Hardware key also called a “dongle,” it is a copy protection device supplied with software that plugs into a computer port, often the parallel port on a PC. The software sends a code to that port, and the key responds by reading out its serial number, which verifies its presence to the program. The key hinders software duplication, because each copy of the program is tied to a unique number, which is difficult to obtain, and the key has to be programmed with that number.
The key also acts as a pass-through to the printer or other peripheral. Multiple hardware keys can be used, each plugged in one after the other.