What (if any) sort of pilot authentication systems do fighter jets have?
Could anyone with the required skills simply take the aircraft or is a key or authorization code required?
Military aircraft rarely have any sort of keys or any sort of authentication systems, they rely on physical security measures to prevent people from getting to the airplanes in the first place, like fences, dogs, cameras, guards, and of course guns.
If there were no physical barriers then you have 3 major barriers to being able to steal a fighter jet:
If you know how to start and fly the airplane there's nothing stopping you from doing it, provided you can solve the problems of prepping and getting in the airplane. In fact, it's been done when pilots defect with their airplanes, see the comments below for some great examples.
I was a pilot attached to Attack Squadron 86 flying A7-E's during the mid-1980's and here is a case where one of our jets was nearly stolen. It is not what you might guess, and as GdD pointed out above, maintenance support nabbed the pilot. To this day I am amazed at the guy's audacity. Don't know what happened to him after this encounter. I was the Officer of the Day when this event occurred.
Our squadron was going through qualifications and we were responsible for providing the aircraft for the referee/spotters on a particular flight. The referee showed up from down the field and checked himself in with maintenance, going through the Aircraft Discrepancy Book. He went out to the aircraft on the apron and hopped in after his preflight.
Shortly after he left maintenance another pilot came scrambling in all suited up and ready to go. He told the maintenance chief to give him a plane, and our chief obliged not knowing who the heck he was. After going through the discrepancies the pilot headed out to our aircraft and the Chief gave me a call.
I went down to the flight line and found the pilot getting ready to climb into the cockpit after doing a kick the tires, light the fires pre-flight. I was a bit surprised to see a guy I had come through Replacement Air Group (RAG) training with. I asked him who he was flying with, and he said he was part of the flight going out. "You are not listed as one of the referees Bob." He came back, "Oh, yeah, I'm not. I was just gonna catch a flight out in one of your birds."
This is like walking up to your neighbor's house, the neighbor you never talk to, grabbing the keys to his Ferrari and taking it out for a test spin. "Hmm, no, I don't think so. Go home Bob."
Absolutely, no key required. There was the case of a lance corporal who took a USMC A-4 Skyhawk for a joyride in 1986. See: Marine Mechanic Stole An A-4M-Skyhawk.
21-year-old Lance Corporal Howard A. Foote Jr. went for a joyride after learning he can no longer pursue a flying career in the military. He was only experienced in gliders.
A load of charges were brought against the Lance Corporal, including misappropriating the truck he used to get to the aircraft and the Skyhawk itself, as well as damaging an aircraft and disobeying regulations. He was also charged with hazarding a vessel, flying without proper training or approval and recklessly disregarding the plane’s mechanical condition at the time of flight.
Many of these charges were dropped, but Foote still could have faced nine years of hard labor, forfeiture of all pay, demotion to private and a dishonorable discharge.
Could someone steal a jet fighter? Sure. But as mentioned above there are a few obstacles to doing this.
First you're going to have to get to a jet fighter which means you're going to need access to the flight line of a secure military facility and have to pass a number of sentries and obstacles guarded by big mean men with guns who are authorized, if necessary, to shoot trespassers; you can't just walk off the street and onto a flight line without garnering attention and apprehension by the authorities.
If somehow you could get through the defenses of such an installation and get access to a jet, you're going to have to suit up in the proper life support equipment or there's a risk the jet itself could kill you during operations, vis a vis hypoxia, G induced loss of consciousness followed by CFIT, etc. It also has on-board equipment like pyrotechnic charges for canopy jettison and rocket powered ejection seats which can kill if mishandled by a neophyte. You're going to have to have a good understanding of this equipment, how to connect and use it in the jet and how to handle various emergencies involving it.
After that, you're going to have to make your way onto a line with jets which are properly maintained in a flight ready condition, pre-flighted, and fueled. The latter is rarely done unless the aircraft is scheduled for a flight. Failure to properly pre-flight the aircraft will probably result in mechanical failure and departure from controlled flight. An armed jet with ordnance on the wing/weapons bay stations or ammunition in the gun magazine is even rarer. Such jets do exist but are usually kept in even more sterile and secure places places e.g. HAS, etc.
