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:
Step 1: Measure and cut
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.