Page "Product key" Paragraph 4

Because of this, software publishers are increasingly turning to alternative methods of verifying that keys are both valid and uncompromised.

One method, product validation, assigns a product key based on a unique feature of the purchaser's computer hardware ( such as its MAC address ), which cannot be as easily duplicated since it depends on the user's hardware.

Other newer methods may involve requiring periodical validation of the CD key with an internet server ( for games with an online component, such as Battle. net, this is done whenever the user signs in ).

The validation can then be performed on the server side, preventing cracks tampering with it ( as they do on the client side ).

The server can also keep a blacklist of all CD keys known to be invalid or which have explicitly been banned, and deny them access.