If you only need to guess 1 byte that means you have 256 options (from 0x00 to 0xFF) and except if your teacher is cruel and is using the bible as plain text you shall be able to try all of them in seconds.
I think the problem is that you are iterating from 0 to 99999999... and that is 390000 times more effort.
I think the problem is that you are iterating from 0 to 99999999... and that is 390000 times more effort.