I would recommend paying attention to the second of those two theories. Note that you're looking for a _hidden message_ in the letter, so you're going to need at least one more step besides just thinking "prime" or "eleven" or whatever.
I tried taking every prime letter, every prime word, and every eleventh letter and word, but couldn’t make sense of any of it.
Unless it’s not supposed to make sense, just to be used as a key? Though it does say hidden message, so I’m guessing it should make sense.
