Authentication can be based on three things: who you are (biometrics), what you have (keys), and what you know (passwords). Things dependent on who you are can be used against your will—lopping off your finger, for instance, to use your fingerprint, or taking a DNA swab. Things you have can be stolen. Things you know can be guessed or tortured out of you. By using more than one of these factors, of course, you can increase the security. Something that requires your fingerprint and your password is going to be more secure than something that requires either your fingerprint or your password. But there’s no perfect security.