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How to Protect Radio Signals Over Short Distances?
anth_007 asks: "I'm a software developer seeking to delve into the hardware world. I am trying to create a prototype for a new product that would broadcast signals over a very short range (100s of meters). The important part is that only authorized users (ie. no hackers) would be able to broadcast signals. Well, a more accurate description would be that anyone could broadcast (it's just a radio transmitter after all), but receivers would be able to differentiate between authorized signals and those which are not. I've been looking to find information on Satellite Radio (XM, Sirius, etc) and how they protect their signals, but I haven't had much success. I realize that I'm talking about two different problems here (I want to stop unauthorized people from broadcasting, satellite radio is trying to stop unauthorized people from receiving), but I need to start somewhere. Any ideas? How do the big boys protect their signals? Is there any readily available technology out there that would allow me to accomplish this?"
Can't you just ask Q?
Public key cryptography solves this. Only transmitters get the private key, they sign the communications with it, and anybody with the public key can verify that the private key signed it.
if you want to make sure that the RF is only being sent and recieved by you, you have to set up a system where the frequency "hops" - changes very rapidly in what appears to an outsider as randomness. You do this by using cryptography to establish a connection and share a seed value for the variable that initiates the hopping sequence and a synchonization pulse for their internal clocks.
Then you need to build a transmitter and a reciever that will be able to change frequencies very very fast (hundreds or thousands of times per second), keep in sync with one another, and send packetized digital data between them.
If you are really paraniod, you encrypt the signals going to the transmitter, allow the transmitter to encrypt them again, the reverse the process at the receiver side.
If you are really, really paranoid, you iterate that cascading pattern over several layers of the network, multiplex the signal to the radio, and broadcast a solid stream of encrypted information, filling it the gaps between "real" information with garbage so that there isn't any variation in the RF between when you are saying something and when you aren't.
If you are more paraniod than that, you unplug everything, use very low tech methods executed by fanatically loyal zealots who would rather die horribly painful deaths than bring shame on their families, betray the cause, or endanger their eternal soul.
If you want to stop the paranoid, really paraniod, and really really paraniod folks from communicating, you build your own frequency hopping device, add a modulation hopping function, connect it to a really big tesla coil mounted on an electronically isolated & shielded truck, and broadcast megawatts of RF into the atmosphere while driving around in arbitrary paths.
If you want to stop the zealots, pull a Keiser Soze on them and kill them and everyone they've ever known, their dog, their neighbor, the guy who sold them coffee this morning, etc. Iterate until there aren't any more zealots.