[morsch]

I like the thought, but encryption is not enough to hide it from us, it'd require steganography.

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 something 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.

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 something. I'd have to discreetly tap my plate in order to mask it.

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.

[kabouseng]

Not so much steganography, but spread spectrum communications does achieve the effect you are looking for.

https://en.wikipedia.org/wiki/Spread_spectrum