Great job on this game! The sound design is fantastic and exactly what I would expect in a game like this. It really set the mood. I'm glad that when I added a letter it automatically typed in all of the same letter. It would be great to be able to use the keyboard to enter a letter instead of only the letters at the top. I'm not sure if the cipher is the same between levels - if so I think it could be good to let the player go back to the page showing the entire message so they can reference it going forward. That will let them build progress on what they already decrypted.
Again, fantastic game! I'm excited to see where you go with it.