So if you have an alien civilization trying to listen for other civilizations, or our civilization trying to listen for aliens, there’s only one small period in the development of their society when all of their communication will be sent via the most primitive and most unprotected means.
So when we think about everything that we’re hearing through our satellites or everything that they’re hearing from our civilization (if there are indeed aliens out there), all of their communications are encrypted by default.
So what we are hearing, that’s actually an alien television show or, you know, a phone call … is indistinguishable to us from cosmic microwave background radiation. —Edward Snowden tells Neil Tyson how aliens may use encryption - Tech Insider
Well, that’s good point :-)