A word to the writers of the world:

[conuly]

If the codewords are simply English words written backwards, or the hidden message in the letter involves reading the first letter from each word (or the first word from each sentence), and it takes your adult or "super intelligent" protagonists more than three minutes to figure this out, I'm gonna seriously question both your and their intelligence. I know you need to make a code that your characters and your readers can figure out, but by now I habitually read all unfamiliar words and names backwards, and also automatically read just the initials of any in-fiction letters. You can do better than this.