A common technique used for software copy protection. It’s comprised of an exchange of passwords that contain hidden information that permits software to run and to be stored on a computer’s hard disk. Usually, when an application is first run, the user is presented with a unique challenge password. This password is submitted to the manufacturer (or a service company they employ) who then provides a response password that can be used to “unlock” the software so it will run on that machine. In some cases the same response can be used to unlock the software on other machines, but in many cases the challenge (and consequently the response) are unique to a given machine, or even to a particular instance of installation (in other words, if you wipe the software from the machine and install it again you may be presented with a different challenge). This system has some advantages over key disk/CD or dongle copy protection methods: you never need to deal with anything physical and the entire authorization process can be performed by e-mail or phone. However, there is one disadvantage, which is that your authorization is not easily transportable from one machine to another. Challenge/Response, while still in use, seems to losing popularity among software developers.