How about through a wall? That would make security applications in an apartment a bit less attractive, and I’m sure there are others. I wonder if encryption would be possible without much extra power overhead.

Some sort of directional shielding? Isn't backscattering light directional? Someone knows more about backscattering can have a better answer.

Analog encryption might be possible if the sender and receiver measure some common optical signal (assuming optically local security is what you're after).