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?"

You're looking at the problem from the wrong side

[inio]

There is no practical way to provide secure RF transmissions. However, there are many many ways to protect the data carried on those transmissions. What you're interested in is public-key cryptography as it applies to message signing. It works like ths:

1. Send block of data
2. compute fingerprint ("hash") of the transmitted data
3. encrypt that fingerprint with the transmitter's private key
4. transmit that

on the receiving end, you do this:

1. receive data
2. receive encrypted hash
3. compute hash of received data
4. decrypt received hash using sender's public key and compare to actual hash of data

This process is called signing, and is used in many many places. The use you've most likely encountered is to verify the identity of online stores. (the store's encryption certificate is signed by the trusted certificate authority that issued it. Your browser has all the authorities' public keys built into it, and uses it to check the signature against the certificate contents.)