Are the two characters who play major roles in some theoretical considerations. I guess you’ve met them before. Above all, they communicate a lot, so let them do so once again!
Alice is sending a Sudoku puzzle to Bob. He solves it, and finds where in the table of all Sudoku solutions the one Alice sent him, is. Then Bob converts this number into about 8 characters which make Alice’s message.
In fact, Alice sent him a whole book of Sudoku puzzles, and Bob, after solving them all, has quite a long message from Alice. He gives Alice his response in the form of chess problems. After solving one, the final positions of kings are a letter or two of Bob’s answer to Alice’s question. So Bob might send a whole book of chess problems to Alice, depending on how much he has to say to her.
Of course, they are sending those books as files and they both have an automated way for solving the puzzles. TBA have no clue what’s going on. Or do they? They could have.
Alice and Bob take part in this arms race of coding. Alice just stops signaling to Bob. Instead, she sends some angry comments to various bloggers worldwide. Now, if a blogger is irritated by her in the response, Bob knows that Alice meant 1. If a particular blogger is happy, Alice wanted to say 0 to Bob. But since this folk is highly unpredictable, Alice uses a lot of parity code to surely convey her message to Bob, this way.
Still, clever spooks can hear it as well.
It just so happens, that Alice lives near a famous writer. She has ways to influence his mood. Bob has to buy his book when it’s out, and decode the hidden message from Alice within. Somebody might still be listening in, though.
This was in fact John Nash’ premise in the movie Beautiful Mind. He wanted to intercept such hidden Soviet messages from newspapers. For the most part, he wasn’t able to. But his premise was correct, messages were there. And still are.
There was a secret, for all of us to see and get rich knowing it. But we mostly failed to spot it. It was encoded into the buying/selling habits of some large Wall Street players. The secret were some fixed dates when to buy and when to sell. A simple recipe. Forbes has a story on it.
There is a much bigger concern, however, than some minor human’s secrets, which can always be told over a beer. Has the intergalactic great war already begun? Or are we still in the preparation phase?
Only if we are alone can there be peace in the Universe. If we are not, some alien Alice and Bob are negotiating our fate right now. We should listen and decode everything that might be hidden in the incoming noise. This is much more urgent even than an asteroid defense system, because the damage caused by aliens could be much greater. I wish that there is nothing SETI could possibly intercept. But if there is, I hope SETI catches and decodes it as soon as possible.