Two-Factor Authentication! Can you broaden your acceptable answer to allow the key to be combined with something else? For example:
* The lock requires the physical key plus exactly one drop of blood placed on the tip. Perhaps it has to be a female goat's blood or something specific.
* The lock requires that the key be inserted into the lock at a particular time of day, or day of the year, or when the sun is shining (or not).
* The lock requires the key to be inserted and turned exactly 1/4 to the left before being removed.
* There are 32 indentations into which the key fits. You must insert it into a specific one. This one may depend on the day of the year or the hour of the day.
* Besides the key, there is a floor pressure plate beneath the lock which weighs you, and you must weigh exactly 195 pounds when you insert the key for the lock to open. (Which is heavy for the era, which means you plus some sandbags on your shoulders.)
That's how modern two-factor authentication works: one thing you know and one thing you have with you. So your password can be compromised, but that's not enough. 242
In like manner, your key could be duplicated, but it's the key plus how to use the key that matters.
The details of the extra information you must have would determine what happens if you are wrong. For the blood, having no blood or the wrong blood simply does nothing. The key appears worthless. Same for the time of day or the 1/4 turn.
For the 32 indentations, you'd have to do something more severe to avoid them trying each indentation in turn: things like poison gasses, flames, crushing rocks, etc. Or maybe the use of the wrong hole locks things up permanently, or forces you to know the "reset" hole that will reset the lock so you can then try a different hole. (In the extreme case, the key could be required to be inserted into the 32 holes in a prescribed sequence or it would not work -- like a combination lock with the addition of a key.) 242