Years ago, when I was still in the Army and working at NSA, I came across an article with obvious use in game (at least if you're the type of DM who likes to make maps with cryptic notations or other similar props). Luckily, it was not only unclassified, but also available on their external website.
I don't know what made me think of it today, but here it is. Enjoy.
https://www.nsa.gov/news-features/declassified-documents/cryptologic-spectrum/assets/files/unsolved_puzzle.pdf
