Secrets secrets (in this case) are so fun!

Create a shadow organization that deals in information. Your “doors” are members of the organization that guard entrances, and your “keys” are interesting secrets.

Requirements:

Any "door" can be opened with any "key"

Any member of this organization will accept any valuable secret you have.

Once opened the "door" stays open

The guard to an area recognizes your contribution, and always allows you through (after paying, of course).

It must be possible to purposefully construct a "door.”

Naturally, this organization can be hired out given the appropriate payment.

The "key" can only be used once.

Once the organization is aware of a secret, it is no longer a secret to them.

Desired but not required:

The "doors" can appear randomly (like vines growing over a normal door)

The motivations of this organizations can seem random to the player.

The "keys" should be rare but it should not be obvious where to find one (for example if only rich people could possess them, it would make no sense to search for a key in a poor man's house)

This should certainly be true about meaningful secrets.

• A sacrifice or votive offering. Burn a pinch of incense, and the portal opens.


• A rusty door and a can of WD-40. The door is sticky, and requires lubricant. Due to the environmental conditions, the effect won't last.


• There is a spell on the door, but certain folk remedies like cold iron horseshoes will dispell it for a time. The horseshoe must be nailed to the door, or perhaps buried under it.

The "doors" are limbs of a massive interdimensional eldritch being. A magic-user can bind them to our reality and coalesce their power into some sort of token, and those tokens can then be used to banish it back. For bonus points, this being could have some relevance to the game's overall plot (or it could be a Giant Space Flea From Nowhere bonus boss).

Requirements:

Any "door" can be opened with any "key"

The tokens are interchangeable; they all contain the same magic.

Once opened the "door" stays open

Unless someone binds it again, at least.

It must be possible to purposefully construct a "door".

With magic.

The "key" can only be used once.

Using a token discharges its magic and destroys it.

Creating a "door" may require the use of magic but opening a "door" with a "key" should not require magic abilities

Anyone can use the tokens to banish it, all it requires is physical contact.

"Doors" are not portals. You can not teleport to a different location by walking through a "door".

Banishing it just removes an obstacle, you're left with an ordinary opening.

Desired but not required:

The "doors" can appear randomly (like vines growing over a normal door)

Perhaps it can occasionally manifest itself into our reality without assistance? Though that would still require explaining where the tokens come from. Maybe the token manifests nearby in the form of a shiny object, which some critter takes away and hides.

the "keys" should be rare but it should not be obvious where to find one (for example if only rich people could possess them, it would make no sense to search for a key in a poor man's house)

Tokens would be wherever the magic-users decide to put them, so that isn't a problem unless they all have consistent habits (or see above).

Recently the local area has become infested with Barhools! Barhools are large bearlike creatures that spend most of their time sleeping. They're nearly indestructible and immune to all magic, but fortunately they're pretty harmless - except when they first wake up, whereapon they immediately seize and devour the nearest creature before wandering off.

Fortunately for you, Barhools are rather partial to Foostirs - large grublike creatures which can be found just about everywhere. They're in rather short supply at the moment due to Barhool predation, but I'm sure that you can find some hidden in out of the way places, right?

(More generally, any sort of bribe for a group of people or creatures works well for this sort of thing.)

A simple mechanical solution would work just like a vending machine. That vending machine could be configured to not accept any coin, but only a very specific and unique one, which is used as a key, and will disappear deep in the bowels of the machine, never to be retrieved by any simple means.

In case you wonder, vending machines have been around since ancient times, so it wouldn't be anachronistic.

Mechanic solution is that key is used up in opening the door. E.g. key is actually a shaft that pushes the latch to open position, and gets permanently wedged in the mechanism. But this does not explain random appearene of doors or keys.

A magic solution is spirits. Locks are hungry spirits that need "food", and keys are the spirit food, which appears randomly in the world. Lock spirits are lazy, or very slow, so rather than look for their food, they hold the door hostage until somebody brings them the food.

In computing we call such "keys" names like:

• One Time Password (OTP)


• Access tokens


• Secrets (such as those exchanged in HTTPS connections)


• Keys. Literally. Such as a license key, which activates a license only once.


• Lots of four letter words, when we need one but have forgot them or have no way to get them.

You could name them tokens, or secrets, or just keys anyway and by the time a player/reader has seen it in action once or twice they will have figured them out. The physical representation of the objects you use is just a detail; Much like portkeys in Harry Potter's world, you can use anything, restrained only by your choices as the world builder.

If you go with "key" and want to qualify it in such a way as to differentiate from a regular, everyday key, call it a "magical key", "mana key", "ether key" or something like that, and be done with it.

All the doors are locked, and you don't have a key. You don't even know how to pick locks! But what you do have is a snap gun that can open any lock by force.

Recently, though, there's been a rash of crime using snap guns, and the government is trying to cut down on it. So all snap guns available to those outside the police and military have fragile and temperamental parts. Yours is particularly bad, and you have to replace the lever or the spring between each use.

The key is a special liquid or powder that is poured or splashed onto the door, or poured into an opening of the door.

The door then dissolves or disappears, using the liquid/powder component in the process. Since the door has dissolved, it stays open.

The liquid could be a variety of special substances that would react with the door to cause its dissolution. The liquid could range from easily available, to incredibly difficult, and could force someone to search in a specific area (like a black dragons cave). For example

• There are many different types of acids that literally melt things. You could use scientific acids (hydrochloric, sulfuric, etc), or something a little more fantastical like the acid from a black dragon's breath weapon.


• If the doors were constructed of something like salt of sugar, then almost any liquid would cause them to dissolve.


• The liquid could contain microorganisms (bacteria for example) that eat away at the material of the door. Similar to the extraterrestrial bacteria in The Andromeda Strain.


• In a sci-fi setting, the liquid could be nanobots that eat the door or convert it to a different shape.


• Thermite makes a very good door opening agent. The components of the door could cause an exothermic reaction, igniting the thermite, and melting the door.


• Salt or sugar, or some powdery concoction, could cause the door to melt when applied. In Supernatural, we know that salt affects the demonic and supernatural, so if the door is made with demon flesh, it would surely melt when salt or saltwater (aka holy water) is applied to it.

The door, or at least the keyhole, is made with Litium-ion, or a similar substance that burns when exposed to air. Or maybe just ignited with the key.

They keys are specially shaped to fit the keyhole, puncture a barrier, and then the door bursts into flames. When the fire is done, the door is permanently opened.

The keys, being mass produced in the same factory or by the same keymaker, are all the same, and only have one purpose. Since the door immediately starts shooting flames out of the keyhole, the key is consumed in the fire.

A key could be an intricate coin. To manufacture such a coin requires expensive metals and precise tools. The design should be complex enough that you couldn't simply use a mold or die to smelt a coin into the correct shape.

The door has a coin operated locking mechanism. Incorrect coins fall through and come out the bottom. Only coins with the perfect pattern of bumps and holes will fall into the correct place, completing the mechanism and opening the lock.

For added effect, make the door look like a giant gumball machine. 😁

What you describe sounds like a kind of single-use skeleton key. This could be justified by having the "key" contain some active component that can open any lock, but expires when used.

In magical settings, this might be a Scroll/Wand of Unlocking; in scientific settings it could be some nanotech (or bio-engineered) "living lockpick" that permanently molds itself to the lock.