Make it so complex that nobody wants to go through the incredible hassle of impersonating anyone, but so simple a child could use it. That’s the point of encryption, kind of. Sound increases the complexity of input but voices change and legs get problems. Fingerprints are very convenient in that they don’t really change much. Keys allow you to take a massive password with you wherever you go. I really like the idea of keys, and keys could easily be put on phones. Is there anything better? Ooh, how about a pair of augmented reality glasses that use a see through visual system that both tracks the eyes and, without a camera that made glass fail, keep track of an item like a ring based on multiple transmitters that each should send at different speeds depending on where the ring is in relation to the glasses in order to have a pointer system. The eyes could help guide the ring. Idea sounds cool. Let’s do that.