Your question is off a bit. It isn’t exactly either of those.

Each combination of room, mic, keyboard, and letter is going to have a unique sound signature. But, you don’t need prep work.

If you can just record enough keystrokes, it’s possible to figure out what sounds go with what letters. This is the “substitution cipher” the other comment mentioned.

For example, the sound that comes every 2-10 letters or so but rarely twice is spacebar. The one that sometimes comes between two spaces is I. It’s obviously more advanced, but just to give you an idea.