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.
Very cool. I like ciphers :)
This is a very a sweet litle project! It makes me want to send post-cards and analog ciphers!
Oh how fun!! I am thinking of how I can put this to use in my classroom next year along with a game I'm working on to put artist, art styles, names of works and so on together like memory. I want to do it as a center, so I'm thinking I could perhaps use your instructable for the key to see if they have it correct. (That's if you don't mind I will completely give credit where it is due.) Well done!
Do it, I'd love to see your results!
Interesting! <br> <br>If CIA hires me, I am ready to work.

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Bio: I'm Mike and I make crazy things at Instructables HQ in San Francisco. Follow me and try a few of my projects for yourself!
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