How does your phone protect other people from pressing your volume buttons, or using your touchscreen? Security is pretty irrelevant for a HID.

it's obviously pretty different when you don't have to be touching the phone to interact with it

Not when it's a few inches away. It's less distance than voice interaction.

A lot of people are a few inches from my pocket every day. And I can easily imagine software listening for such sonars and sending back fake responses. I think debarshri has a valid point.

> A lot of people are a few inches from my pocket every day.

Really? And your phone is on? With the screen visible for them to see what they are interacting with. And as they are fumbling around in your personal space inches from your hand or pocket, no one notices.

Get real! If true you should be more worried about pickpockets than some random gestures.

According to their paper (which is written well IMO), their prototype uses a double swipe gesture to trigger or stop the detection. I suppose the idea could be extended to use something like a lock/unlock pattern, similar to the swipe patterns on Android lock screens.

I wonder about the other kind of the security. I doubt there's any serious protection against a rogue background-running application tracking your finger and gathering sensitive information, like keystrokes.

You know, check for what's the foreground process, notice it's a password manager or bank application (or even a lock screen would do, who knows, maybe user had reused the same PIN elsewhere), blast the speakers with ultrasound and wait for the typing-like movement patterns.