Sometimes mediocre encryption is better than strong encryption, and sometimes no encryption is better still.
The Wall Street Journal reported last week that Iraqi, and possibly Afghan, militants are using commercial software to eavesdrop on U.S. Predators, other unmanned aerial vehicles (UAVs) and even piloted planes. The systems weren't "hacked" — the insurgents can't control them — but because the downlink is unencrypted, they can watch the same video stream as ground coalition troops.
The naive reaction is to ridicule the military. Encryption is so easy that HDTVs do it — just a software routine and you're done — and the Pentagon has known about this flaw since Bosnia in the 1990s. But encrypting the data is the easiest part; key management is the hard part. Each UAV needs to share a key with the ground station. These keys have to be produced, guarded, transported, used and then destroyed. And the equipment, both the Predators and the ground terminals, needs to be classified and controlled, and all the users need security clearance.