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Comment by jessaustin | Hacker News Reader

Isn't GPS supposed to be a military technology? Maybe there's something they're not telling us about how they really use GPS. However, I'm not reassured by the wartime durability of any tech that must be protected from all interference while at peace. Not that we actually have any real enemies anymore (rustic Muslim goat-and-flammable-underwear aficionados are more dangerous than the Axis powers were, really?), but if we did why would they obey the FCC?

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1) The GPS that we know and love is the Coarse/Acquisition (C/A) code[1] which is not encrypted and thus is relatively vulnerable to spoofing. Note that spoofing (as opposed to jamming) even a C/A signal is non-trivial, but equipment to do that is available[2].

2) The military uses the Precision (P) code which is encrypted and should be impossible to spoof unless you break the encryption. That is quite unlikely.

3) All radio signals are vulnerable to jamming. In order to jam, you must transmit, however... which means that your jammer is a beacon that the military can direction-find on[3]. If you piss off the military too much, they will turn off your jammer. Permanently.

[1] http://en.wikipedia.org/wiki/Global_Positioning_System#Satel...

[2] http://www.google.com/search?q=gps+simulator

[3] http://en.wikipedia.org/wiki/Wild_Weasel

@2) Wouldn't it be possible to reverse engineer a military receiver? Or is it a changing passcode that must be input manually?

http://en.wikipedia.org/wiki/Public-key_cryptography

Yeah, but that doesn't help if you have the device which decrypts the signal. Then you should be able to get the key (which would probably still be fairly difficult).

A receiver/decrypter only contains the public key, which can't be used to encrypt a new message. Perhaps you aren't familiar with asymmetric / public key cryptography?

And that is one reason the US military spends so much money and effort on anti-tamper[1] design and technology.

[1] http://www.at.dod.afrl.hpc.mil/

I don't know much about the cryptology they use, but I do know that the keys are time-limited and I'm pretty confident the encryption is strong.

* If you steal a key, it will stop working after a brief time (probably hours to days).

* Knowing the decryption key does not (necessarily) mean you can perform the encryption.

It's interesting to note that the drone that the iranians spoofed and forced to land relied on encrypted gps, however they were successful at what is assumed to be a gps spoofing attack. The likely scenario is that systems are designed to fail gracefully - if encrypted streams are unreadable (jamming) unencrypted data may be used.

"Assumed" is the operating word. A much simpler explanation is that the drone crashed due to a malfunction, the Iranians found the remains, glued and taped it back together (note that they did not show the underside), and then made up the spoofing story for propaganda purposes.

Here is a more skeptical article: http://www.theregister.co.uk/2011/12/21/spy_drone_hijack_gps...

There has been at least one valid spoofing demo: http://www.ae.utexas.edu/news/archive/2012/todd-humphreys-re...

"During the spoofing demonstration at White Sands, the research team took control of a hovering UAV from about a kilometer away. Next year, they plan to perform a similar demonstration on a moving UAV from 10 kilometers away."

That demo was a much simpler scenario than what the Iranians claimed to have done. The demo was a stationary UAV and it was most likely using the civilian (C/A code) signals rather than the (classified) encrypted P-code. They also knew exactly where the UAV was, and could likely see it.

They have always had other methods of determining location such as Inertial Navigation and terrain contour matching. The question is how well things are tested.

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