You will also have to know how to operate the thing. As mentioned above, high speed flight in a high performance aircraft would probably saturate and overwhelm the average Joe of Jane on the street, leading to a catastrophic accident. Sometimes jets require external start equipment and additional personnel to power up, so this might not be a one man job. Ironically combat aircraft are fairly easy to start - if you know what you're doing and what you're looking for.
And starting up a jet will attract a lot of negative attention; again the big mean men with the M-16s aren't going to casually sit back and let you taxi the jet from the parking apron to a runway.
And even if you did get the thing airborne, where are you going to go? You only have a finite amount of fuel on board and that jet goes through it pretty quickly. You have, at best, about 1000 NM of range before you run out of gas and into the arms of the law on some very serious charges. You're also likely to be intercepted by armed jet fighters flown by trained and experienced fighter pilots who will either motivate you to return the aircraft or shoot you down if they or their commanders feel you pose an immediate threat.
As mentioned above, aircraft theft of that nature usually is an inside job such as an existing military officer and appropriately rated pilot stealing a jet in order to defect to another country. There have been a few unauthorized appropriations of a military jet in the past such as this USAF mechanic taking an F-86 on an inadvertent solo flight after a botched high speed taxi test. Fortunately most thieves have been dumber than a bag of rocks and totally ignorant to operations of an aircraft, so their attempts usually don't go far as this car thief's attempt to steal a life flight helicopter from a hospital went.
Anything can be stolen.
If by "steal" you mean get into a fighter and fly it off by yourself, it depends on which aircraft it is and various other factors.
Many older jets cannot be cold started by just the pilot. They require a special device called an APU to spin up the engines. You would need help to operate the APU. Newer jets can be cold started by the pilot, but there are still various obstacles. The first problem is that unless the jet is on standby, it will likely be unfueled or only be partially fueled. So, if you just picked a random hangared jet to steal, it might only have 20% of a tank or less of fuel, which would limit you to a range of a few hundred miles.
Of course, if you are stealing a jet, why not steal some fuel, too? Usually fueling a high-performance fighter is a two-man job because the hose and nozzle are wicked heavy, but I suppose if you don't mind getting a hernia, you might be able to do it by yourself.
Another problem will be getting into the plane. How are you going to get up there? Get a rollaway ladder you say? Ok, and now that you are in the cockpit, what are you going to do with the rollaway? Drive over it with the aircraft? Nice move top gun.
Once you are in there, you are good to go. Military jets do not have keys. (Even most civilian aircraft have pretty rudimentary locks. Old Cessnas have ignition locks that can be picked by an 8-year-old with a bobby pin. Also, the panel is just a sheet of plastic, so you can always just reach behind the panel and connect the wires.)
Why do you ask? Planning on sneaking on to the local air base?
I won't get into specifics, but newer aircraft are taking this into consideration and employing authentication/authorization features. This could be as simple as a key you turn like in your car or as complicated as your helmet performing a retinal/iris scan before allowing the engines to ignite, weapons to arm, etc.
The other benefit of these measures is that preferences can be saved and it could allow a pilot to take their preferences with them from plane to plane.
So, older planes are totally steal-able (Discovery or someone actually has an airplane repo show where they kind of do just that), but newer planes will be more secure and "thieves" (or repo men or whoever) will need various "hot wire" like techniques to get around the security.
Once, in 1997, an A-10 pilot 'stole' the aircraft he was flying, (broke off from his flight and his assigned training mission), and disappeared from ground radar. His wrecked aircraft was later found in the side of a mountain in Colorado. It was assumed that he committed suicide.
See this article for details.
In order to 'steal' a military jet, you would have to be proficient enough in that type of aircraft to safely take off and fly the aircraft. So, the pilots who would be capable of doing this are necessarily limited to a small number of people, most of whom are military trained pilots.