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The things that thieves actually use to open doors is called a lockpick or picklock. These are actually quite tricky to use, and won't work on all locks, although they do work on many of them.

A master key is something that is made to open all the locks in one building or set of buildings. Generally, managers and janitors have these. But these (or at least, the ones in actual use) don't open all locks; just locks that have been made so as to be opened by this specific key.

A skeleton key is a key that has been (probably illicitly) filed down so that it will open all of some particular class of locks. I don't believe that skeleton keys work on most modern locks, but they do work on old-fashioned types of locks, and so in the past they would have been very useful to thieves.

If you are talking about some mythical key that opens all doors easily, I don't believe there is such a myth in the English-speaking world.
If such a myth actually existed, it might be called a

universal key,

and in fact, Googling universal key shows that it indeed has been used with this meaning

Skeleton key has been suggested by many other answers, but always as a secondary suggestion. That would be incorrect: it is the correct answer here. No other answer suggested, possibly excepting the archaic passe-partout, has this meaning.

A skeleton key is a key which has been specially made so that it will open many different locks.

^(Collins)

Many dictionaries, as well as Wikipedia, will tell you about the original design of a skeleton key: a key with all the ridges filed away so that it can open many locks. It’s actually where the name comes from, as stripping off the ridges was said to leave only the key’s “skeleton.” Anyway, that design only works on rather basic locks known as warded locks. Warded locks are not much in use anymore, largely because of the vulnerability to skeleton keys, therefore the original skeleton key design is not generally all that useful for theft.

On the other hand, the term skeleton key has outlived the original design, and is often used to describe a hypothetical key that opens any lock, rather than just those susceptible to the original skeleton design. For instance,

Many times refers to a magical key that will sucessfully unlock anything.

^{(Urban Dictionary—note that by the third definition, the term has become a euphemism for sex, as most Urban Dictionary definitions eventually end up doing)}

Used metaphorically, then, skeleton key means a key that will open any lock. Given the sophistication of modern locks, and the many different designs for them, this is generally treated as a magical ability, since no actual key could open all of those different locks. In my experience, the metaphorical usage of skeleton key is far more common than the specific historical design that it is named after.

Master Key

Master keys are legitimate, intentional tools used by property owners, not thieves’ tools. Typically, a set of locks must be produced together, along with a master key that opens all of them; you usually can’t take some random set of locks and make a master key for all of them. For example, if you own a building, you might have the locks for your doors made with a master key, and then give it to a janitor for cleaning. If a thief got their hands on a master key for a building or whatever that they wished to rob, it makes the theft very easy, of course, but ultimately this is no different from stealing the key or keys that open whatever you need. The master key is easier because it’s one key that opens many locks.

A master key is a key which will open all the locks in a set, even though each lock has its own different key.

^{(Collins Dictionary)}

Passkey

The word passkey can mean master key, or it can mean skeleton key. When used to mean skeleton key, it can have this meaning, but the fact that it can instead mean master key makes it less than useful here.

1. MASTER KEY


2. SKELETON KEY

^{(Meriam-Webster)}

Universal Key

Universal key is not a common term, and I can find no dictionary that includes it. A quick Google search suggests that by far the most common usage for this term is a “universal handcuff key,” a simple cylinder with a little “flag.” There may well be many handcuffs made to open with such a key, but it’s not going to open much else (and I suspect police actually use something else for their handcuffs anyway).

Anyway, not what you are looking for.

Lock picks and bump keys

On the other hand, lock picks and bump keys are definitely theives’ tools, which can be used to open up locks with some skill and effort. Many locks are very easy to pick; many locks are nearly, if not outright, impossible to pick. Either way, lock picking is a skill and these tools help; they don’t just open the lock automatically for you.

Importantly, even though a bump key is called a “key,” it doesn’t really work like one—you can’t just stick it in, turn it, and expect the lock to open. Instead, you have to “bump” the lock to get it to open. However, it’s worth noting that the bump key does bear some similarities to the original design of the skeleton key—like the skeleton key was filed smooth to foil cylinder locks, the bump key consists of several unusually-sharp points which can be used to bump pin-tumbler locks. Like the original skeleton key, the bump key is not useful for other lock designs. Unlike skeleton key, though, bump key has not reached the quasi-legendary status that skeleton key has, and it is not used for a hypothetical key-for-all-locks.

You are probably looking for passe-partout:

something that secures entry everywhere, esp a master key

Also skeleton key or passkey:

A skeleton key is a key which has been specially made so that it will open many different locks.

(Collins Dictionary)

You could call it a master key

: a key designed to open several different locks

• Similarly, hotel rooms around the world are vulnerable to a hack that lets an intruder mimic a hotel's master key and open any door.

Brian Barrett, WIRED, "Security News This Week: The Biggest DDoS For Hire Site Goes Down," 28 Apr. 2018

or a skeleton key

a key that will open several doors

Thieves may have used 'skeleton key' in Wigan break-ins

What about a master key

master key (noun)

1. a key that opens several locks, each of which also has its own key.

In terms of what modern thieves are likely to actually use, consider bump key.

Bump keys are specific to a lock type, effectively single use and they require considerable force to work. They are quite inelegant, although very effective and extremely fast to operate - they work in seconds.

Lock bumping is a lock picking technique for opening a pin tumbler lock using a specially crafted bump key, rapping key or a
999 key.

...

Lock bumping is a trend in burglaries, which can make it easy and fast
to break into homes without needing too much special equipment or
leaving any trace of forced entry. It works almost as well as actually
having a key, and a set of ten rapping keys can make the criminals
capable of opening 90% of common tumbler locks. However, criminals
might choose to avoid lock bumping when stealth is required, because
of the noisiness of the process. Almost all tumbler locks are
vulnerable to bumping.

(However, I think you have already got an equivalent expression to your thief's key in skeleton key, which is dated enough to have charm, is a word children would know, and also has a hint of the sinister about it.)

If we get away from the practicalities of whether such keys exist in reality and look at keys which do exist in fairy-tales and other fiction, we find a clear distinction between a master key which is made with good intent, and only available to the thief by theft, deception etc., and a skeleton key which is made with evil intent. Supernatural forces are often used, either to actually make the key or to find out how to. A third method is to take an ordinary key and cast a spell on it. Whatever method you use the principle is that a skeleton key is the most widely understood term for a universal key made with evil intent and found in stories.

Other answers have suggested this, but to make it clearer, no such key exists. A master key can open many locks, but not all. And even when there were skeleton keys in common use, there were also locks of other types, just as today there are tumbler locks with flat keys to lift pins, flat keys where the pins fit in holes on the flat side, and tubular keys (and several other types).

Perhaps, people might consider it a pass key.

pass key noun

1. a key to the door of a
   restricted area, given
   only to those who are
   officially allowed access.




2. a master key.