Substitution Cipher Puzzles

by Mike Shea on 24 November 2014

Coming up with interesting puzzles for our players to uncover and solve can be hard work for the busy dungeon master. While there's an infinite supply of potential puzzles, finding the right one and customizing it for your game can take a lot of time and much of that time may be wasted if the puzzle never comes into play.

Today we're going to look at one type of puzzle, a simple cryptographic system known as the substitution cipher that brings both simplicity and versatility to your DM's bag of tricks.

can you decode this dark message?

Understanding the Cipher

A substitution cypher is all about replacing a letter with another letter or a symbol. As a method for obfuscation, it's actually a terrible way to hide the text, but it can work nicely for a puzzle.

Ciphers can represent many different things in your game. It might be a strange language the PCs don't understand. It might be a clue written on a wall in blood. It may actually be secret messages passed between enemies, though smart enemies will know how weak it is and choose something like a one-time pad instead. It's this versatility that makes the substitution cipher such a useful puzzle format for your game.

Building a Cipher

To build a cipher, you need to have a message you want to hide. As mentioned, this might be a prophecy, a secret message, a riddle, or a word that unlocks a magic chest. It can be any useful bit of knowledge for your PCs. Secrets are perfect bits of story you can feed out in a recovered bit of cipher text.

The easiest way to build a cipher is to download a crazy fantasy font, write a message in your word processor of choice, and change the font to the new crazy font. Here are a couple of good sources for interesting symbolic fantasty fonts:

Printing out your messages on some fancy parchment resume paper (try the copper colored) gives you a nice handout that's easy to create.

Decoding a Cipher

As a GM, it's important that you understand how such puzzles are decoded so you can tune it for the appropriate difficulty. To decode a simple substitution cipher, you look over the text and through probability and Wheel of Fortune-like contextual guessing, you puzzle out the words.

For example, the first word to look for when decoding a substitution cipher is the word "the" because it's common and it contains two very common letters, T and E. When you have a good guess at the word "the" you can plug it back into the rest of the phrase and start to puzzle out the other words as well based on commonality.

For this reason, you'll likely want to have the word "the" somewhere in the puzzle phrase.

Here's a simple tutorial for solving simple substitution ciphers.

Tuning Difficulty

You can tune the difficulty of the cipher in a few ways. Overall, the more words available to the players, the easier it will be to solve the cipher. The more you use short common words, the easier it will be to guess the words. Proper names and names with a strange mix of odd letters will be harder to guess.

Since it's easier to solve a puzzle with more material, this means you might keep a puzzle interesting by giving out only small phrases piece by piece. As the PCs collect the pieces, they have more material to build a primer they can use to decode all the pieces.

If you want to make things hard you can remove the spaces so players can't figure out the boundaries of words. This will make it too hard if you don't give them any examples where word boundaries are clearly defined but it can work well if you want to give messages early on that they can't decode until later.

If players are having a hard time doing any sort of decoding, you might give them clues consisting of one or two letters based on skill checks they perform or bits of information from working with NPCs. This way they can recover letters that might not be replicated anywhere and are part of words they won't easily guess.

Not For Everyone

Some players may not enjoy the use of cipher puzzles and they're easy to overuse because they're so easy to put together in the first place. Cipher puzzles are also not accessible to people who have impared vision. Be ready to substitute your cipher puzzles for other comparable challenges such as skill checks, NPC interaction, or other methods of discovery. You can also narrate the deciphering of something like this through use of the character's background and appropriate skill checks. Like all things in this hobby, remain flexible and don't force anyone to do something they don't enjoy.

Flexible and Easy

Substitution ciphers have a lot of potential uses in your game. They can represent ancient text or secret messages. They can represent words of power for unlocking magical weapons or the true names of demons. As versatile as they are, designing substitution ciphers takes very little time. It's a great tool for the lazy dungeon master. Give it a try.

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