> then no, you can't distinguish encrypted data from random noise.

But, would there not be a noticeable difference between random noise in space, and that same space PLUS someone broadcasting random noise? (And would it depend on whether or not they cared about being heard, even if they still wished their transmission to be encrypted).

It just seems to me that if there is a particular point that is continuously broadcasting randomness, would it not be noticeable? Or is what you are saying that that is essentially what stars are doing is broadcasting randomness, across a broad spectrum, and we can't distinguish between a star and an intelligent race....they are literally indistinguishable.