How to Make a Letter With a Hidden Puzzle / Code




Introduction: How to Make a Letter With a Hidden Puzzle / Code

I love going to "Escape from the Room" games with my friends.

For my birthday this year, I invited them to join me in another. After coordinating the event and our schedules, I decided I wanted to help establish the atmosphere and story ahead of time to get them hyped.

I figured that the best way to do that would be to give them some real life puzzles that are similar to the kinds one may find in an escape room (this way, my friends who haven't been in an escape room before could get a sense of it).

However, I also wanted to provide a reward for solving the puzzles, so I encrypted a download code for the video game "The Room."

So, I made this letter as though I was inviting them on a quest, while giving them both a taste of puzzle-solving and a party favor at the same time.

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Step 1: Materials

Most of these I picked up at Michaels and Staples. A lot of these can be altered because they were chosen just for aesthetic purposes.

  • Old-looking paper (I found some scrapbooking paper that's about the thickness of cardstock. This happened to be 12"x12", which ended up being the perfect size to write the content of the letter, and provide a square Sudoku board. The thickness also helped keep the highlighter ink from bleeding)
  • Vellum paper (it must be transparent. I thought silver was a cool color)
  • Yellow Highlighter (I would pick up several so you can experiment getting the appropriate level of diluteness. Sharpie brand works best I found)
  • Purple Sharpie
  • Blue Sharpie
  • Clear Scotch tape (I found the glassy, non-matte kind is best)
  • Sealing wax
  • Sealing wax stamp
  • Pen with thick ink. (I used the black ink Uni-ball vision elite. Its writing looked most like a fountain pen. I'd guess a 1.0mm thickness would work well for that effect)
  • Silver Metallic Gel Pen (Something to make it look a little mystical and different from the black ink)
  • Ruler
  • Scissors
  • Pencil
  • Computer
  • Steam Account (for the gift)
  • Sudoku puzzles and their solutions

Step 2: Preparation: Get the Steam Gift Code

Before you dive into creating the puzzle you have to establish the solution first. Here, the solution is a code to download the prize.

  1. Create a Steam account at
  2. Next, find the game you want to gift. I chose "The Room" (
  3. Add it to your cart.
  4. Select you want to "Purchase as a gift" (then sign in if you aren't already, when you're prompted)
  5. Select "email gift" and enter YOUR OWN email address. (It should look something like this:
  6. Continue through the purchase. It doesn't matter what you enter as the gift message. Complete your purchase.
  7. You should receive an email that looks something like the image attached.
  8. Scroll down until you see the link that says "To Receive Your Copy..." and copy that link.
  9. Go to
  10. Paste the link in the space prompted, and record the shortened URL. Capitalization matters. Make sure that there's a number in the URL--that will become relevant in the next step.

Step 3: Preparation: Finding the Right Sudoku

The way I wanted to conceal the Gift Code was by removing a number in the URL. The puzzle to conceal that number is a Sudoku.

So how do you find the right Sudoku?

  1. Choose a number in the URL.
  2. Look through the solved Sudokus until you find that number on one of the main diagonals. I chose one that was more centered, in the lower left corner of the center square. This was because when I drew the outline of the key (described in a later step) the outline is spread out across more corners.
  3. Then find the unsolved version of that Sudoku. The only requirement is that the unsolved Sudoku must NOT have the chosen number already given. If its number is filled in already, go back to step 2.

So what I did was I looked at the URL and noticed there was a 2. I found a 2 in a completed Sudoku (in that lower left corner of the center square) and made sure that it wasn't a "given" number in an unsolved Sudoku. I then marked it so I could return to it later.

Step 4: Preparation: Make the "Black Light" Lense

Next is to prepare a filter that will help turn a phone into a "black light" that will illuminate the invisible ink you'll make in the next step.

You'll need:

  • Blue Sharpie
  • Purple Sharpie
  • Clear/Glassy Scotch Tape

Follow the instructions in this Instructable:

Note: I recommend drawing it in the shape of an eye, or whatever best matches the riddle you compose in a later step.

Make sure that one layer is a little longer so you can fold the edge back on itself to make a little tab. This will allow the lense to be taped down and pulled up again.

Step 5: Preparation: Dilute the Highlighter to Make Invisible Ink

Now that you've made the "black light" lense, you need to make the invisible ink it will illuminate. This will be used to write the hidden Sudoku and gift code URL.

Use Yellow Highlighter Sharpies (I used several because I couldn't get the concentration right)

Follow the instructions in this Instructable:

Test your invisible ink on paper using the black light lense.

You want it to be light enough so it isn't very visible against the yellow-ish paper, but strong enough so it glows under the black light lense.

Step 6: Preparation: Make Your Messages

The longer letter was designed to establish the atmosphere of the Puzzle Room and provide motivation to solve the room. The second one (the riddle) is the first clue to solving the letter:

When the eye within
Eclipses the torch
Behind the electric library
In your pocket,
The path upon this paper
Shall be revealed
By the black light of truth.

To explain my thoughts in constructing this...

  • The "eye within" refers to the eye design of the black light lense, because that's where the lense will be taped.
  • "Eclipses" is to put one thing over another, and "the torch" is a term for a flashlight.
  • "Electric library in your pocket" is a smartphone, which usually have flashlight LED's on the back.
  • "Black light" references the blue-purple lense.

Step 7: Now, Let's Put It Together! Begin by Folding the Paper

Okay! Arts and crafts time!

  1. First, figure out if one side of your paper is better for writing (using the 1.0 black ink pen)
  2. Put that better-writing side FACE DOWN, and fold the 4 corners in so all together so they touch in the center. This is how the paper will be folded when you write the Sudoku.
  3. Open the paper, flip it over, (now the side that writes better should be face up) then fold it so two sides touch. This will allow your letter to be fit in an envelope, while protecting the puzzle and message.

