Let's say we do detect encrypted signals.

If we are talking radio signals that are being transmitted from point to point on an alien planet, from which we pick up the stray signals, then if it is encrypted we know something interesting:

-There is someone on the alien world who doesn't want someone else on that world to know what they're saying. A number of sociological conclusions can be inferred from this, including the possibility of conflict (military codes), or commerce (cable TV!).

If aliens are transmitting information in powerful signals across space, to be heard by others in that solar system or beyond, and that is encrypted, then we know something even more interesting:

-The aliens have space travel, or know of someone else who does

-They are not on good terms with others out in space