, the number should be around 6 characters long.

Ideally, that procedure will validate the serial number without having to index it against a huge database of valid numbers stored somewhere in the sheet.
Web connectivity, to tap into a central server for number validation, cannot be assumed.
In a nutshell, what is needed is something akin to serial numbers used to protect commercial software.

Any help is appreciated.
alseikhan