Introduction: Secret Postcard Decoder

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Ciphers are a secret or disguised way of writing. Ciphers and codes have been around a very long time and have seen endless variations and iterations in order to confuse those who aren't meant to read its contents. In recent times we use computers to come up with complex algorithms in order to encode our private transmissions, however I am partial to the old-school, analog method of encoding. This paper postcard decoder is a very basic, entry-level approach to understanding ciphers and is great fun for postcards.

This postcard decoder is placed over your postcard and can be used to encode a message, then decode the message after it's been sent. The cipher code can be sent either in a separate transmission or hidden somewhere on the package itself, allowing savvy codebreakers a clue on how to decipher your message. Though this cipher is not as sophisticated as some digital ones, it's a fun, low-tech and effective!

Here's what I used to make my postcard decoder:
  • pencil and rule (or computer printer)
  • pen
  • measuring tape
  • postcards
  • card stock

Step 1: Measure and Cut

This paper cipher works by filtering out (or omitting) a portion of the plaintext to reveal the secret message (ciphertext). It may sound complicated, but it's easy once you see how it works. I chose to make my cipher sophisticated and use all four alignments of my card (top/bottom facing up, and top/bottom facing down) ∴ four ciphers will be created from one sheet of cardstock.
You can create more or less ciphers depending on the complexity of the message you want to send.

Start by measuring the width and height of the space you want to write your message. Transfer these measurements to your cardstock, creating a boundary box for your cipher to fit. Then, divide the width by four and draw vertical lines to separate the box into four columns (as we are using four ciphers to decode this message, which corresponds with the four alignments of our cardstock). Next draw horizontal lines about the height of your writing down the boundary box to create a grid, you'll need an even number of rows. You should now have a grid of equally sized rectangles inside your boundary box.

Next, label the rectangles with numbers. Starting by labeling the four corner boxes with the number 1, then move towards the centre of the sheet labeling with numbers. Eventually the numbers will converge, at this point move on to the next column and continue numbering towards the centre. Continue until your grid is completely filled in.

Next it's time to cut out some rectangles and create the cipher. Using your hobby knife, carefully cut out one rectangle of each number shown on the grid. Soon you'll have revealed your super secret cipher.

I've included a template for a grid that can be printed on your home printer (A4 / 8.5"x11") that will fit the writable area of most standard post cards. Just "save as", print, number and cut out your cipher.

Step 2: Label Sides

You can now label your decoder. I put a 1 on the top of one side, then flipped it over and placed a 2 on the top of the other side. The decoder was rotated 180 degrees and the front and back were labeled again with 3 and 4. These numbers now represent the four ciphers you have on your cardstock.

Step 3: Start Your Message

Starting with the 1 cipher, place the cardstock over the portion of your postcard you want to write on, then fill out your message in the cut-outs. When you reach the end of the cardstock, move on to cipher 2, align the cipher with the postcard and continue filling in the cut-out boxes. Repeat this process for ciphers 3 and 4. 

After, you should have a block of plaintext of nonsense. your encryption is complete!

Step 4: Send and Decode

With the message encoded you're ready to send your super secret message.  If you're feeling very clever you can hide your encryption method somewhere on your postcard. For example, you could hide a list of numbers which correspond to the box cutouts in an inconspicuous place on the postcard itself (cipher 1 would be: 1,4,2,4,3,2,4,2,1,3,4,4,3,1,2,3), this would allow the receiver to make their own cipher cut out and decode your message. Or, you can just send the cipher in a separate email or post. 

This classic method of encoding has been around for ages and been remixed countless times with varying degrees. Did you make your own paper codec, maybe an improvement on my design?I want to see it! Share a picture of your version in the comments below.

Have fun!

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