Yes, as I understand it (I'm no expert though), we can't distinguish between the background noise of the universe and an encrypted alien signal. I believe the main problem has to do with all the various possible sources of background noise constantly moving and interfering with each other through the galaxy.
That being said, from a physics perspective perhaps there's some way to say "these bits of random noise are unusually powerful, and based on our multiple noise detectors over a large area we've managed to estimate the path these signals travelled and there's only a planet on the other end with nothing else appearing in in the way (over the course of time it took for the signal to arrive) to explain them as mere background noise."
That being said, from a physics perspective perhaps there's some way to say "these bits of random noise are unusually powerful, and based on our multiple noise detectors over a large area we've managed to estimate the path these signals travelled and there's only a planet on the other end with nothing else appearing in in the way (over the course of time it took for the signal to arrive) to explain them as mere background noise."