## Straddling Checkerboard Cipher

The Straddling Checkerboard Cipher is an awesome substitution cipher that turns plain text messages into numbers.

### Encrypting

The secret key for this cipher is a randomised grid of letters (see below).

To encrypt a message go through your plain text letter by letter. Find the letter in your grid:

- If the letter is in the first row then write down the column number (0-9)
- If the letter is in one of the other two rows then first write down the row number then the column number

Watch out for spaces! In the grid below, the space is in column 8 in the third row. The gaps on the first row, are gaps, not spaces!

When you have gone through your entire plain text message you will have a long row of numbers. This is your cipher text!

### Decrypting

To decrypt a message, go through the cipher text number by number. Look in the column with that number:

- If there is a letter in the first row in that column, write it down
- If the first row contains a gap then take another number from your cipher text. The first number tells you which row to look in, the second tells you which column. Find the letter and write it down.

When you have gone through all of the numbers you should have your plain text back again!

### Making Your Own Key

To create your own grid (and therefore your own secret key) draw out a grid with 10 columns and 3 rows. Number the columns 0-9.

Now pick 8 random letters and fill in the top row with them. Leave 2 random columns empty.

Label the second row with number of the first column you left empty. Label the third row with the number of the second empty column.

Now fill in the rest of the grid using the letters that you haven't picked for the top row. Use the letters A-Z as well as a space and a full stop.

### Example

Here's a grid that you can edit. When you change the letters in the grid the key will update and the example plaintext will be encrypted for you. Can you see how it all works?

0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | |

HELLO WORLD