Or, more likely, civilizations more advanced than us have long since stopped splattering radio waves out into the cosmos willy-nilly, just like we stopped back in the '70s. I think it's funny that our hopes for detecting extraterrestrial intelligence are predicated on monitoring the hydrogen line with a small, underfunded, scrappy network of volunteers that have only scraped enough cash to do large-scale broadcasts on that same line a handful of times in history... yet we assume that these aliens are wholeheartedly focused on blasting out constant 10 PRINT HELLO WORLD on that same line just in case we're listening. That we don't consider alien intelligences to be every bit as disinterested in us as we are in them strikes me as narcissistic hubris of the first order.
That's an interesting theory for communication we aren't meant to hear, but it doesn't explain why they haven't tried to contact us directly, because if they were to (like in the cases we have sent out information) they would probably would not be encrypting it, and would instead be sending out very simple signals which we could actually understand.
This is a pretty cool concept. Also, why would they want to communicate anyways? If aliens are super-advanced they might see us like we do ants. They know we are here but not interesting enough to bother with. Or if they are slightly more advanced maybe they use encryption assuming that other civilizations are hostile. To visualize (crappy metaphor coming!), take yourself blindfolded into a forest at a random time and place in history. You could yell out for fellow humans. There is a chance a medieval peasant will come, but you are more likely get eaten by a T-Rex / predator. Regardless, the Fermi Paradox is hard because we only have a sample size of 1 with an extreme bias (since the sample is us). There could be considerations we haven't imagined yet.