Even in an age of vanishing privacy, people using Apple's digital assistant Siri share a distinct concern. Recordings of their actual voices, asking questions that might be personal, travel over the Internet to a remote Apple server for processing. Then they remain stored there; Apple won't say for how long.

That voice recording, unlike most of the data produced by smart phones and other computers, is an actual biometric identifier. A voiceprint—if disclosed by accident, hack, or subpoena—can be linked to a specific person. And with the current boom in speech recognition apps, Apple isn't the only one amassing such data.