Jonathan Altar, on the Daily Beast, lays out the terms of the new debate over smartphone (and end-to-end or “full-disk”) encryption in an article “Manhattan DA: Smartphone encryption foiled 120 cases”.
With newer encryption, all the relevant information is kept on the device only, effectively throwing away the key.
The main argument from Silicon Valley is that a “back door” would ultimately be hackable by criminals, or possibly foreign powers or terrorists, exposing “ordinary people” to unpredictable catastrophe as well as criminals.
But the counter argument is that law enforcement, in other situations, can always “break in” if necessary. It’s asking for the digital equivalent of breaking down the door of a house when having a search warrant (authorized by Rand Paul).
So you could imagine the decryption keys being kept somewhere in a digital Cheyenne Mountain.