Sounds a lot like your average car key. Unfortunately, it's not something that an "ancient civilization" would be able to do.

A regular car key can be forged though. If I get your key, I can cut one exactly like it. My question was about if you could create a key that cannot be copied.

It will not start with a key that is only shaped the same way. You would also need to have a transponder chip programed to give the right signal.

Absolutely cannot be copied, no matter what? Or, do you mean that it has some non-obvious 'keying' characteristic that is very hard to spot and/or very hard to reproduce.

@Catalyst I mean that if you were given the key, you wouldn't be able to reproduce it. There is one key, and one key only. If it was lost or destroyed, the door would be locked forever.

Maybe you could do something with quantum entanglement? Not sure if quantum physics fits well into a story which also has magic, though.

@Philipp Perhaps not in the setting I provided... It would work in a different setting though. Nice idea!

If the locking mechanism can be magical, but not the key itself: Make it so that the lock has an amorphous shape (or just a circular indent), but responds when a key of the correct shape is placed "in mid air" in front of it. With a sufficiently complex key, it's impossible to guess it's shape, so one must have access to the original to make a copy.

@Laurel It's totally possible to copy a modern car key. You need some specialized equipment to program the transponder chip, and blanks are unlikely to be handed out willy-nilly to anyone who asks, but I'm pretty sure that with sufficient perservence any present-day car key with transponder can be copied. (In the specific case of a car key, convincing an authorized workshop that you should be given a key is probably easier. AKA social engineering. It's surprisingly effective, by the way.)

Uncopyable is simply not possible if the lock is purely physical...but you can use deception to focus attention onto a fake lock or fake key, exposing intrusion attempts and wasting the intruder effort.

You could potentiality have a chip in the key that has a digital key inside the physical key, such that both are required for the lock. It wouldn't be uncopiable (if you can make it once, you can make it again), but digital codes can be made such that they would take millions of years to crack without the key. So it'd be a whole heck of a lot harder. And if people didn't know a chip was embedded in the physical key, it's unlikely they'd copy it. Just make it such that the chip doesn't emit any signals unless the physical key it fitted in the lock so it's harder to detect.

In addition, if you're going for material, you could have it such that the key is made of a magnetic material that is finely magnetically aligned, such that it wouldn't be easy to replicate the alignment. I believe that's possible. As an alternative, you could make the key out of a radioactive material, such that the lock with recognize the amount of energy hitting it. Ofc, the issue with this is that you'd actually need a new key each time, as eventually it'd decay. You could try it with a conductive material that retains energy, and releases it slowly over time, but you'd have to "charge" it

You propose that just by filling in a hole in the wall, some mechanism is activated that unlocks the vault? How do you propose to do that without either magic or advanced sensing technology?

What tech range are you talking about? "Cannot be copied" in many cases is not a matter of it being physically impossible, but rather that the attacker simply lacks the ability to do it with their resources. Also, how long does the attacker get to have the key, and how much are they willing to risk damaging the key?

Please state the technological level of the unlockers. For example, now we could manufacture a lock and key which would be completely impossible for pre-industrial people to copy. Make the key out of high-grade steel or some other modern alloy and make the mechanism require extreme precision and high forces applied to the key. A medieval smith, no matter how experienced, would not be able to copy it, as the materials necessary for it were not available.

Testing DNA, reflecting light, drop of blood, detecting radioactivity... all these are "magic" with respect to ancient cultures and thus do not answer the question. Example, how does an ancient test for radioactivity and use this to unlock a door?

Even modern cryptographic methods are not unbreakable. But they are designed to be immune to attack for so long the message is no longer valuable. For example, let's say someone In The Know knew how JFK really died, and encrypted it. In the early 60s, this would have been critical to know, to prevent another attack. But if the encryption was broken 1,000 years later, the exact same information would be nothing but a curiosity for academics. Thus the OP's key just has to resist forgers for as long as the protected room contains goodies.

Did you already figure out how to prevent them from blasting their way through the wall next to the door?

A key could be a small and tamper proof computer which has a private key which it won't reveal, and the lock tests the key by using secure key exchange (like in SSH)

I'm having some trouble with your premise. If the lock itself is magical, can't part of that magic simply be detecting that the key is the True Amulet of McGuffin and not simply a copy in the same shape?

@Sirama It knows by magic. And I'm not being flip. Knowing the true identity of something is definitely in the very standard domain of magical tropes.

Require it to be made of a substance whose production techniques have been lost to time.

I got tired of downvoting all the answers that boil down to "its really hard to copy the key".