Note: There's one more fold at the end, but I'd wait on that until then.

Step 8: Write the Message

This is the longer message (not the riddle).

You can write anywhere in the middle that doesn't cross over the diagonal lines (because if you do, it will overlap with the Sudoku puzzle).

I kept it in the middle column to make it seem like a normal letter, which made the ending cramped. A shorter message would have been better.

Step 9: Make the Invisible Sudoku Puzzle

Time to break out the highlighter / invisible ink pen you made earlier!

Your goal is to copy the Sudoku board you identified earlier.

  1. Fold the paper back so the 4 corners are touching
  2. Measure the length of one side. Divide it by 3 and make marks so you can make straight and even columns and Sudoku rows
  3. Use the invisible ink highlighter you made and draw the main divisions between the blocks. If you do it right, the corners of the blocks should align with the edges of the paper.
  4. You don't need to draw all of the smaller rows and columns; I thought that doing so would make the highlighter too visible, and the numbers too hard to read.
  5. Copy the numbers from your chosen Sudoku into their positions within the blocks. DOUBLE CHECK. YOU DON'T WANT TYPOS.

Note: If some numbers went between two edges, which I thought was fine. It makes the player know they're on the right path when two halves of a number come together.

Step 10: Finish the Invisible Writing

On the back, write your gift code URL. However, remember that number you chose to swap out? Now's the time to swap it out with a symbol. I chose the Null symbol from "The Room" because of its prominent role in the game. It doesn't mean anything outside of the game to my knowledge, so players may not read too much into it.

On the front, I wrote "SUDOKU" and "SEE BACK" so the players would know what to do to solve the puzzle, and also that there was more writing than just on this front page.

Step 11: Make the Transparent Key

The goal of the key will be to provide an outline for the blank Sudoku square that must be solved. It must be asymmetrical length-wise and width-wise so there's only one way to place it.

  1. Check to see which side of the vellum paper is easier to write on, this time with the silver gel pen.
  2. Outline the shape of the key with pencil on the vellum paper. (I chose to make the teeth of the key a "K" because that was the initial of the letter's recipient. The base must be big enough to surround the blank space of the Sudoku square.)
  3. Cut out the key.

Step 12: Outline Transparent Key

  1. Shine the black light so you can see the Sudoku board, and get heavy things to hold the board in place.
  2. Position the base of the key so it surrounds the chosen blank square (the square that, when solved, reveals the number you swapped out for the Null symbol on the back). I chose to make the key go over as many edges as possible, so the solution was evident to the player when all of the edges were lined up correctly
  3. Outline the key in the silver gel pen.

Step 13: Mark the Transparent Key

Now that you've outlined the key, you want to outline the chosen square so the player knows that they are solving for that square.

  1. Place the key back in its outline.
  2. Imagine where the square would be, and put dots in its corners using the gel pen (so, one is on the corner of the block, 2 are on the edge lines, and 1 last one completes the square of dots).
  3. Connect the dots with the gel pen.
  4. Next to the square, write " = [Null Symbol]" (or whatever symbol you chose).

This indicates that the symbol corresponds to what goes in the square. Because the key is transparent, it the symbol matches what's on the paper underneath that spot of the key.

Step 14: Fold It One More Time

Close the letter and fold it into thirds, so it can fit in the envelope. I chose to have some overlap so I could put the wax seal near the middle.

Step 15: Write the First Clue: the Riddle

Now, in the middle third you just folded, write the riddle you composed in your preparation. I chose silver to give it a mystical quality. Silver was more visible in gold in this case.

Step 16: Double Check It and Place Everything Inside

  • Place 1 or 2 pieces of tape inside the letter to create a smooth surface so the black light lense can be easily removed by the player.
  • Remove the black light lense from your phone and place it on the surface you just made.
  • Place the key inside the letter
  • Fold it up!

Step 17: Seal It!

I folded down the letter and held it in place with some weights while I sealed it.

I followed this video:

Note: It may be better to seal the envelope's edge rather than the letter's edge, because the wax makes it difficult to fold. My friends had to cut off the seal, which cut out part of the Sudoku board. But I was afraid that the seal would be broken in the mail.

An alternative would be to avoid sealing the edge and use a ribbon or string like so: This would mean the seal would not interfere with the Sudoku.

Step 18: Epilogue: the Solution (How Others Will Solve It)

YOU'RE DONE! You kind of worked backwards to create the letter, but here's what the players will do to solve it:

The walkthrough:

  1. Break and open the seal.
  2. Read the silver riddle and story
  3. Take the blue eye and place it over a small flashlight (like a phone flash light) which is the answer to the riddle.
  4. Lower the lights and shine the light (which is not a black light of sorts) all over.
  5. The highlighter (diluted) will turn bright.
  6. Fold along the diagonal creases so the shape of the key (that has the letters as the teeth) fits well, and black light it up.
  7. Note the numbers and the grid are like a Sudoku puzzle. Solve it.
  8. Put the key in its spot. The square in silver outlines one number. As the key indicates, that number is that half-hexagon symbol (the “Null” from “The Room”).
  9. Turn the page over and shine the black light to reveal a URL for a Substitute the Null symbol for the number learned in step 8, and go to the site.
  10. If all goes right, it should say you can redeem your gift of “The Room” on Steam.

I attached a document that I used to give hints to the players in my game. I posted it as a Google Doc and gave my friends access to it in case they got stuck.

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    2 Discussions


    3 years ago

    Badass man. Great work on this, as a live game builder, I know how long this takes to design and make. Thanks for the awesome idea and instructions


    4 years ago

    This is so fun!