Alcaray and Gary, I agree that there is more than one way to encode a message so that it can't be decoded by anyone other than the intended recipient.

And that's my point. I think societies get better at protecting and using information as information takes on an increasing importance. In other words, as society evolves from hunter-gatherer prehistoric to industrial, society's expertise with information-handling only gets better.

An advanced civilization is likely to have very secure and efficient methods for communicating information.

When you heap on top of that the fact an advanced extraterrestrial civilization also possesses a culture completely alien to ours, the chances of our deciphering an alien signal drop precipitously. If you go further, and add in the scrambling effects of interstellar scintillation, the odds of SETI detecting an intelligible signal drop to almost zero.

I say 'almost zero' only because one must assume there's always the slim possibility we can both detect and decipher an alien broadcast. In my opinion, however, this possibility is so slender as to be indistinguishable from zero. I reckon this to be impossible, but also accept I could be wrong.