Method and apparatus for protecting multiple copies of computer software from unauthorized use

A hardware device containing a first permanent key and a second key is plugged in a specified port of a computer to establish a communication path between the hardware device and computer. The first and second keys are presented with an access sequence from the computer in the software which is desired to be controlled generating outputs which are applied to an algorithm decoder which provides a control key which is used to protect the software from unauthorized use. The control key is the resultant of the algorithm decoder and the first and second keys. One of the keys is supplied with the hardware under the control of the developer whereas the second key may be supplied with the hardware device but is not restricted to the hardware device and may be entered through a separate port of the computer. The algorithm decoder can be included in any part of the software, the computer hardware or any combination thereof. Many combinations of the first and second keys may be designed to produce the same control key when decoded. One of the keys may be altered without regard to security, but there are certain values of the second key for each given value of the first key which will operate the software. Providing a plurality of first and matching second keys which produce the plurality of different valid control keys allows the use of additional software that works in conjunction with the original distributed software acquired at a later date without the addition of additional hardware devices.