Marcia Hoffman worries that Apple’s fingerprint-ID iPhone will make it difficult for owners to invoke the right against self-incrimination. Why? Because while people can’t be forced to share information such as PINs, they can be compelled to provide fingerprints:
Take this hypothetical example coined by the Supreme Court: If the police demand that you give them the key to a lockbox that happens to contain incriminating evidence, turning over the key wouldn’t be testimonial if it’s just a physical act that doesn’t reveal anything you know. However, if the police try to force you to divulge the combination to a wall safe, your response would reveal the contents of your mind — and so would implicate the Fifth Amendment. (If you’ve written down the combination on a piece of paper and the police demand that you give it to them, that may be a different story.)
The important feature about PINs and passwords is that they’re generally something we know (unless we forget them, of course). These memory-based authenticators are the type of fact that benefit from strong Fifth Amendment protection should the government try to make us turn them over against our will. Indeed, last year a federal appeals court held that a man could not be forced by the government to decrypt data. But if we move toward authentication systems based solely on physical tokens or biometrics — things we have or things we are, rather than things we remember — the government could demand that we produce them without implicating anything we know. Which would make it less likely that a valid privilege against self-incrimination would apply.