we aren’t searching for messages intended for us. we’re trying to eavesdrop on normal communications... because anyone trying to find random civilizations would have to send signals in all directions in the hopes of finding someone.
this has always been my issue with SETI.
the amount of energy needed to send an interstellar message would be fairly significant in order to avoid dissipating over the vast distance. as such, in order to be efficient, anyone communicating like that would do so in a very tight beam. eavesdropping on such a beam would have a very low probability.
btw, snowden’s concept that the signal would be indistinguishable from background radiation doesn’t work as such energy is extremely weak and wouldn’t travel very far once it hit any kind of obstruction.
as such, encrypted or not, if we happened to be in line with the transmission... it’d be very powerful and would definitely stick out.
the trick is getting in line at the right moment
Even if you beamed a signal straight at another planet in another galaxy, the planet would not be ‘there’ when the signal got there.