That depends on what you want to do. Do you want to authenticate the system of the user or the user him/herself? If its the latter, then how do you protect the

That depends on what you want to do. Do you want to authenticate the system of the user or the user him/herself? If its the latter, then how do you protect the link from other uses of this system? Encryption? And how are you encrypting something? Password? If you do it via a keyfile then how do you protect the keyfile? If you do it via a hardware token, how do you prevent other people from stealing and using it?