The proper analog is a letter v. a postcard. There is no reasonable expectation that the contents of a postcard will remain private -- the message is right ther

The proper analog is a letter v. a postcard. There is no reasonable expectation that the contents of a postcard will remain private -- the message is right there, and people will see it even accidentally. A typical plaintext packet is the same way. Stuffing your papers in a sealed envelope is analogous to passing your plaintext through an encryption algorithm. If you are "vibrating the air" in public, then yes, you should realize that there could be a microphone nearby that records what you're saying (and in fact, in modern practice, there is a microphone on pretty much every person you see, any of which could be recording at any time).