You buy a program, paste a long string of letters into a small box, and watch a spinner. Ten seconds later the program unlocks, and you own nothing you can hold.

That box is doing more work than it appears to. It checks that the string looks valid, asks a server whether it has been used before, and writes a small record on your machine. This piece follows the trip and explains why the same key may stop working years later.

Simple analogy

A license key works like a numbered ticket at a coat check. The number is hard to invent, the attendant keeps a list, and the ticket only matters while the list agrees with it.

A forged number is easy to spot if the format includes a check digit, much like a bank note with a magnetic strip. The harder question is whether the same ticket has already been handed out twice.

Activation is the attendant writing your name beside the number. The ticket still exists, and the name is what makes it yours.

Core mechanism

Most keys are built so that typos are caught locally.…