I think this discussion is missing something. A broadcast would not be detectable because it says "Hello, we are liitle green men" (plaintext) instead of "sfghkjrtzdfbxcvnbzd,jtsdfò" (ciphertext). It would be detectable because it occurs on a frequency band which does not naturally occur. Anyone scanning the sky for signals would see *a signal* whether they could understand it or not. Suppose your country outlawed encryption - the presence of encrypted data on your computer would incriminate you (unless you can prove you're working on random number generators and need lots of random data). Just because the government could decipher your data wouldn't mean they didn't know it was there.
Yeah, I thought about that, that's why I said
or basically any technological equivalent of signal masking
... it's always going to be difficult to get out of an anthropocentric view of other species technologies but we can use laws of physics, as we currently understand them, to place bounds on the possible/impossible.
So imagine there is a civ. out there that wants to broadcast interstellar signals (don't know why), and wants to be able to mask that signal in some fashion. The only way to do this to avoid what you are talking about is to produce a radio broadcast signal that appears no different than noise at the RF level ... encryption, but not as we know it.
benjamindees:
SETI only looks for beacons eh ... didn't know that. What kind of civ. are they expecting to detect that would be willing to advertise itself I wonder? Someone as stupid as humans maybe?
Edit: Another thought is that human RF broadcast signals that have gone out there already, beginning in earnest around 1930ish (with strong carrier waves) will have travelled "only" 80 light years so far. Using this, you could then calculate the probability that a light cone 80 light-years in extent, centered on Earth, will have already encountered a habitable planet, given the expected density of habitable planets in our galactic neighbourhood. And you could go on further and do a kind of reverse Drake equation and make a guess at how long we've got before those signals encounter an advanced civ. that maybe listening, and then how long to get response from such a civ. .... or when they might show up here?