I like the thought, but encryption is not enough to hide it from us, it'd require steganography.<p>If all communication itself on Earth were encrypted, how much of a visible signal would we still emit? You wouldn't be able to decypher the communication, but you'd still see that <i>something</i> is happening that's requires a structuring intelligence behind it. Unless they pay special attention to not only encrypting their communication but masking it as a natural phenomenon, steganographically.<p>If I shout an encoded message to you in a restaurant, the other people won't know that I told you to order the creme brulee, but they will know that I told you <i>something</i>. I'd have to discreetly tap my plate in order to mask it.<p>And even aside from that, you'd need to mask all other emissions, even those not designed to facilitate communication. On earth, you don't have to intercept and decrypt a rocket launch command, you can tell by the infrared (or whatever) emissions on your spy satellite.