Also called a "dongle," it is a very effective software copy protection device that plugs into a serial, parallel or USB port on the PC. The software sends a code to the key, which responds by reading out a serial number. The key ensures that the software runs on only one machine because each copy of the program is tied to a unique number in the key.

The key also acts as a pass-through to the printer or other peripheral device. Multiple hardware keys can be used, each plugging into the other. See DMCA and PC Card dongle.

The Hardware Key

Hardware keys provide a pass-through so that other devices can be plugged into the same port. Multiple keys can be plugged into each other.

Internal View

This internal picture of the Sentinel Software Protection Key shows the actual electronics. (Image courtesy of Rainbow Technologies, later merged into SafeNet, Inc., www.safenet-inc.com)