HW-key SW protection

Hola a todos! Perhaps you have seen them. Hw-keys are used to protect sw from unauthorized copying. Usually, they come as a piece of HW that goes connected to one of the PC's parallel ports. When you are using sw that is protected, it sends a message to the hw key, and this responds with a signal or data block authorizing (or enabling) the computation to proceed. If you have a copy and do not have the key, you cannot go on. If you have the key then you can make as many copies of the sw as you need (as back up, for example) and be able to use all of them (albeit not at the same time, you have just one key!). Well, the question is: Does anyone of you know what kind of algorithm do this hw keys use? Saludos, Diego