From Logan Horsford
Blind communication protects the identity of the message sender. This offers the perfect tool for villains in your campaigns.
Here are a few blind communication methods:
Dead letter drop
I make a chalk mark on a building that you go by on the way to work. That means, “you have a package at the usual place.” You go to the usual place (some place that people wouldn’t normally mess with) and grab your package.
One time pads
You have a pad and I have a pad. The pad tells what the code is. This method is pretty much unbreakable.
Note, this is not the letter replacement code that GMs are fond of using. I’m talking about real codes that are only used once. Then that page is destroyed and it is time for the next page.
Never would any self-respecting spy use a simple a=b code. If you are truly desperate, you might use a=c, b=d, but that is not how most codes work. Instead, use a group of letters.
Example: AAB DUU V93; or it could be written as AABDUUV93. It just depends on how they do it.
The decoder goes through with a pencil and scores between the letters just to make it easy. Each of the groupings is either a letter, word or phrase. AAB might mean “eliminate”. DUU is “the”. V93 is “PCs”.
See why cracking codes is pretty rough? Now, let’s move it up a step. Nulls are codes they toss in that according to the code book are “null”, which means ”ignore this – we put it in to mess with the enemy”. There could be a lot of null codes. This is why cryptography is a hard, involved field. None of this “e is the most common letter of the alphabet, let’s start there” stuff. Null might be their most common symbol.
Ever wonder why so many political assassins own a copy of Catcher in the Rye? That book was even brought up in the movie Conspiracy Theory. One of the thoughts is that is used for book codes.
For example, I might get you a code (either via mail, phone, dead letter drop, etc.). The code would read 113, 18. 42, 8. That means you go to page 113, word 18 and write down that word. Then page 42, word eight. And so on.
Unless you know what book we’re using, it is pretty much unbreakable. If you want to do some work, you could actually assemble a book code, then have the PCs find the list of numbers. Later, have them find the book and give the PCs a copy of the book. Ensure they suspect it’s a book code or have someone who knows about espionage tell them.
If you want to go for the bonus round, have them intercept that message before it gets to the bad guy. On one of the pages that they were referencing a message, they may notice a faint pencil dot under a different word. Then they can go through the book, find and unjumble the other words. That’s the bonus message.
Make sure both messages are very important though, or the players will lynch you because they just spent a couple hours in decryption.