[bane]

Believe it or not, there's still a lot of utility in putting encrypted messages on physical things (pieces of paper). One time pads work well for this, but imagine if the recognition algorithm was altered in different ways effectively acting as a key s.t. you had to have a similarly altered recognized on your side to see it. Yeah it's symmetric crypto in that sense but you can physically hide all kinds of stuff or divide up the message between different couriers or do other stuff in ways that are a bit unlike digital crypto. The simple fact of a message being the physical object might be enough to confuse an eavesdropper.

Or the message itself could also be encrypted with a more secure system, but then physically presented in an open area so that somebody with a tuned recognizer can get the encrypted data to later decrypt digitally.