Grandpré Cipher

The Grandpré Cipher is easy once you get the hang of it. We will be using a modified version of the Grandpré Cipher for ease of use. The Grandpré Cipher was written in Paris in 1905 by A. De Grandpré.

Introduction

To start, you will need a grid containing all of the numbers in the alphabet. It should look something like this:

You read the grid by looking at the letter at the top, then moving your finger down until you find the letter you are looking for, then moving your finger to the left to find the left number.

The coordinates will be the top number, then the left number.

The coordinates 0,0 represent a space in these challenges.

Encoding

To encode, you must have the grid and the plaintext (flag) that you want to encode.

Plaintext in this example:

Chilly Morning

Now, take the grid and find the coordinates of each letter and space in the plaintext.

(3,1) = C
(2,2) = H
And so on…

To make the encoded message look more encoded remove the parentheses and the commas.

It should look something like this:

31 22 32 62 62 15 00 13 33 63 23 32 23 12

And that is the encoded message!

Now here is how you decode it!

Decoding

Decoding is simply following the steps backwards.

Take the coordinates:

31 22 32 62 62 15 00 13 33 63 23 32 23 12

And plot them out starting with the top coordinate then the left coordinate using the grid:

You should get the original message:

CHILLY MORNING

That is the flag for this example!

Remember, spaces do matter, and remember to start with the top coordinate then the left coordinate!

Happy Decoding!