[mcguire]

Years ago, I attended a talk from a professor about a networking system using the Lego blocks metaphor. In his examples, he had a block labelled "encryption" and another labelled "compression" and talked about how you could arbitrarily compose the blocks in the system.

To my eternal regret, I didn't call him on the fact that those two blocks are not arbitrarily composable.

Both encryption and compression make the resulting data look random (and you can't compress encrypted data), and compression seems to be a much more likely candidate.

P.S. And spread-spectrum transmissions, as someone else mentioned.