Anything that is in the air medium is vulnerable to eavesdropping (a better term I think, using crptanalysis to break encryption) and not just UAVs or fighter jets that use it, but anything that is using the EM spectrum to communicate wirelessly if the intruder has the right instruments and access to it in the right location. Hacking is a more mordern term, and frankly it has been overloaded in popular culture. The type of 'hacking' you are referring is rendering the fighter aircraft useless/disabled/damaged by a 'few key strokes' - something like this can be possible through malware or virus. If an adversary is able to decode your encrypted data then it can compromise your missions plans and ruin it, but then how does that damage the actual system which is communicating? Its just like saying I was able to listen a secret coversation between two persons, so now I have the ability to stop their heart beats or paralyze them etc. (unless you have psychic powers, which may not be that far - see Men who stare at goats). All the intruder did is got the information, which is very useful and can be devastating if used properly (read turing machine during world war 2). Any decent fighter jets controls and management systems are designed in a closed circuit manner - with some critical systems built only in HW such as digital or analogue circuits or even programmed in hardware such as FPGAs. Avionics systems are designed using strict cert and design philosophy guidelines such as DO178B fo SW, DO254 for HW and ARP 4754 guidelines at the system level etc.; well those are must for civil aviation these daya, I think Military also follows these and their own specific MIL standards and guidelines. Just to give you an idea, a lot of SW apps for aviation are these days going into IMA platforms (Integrated Modular Avionics - a type of Virtual Machine) - overall it provides partitioning of different apps running on the platform and, builtin HW and SW features are to ensure that memory and execution of each application is completely partitioned; the platform OS ensures that even a bad loaded application cannot cause a failure for other applications. Here I mentioned about a loaded app not being able to cause a failure in the other apps - so think about how difficult (or even impossible) would it be to introduce a virus or malware in this type of systems (mechanisms in place to ensure data loading of appa is prohibited in air and is only possible on ground through specific maint tools, ports and interfaces).
Open network commercial devices such as cellphones, laptops etc. are open to attacks, as they are visible or acessible on the internet; they need this openess in order to make services like IE explorers, whatsapp, and many other thousands apps function using TCP/IP protocols. High visibility and access of commercial devices combined with less restrictions on SW tech and stadards used on these commercial devices, makes them vulnerable to attacks.
But remove the external interfaces and have a few computers, even stadard windows desktops, in a closed loop network Using ethernet cables, and no 'key strokes' can cause damage UNLESS the intruder has physical access to any of the machines or the network physicallyq. For that the intruder willl have to employ some cheeky ways to get access to the machines or network: such as breaking into the routers by disguising your self as fake maitenance personel or spying or manipulating an employee into doing something stupid like plugging a usb drive with a malware ... the possibilities are endless.
The battle between data encryption and its counter measures to decrypt or decode them is a never ending one, and new measures and then their counter measures will always be invented.
In my opinion Link 16 is still considered a very secure and safe way of communicating, does any knowledgeable member know if there have been instances of it being compromised by eavesdropping or 'hacking'.
Regards
