How to Protect Radio Signals Over Short Distances? 93
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?"
Signatures (Score:1, Informative)
Buy an FCC license (Score:3, Informative)
If you are using some unlicensed spectrum like the 2.4GHz band, you have no recourse. Any unlicenced radio communication is required to not interfere with other people's use of the spectrum, and accept any interference without recourse.
Spread spectrum (Score:5, Informative)
With the right despreading key, you get signals.
Otherwise, you get noise.
...laura
Don't try to authenticate the radio waves. (Score:4, Informative)
It's far easier to secure the higher layers (in the OSI sense) -- the data you're sending over the radio waves.
Authenticating this is pretty trivial, from a crypto standpoint. The simplest approach is to encrypt your data with a shared key; more robust approaches might include signing unencrypted data with a private/public keypair, etc. This would also introduce some noise resistance, since you could tell if the data's been corrupted by checking the signature. (Though for non-malicious interference, you're probably better off using a redundancy check of some sort.)
Good luck!
well technically speaking... (Score:4, Informative)
the only way to actually attempt to control who transmits on what bandwidth would probably require some sort of licensing from the FCC. that way you have some sort of legal recourse if someone does decide to walk on your airwaves.
there are lots of techies out there who know how to build transmitters and recievers that can send and recieve at any frequency they want, so trying to put a technical barrier to the actual airwaves won't get you anywhere, as you mentioned.
an elegant solution would consist of some sort of stream verification, so that you can verify the sender of a signal via fingerprint, encryption, hidden messages, etc. Something that you control and that is difficult to reverse engineer would allow you to differentiate your signals from someone else's.
I think XM and Sirius encrypt their streams, or at least apply some sort of proprietary DRM to their streams to keep idle listeners from tuning in. They also use a fully digital stream, so encryption is pretty easy. And if I remember, they use an odd frequency that must be licensed from the FCC to use. The combination of those things seems to work great for them, a quick search on satellite radio hacking revealed almost nothing useful.
Despreading - yeah that's the ticket (Score:4, Informative)
Further, the transmission by it's very nature is somewhat covert. Now - if you're the NSA, perhaps you can figure out, but it's likely to be able to put together a system that is for all intents and purposes "private" using this type of transmission.
Now -FCC licenses, and spectruma are your REAL problem. The technical problems have solutions.
WCDMA (Score:2, Informative)
Re:Buy an FCC license (Score:3, Informative)
You can complain to the FCC, but they're unlikely to send somebody to investigate unless it's an extreme and repeated violation.
More likely is that you'll need to find the offender yourself, then give the FBI your evidence, and maybe they'll send them a nasty letter that they must respond to within 30 days ...
Hams play this sort of game all the time. The FCC is likely to take a commercial interest more seriously, but even so, they're not going to investigate somebody who transmits on your frequency once in a while.
Depends ... (Score:2, Informative)
I assume it's a digital signal you are sending, but if it's analog audio you can do some cool things by adding something that sounds like white noise to the transmitter, then subtracting it on the receiver. A "sync" signal will need to be sent to get them together (or transmit the "white noise" on another frequency if 2 tranmitters is feasable). If the transmitter isn't adding the "white noise", but you subtract it, then the audio will sound like white noise. (something like this was done for WWII radio encryption).
Weather it's analog audio or digital, it depends on who else will be broadcasting. If it's an attacker who may put lots of effort to reverse engineer your signal then you may need crypto. If you're just worried about a random transmitter or script-kiddie-type trying to screw with you, then you may just need to add some "sync" or "i'm legit" signals occationally (possibly on another frequency).
Spread spectrum-Exploration. (Score:2, Informative)
Some "exploratories" on signal processing.
There's some other good stuff there as well.
Re:Don't try to authenticate the radio waves. (Score:3, Informative)
Not really true.
It's far easier to secure the higher layers (in the OSI sense) -- the data you're sending over the radio waves.
Except that does nothing to protect you from DOS attacks.
This guy was asking for a system where no one else could transmit but authorized users. Obviously you can't actually prevent someone from building a transmitter, but you can design your modulation scheme such that without the proper "key" all your transmissions will be discarded as noise by a receiver.