yep, break it up into words, then letters. It's almost surely a simple substitution. You're off to a good start. The most common letter in English is "e", followed by "t" IIRC. Try to figure out small words. Also look for words that have two of the same letter in a row... there are a limited number of words that have double letters in them, and they're often the same... TT, OO, BB, DD, FF etc.