I saw this talk at Blackhat on vulnerabilities in ADS-B, which scared the shit out of me. The paper is a good read http://media.blackhat.com/bh-us-12/Briefings/Costin/BH_US_12...
tl;dr: once a second every commercial plane transmits its location, heading, and speed over an unencrypted unauthenticated protocol, and other planes and air traffic control take it for truth.
a) is transmitting an AIS identifier that matches the paperwork already received
b) has been transmitting AIS track data that aligns with the paperwork already received since leaving the source port
Furthermore, ships have to allow a pilot on their vessel for pilotage into a port berth. Once they have pulled aside, the area surrounding the berth is, at least for most developed countries, a maritime security zone with barbed wire fences, security patrols and surveillance cameras.
The aviation sector has all of the above controls in place with even greater rigour. Encryption of track data, when factoring in the complexity of key exchange mechanisms and the expense of swapping out old hardware, would therefore have limited effect on either maritime or aviation security.
[1] https://en.wikipedia.org/wiki/Automatic_Identification_Syste...
And there are some _really_ obvious reasons why you wouldn't just go and do that so you can present the results at a conference. If the claims _are_ true, you'll need to get someone with the authority to grant you the privilege to demonstrate your attack against their aircraft - and so far as I can see, the most likely way to get to do that is probably to present a plausible and perhaps overblown paper at a well publicised conference.
https://en.wikipedia.org/wiki/Automatic_Packet_Reporting_Sys...
You could spoof Mode C altitude, of course. You could spoof a distance from the radar that's more than what your actual distance is by delaying your response to the interrogation. But that's about it. You can't spoof direction, and you can't pretend to be closer than you really are. I'm sure you could still make a minor nuisance of yourself, but it's not going to be a very big deal, I'd think.
I think you'd have to either be very close to the radar or airborne to pull any of this off, too, which means that you likely get to play games only once, then the large men in black suits will come ask you to please stop.