## Introduction: Fleissner Grille Cipher

This instructable is part of the June Instructables Build Night with Silhouette at the Taipei Hackerspace.

Cryptography is very popular at all ages, and yesteryear's obsolete crypto methods can be a lot of fun for learning. "Grille" is a kind of transposition cypher: a cardboard piece with holes in such a way, that when laid over a grid and rotated around, every cell is shown only once.

Thus one can encrypt a piece of text by:

• laying the grille over the empty grid
• filling in the text letter by letter going from left to right and top to bottom through the visible cells
• when got to the end, rotate the grille 90 degrees in one direction, fill out the empty cells again
• rotate 2 more times until all the cells are filled in

(this is illustrated on the image above)

Deciphering the text needs an exact copy of the grille, and knowing the start position and rotation direction.

The Fleissner Grille is a variation of grilles that was described by Baron Edouard Fleissner von Wostrowitz in the 1880s, and used in World War I. It's based on the 8x8 grid of the chess board.

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Silhouette Portrait is a small electronic cutting machine, mostly cutting patterns into paper. And it's perfectly suited for making a grille :)

## Step 1: Creating Your Grille

To start off, you can find a design tool for your grille, since it needs a bit of math behind the sceens (the cells cannot overlap as the mask is rotated in the cipher process!) For example can use the Fleissner Grille Cipher online (shown on the image above). Just pick a bunch of random squares until it's quite different from the starting layout. Try not to pick too big blocks next to each other, but in general almost anything works (I think).

Open up Silhouette Studio, set your paper size, eg. to A4, and zoom in a bit. You'll see a grid already that's there to help you position the designs to be cut. It makes the process of making the grille into a cut pattern super-easy! Just pick a node for your corner, pick a grille size (the number of squares on the grid that would make up a single grille cell), and start adding rectangles to lay out the grille you made with the tool in the previous step.

I've chosen 2x2 squares (that add up to 0.5", I think) to be the grille cell size.

Once you copied all the cells, count them! If your original field was 8x8 (64 cells, as standard for the Fleissner Grille), then you should have 1/4th of that, equal to 16 many cells (to account for the 4 rotations). Good to make sure you are creating your grille right!

Then send it to be cut by Silhouette! Make sure you cut at least 2! One for you, one for your partner you want to share secrets with (and maybe can do some spares that you keep safe!)

## Step 2: Write Your Secret

Take a piece of paper, lay the grille over it, and start filling out the cells as follows:

• go left to right, from top to bottom following the rows
• don't leave spaces between words, but you can choose whether to use punctuation
• after filling in all the cells, rotate the grille 90 degrees either clockwise or anti-clockwise, to cover the same area as before - if you've done it well, all the cells will be empty!
• fill in the cells, then rotate again, and repeat until all 4 directions you've filled in

You are done! If you want to prepare, plan ahead to have a message that is 64 characters long, or do multiple rounds (e.g. 2 boxes one below the other, 128 characters altogether).

If you give your secret text to your partner, they can take their grille, lay it over the box, and start reading the message off. For that to work, you have to tell them which orientation the grille starts off, and which way to turn (clockwise or counter clockwise)!

Enjoy sharing secrets!

For advanced use: try to break someone else's grille cipher, and call yourself Bond, James Bond! ;)