It's called a "One Time Pad"
The reason that it's theoretically uncrackable is because the key is exactly as long as the plaintext, perfectly random, and never used more than once. You cannot determine anything about the plaintext or the key, so therfore there's no way to crack it without already knowing the key.
I'm pretty sure that since your key isn't as long as the plaintext, that it's not "theoretically uncrackable."
Yep, I'm a crypto nerd too. Gonna try to solve this cipher~
If the keylength is less than the plaintext maybe you can do some cribdragging stuff.
More likely OP made up some BS mathematic cipher of his own that we can't possibly break without trying everything