It can...

use a key to encrypt Unicode characters (mostly anything you can find on your keyboard) and returns a string of numbers that is useless without the key.

or,

use the key (has to be the same as the one used to encrypt it) to decrypt the numbers and return the original string

That's about it, have fun

Why not

I challenge anyone who reads this to try to decrypt this:

12960​15660​15228​16848​17820​17712​4320​11016​17820​17712​17712​15660​17496​

without the key