It seems there’s still a large contingent of people out there who will believe you can use car keys to turn yourself into Wolverine.
The set-up is simple and there’s a certain elegance to it. The theory is this: you usually have car keys handy, so why not turn them into a weapon? While I applaud the idea of using common items as improvised weapons, putting your keys between your fingers and punching someone with them is going to have less than stellar results.

Both of these are extremely bad ideas. Unless you want to hurt yourself, in which case, go for it.
(Actually, when you get right down to it, both of these examples are bad ideas for self-defense, but for different reasons.)
When I was a kid and reading “authentic” “ninja” training manuals, these things would have looked great. In fact, the first time I heard about putting keys between my fingers and slashing at an opponent, it seemed like a great idea. I mean, why not? Keys are quite pointy in parts and slashing at an opponent seemed guaranteed to shred their face, at which point I could take their wallet.
The problem is it only looks good on paper. There are a couple problems with holding your keys this way and assuming you’ve reached the pinnacle of self-defense. First: your attacker is likely to be covered up and keys aren’t terribly effective against clothes. Truthfully, they’re not great against skin, either. Your dreams of going full Weapon X on a guy in a parking lot with nothing more than the keys in your hand are going to come crashing down quickly when you slash at him and wind up doing almost as much damage to your own hand as you do to his face.
If you do any damange to him at all.
The problem is, none of the keys are stable. Try it. Put your keys in your hands like in the above picture and wiggle them. Not too secure, are they? As soon as you make contact with something, those keys are going to press into the webbing between your fingers with an enormous amount of force. Possibly even enough to make you drop them.
Next thing you know, you’ve got a pissed off attacker, a damaged hand, and you’ve lost your keys.
Which leads to the next problem. In any self-defense situation your primary goal should be survival. The best way to make that happen is to get away and the best way to get away is to get in your car and make tracks. If your car is locked and your keys are tangled up in your hands, it’s going to take extra time to find the right key to unlock your car and skedaddle. It may seem easy when you’re in your living room, but remember when you’re attacked it’s a high-stress situation. In any high-stress situation, adrenaline is going to be pumping into your body to stimulate the fight or flight response. Adrenaline’s good stuff, don’t get me wrong, but fine motor skills disappear when it’s pumping. In other words, your brain turns to mush.
So, how about a better solution? Hold one key – the one to your car or your door – securely between your thumb and forefinger. Make sure you’ve got a tight grip and the tip isn’t sticking out too far – that cuts down on the force that’s coming back into your hand and gives you a better chance of holding onto your keys. Now, you’ve got a secure weapon to slash with and you’ve got your key ready to go, so when adrenaline hits you, you don’t have to think about which key is which.

Think tactically and strategically. The strategy should be escaping in one piece, tactics need to support that strategy, not hamstring it. Although, I would argue the “right” key is still held too far out to be stable.
Of course, you still have to deal with the pesky “getting a key past your attacker’s defenses” part, but if you do it right, you’ll have surprise on your side. Slash at the eyes or throat and get the heck out of there.
Just a quick note on the ring up there, too. Most people don’t know how to punch. It actually takes some time and a lot of practice to get good at punching something. If you slash at someone with that ring, it’s just going to turn on your finger and not do much to your attacker. If you punch someone with that ring on, you’re really going to be in trouble.
The thing about punching is the hand is very good for punching if you punch correctly, namely striking with the big two knuckles on your fist. That ring will put an enormous amount of pressure on the long bone of your middle finger. If you hit a hard enough target – anything on the head, for instance – you’ll break your own finger.
If you want some things to carry for self-defense that aren’t obtrusive, look for a kubotan or something similar. Heck, even a monkey’s fist made from paracord and a big ball bearing would work better.
Don’t believe me? Check these sites for more info:
Martial Arts on Stack Exchange
ACWA Combatives
Think Like A Black Belt