## On the Subject of the Grandpré CM

**This cipher uses 2 pages**

Read the keywords in this order: page 1 screen 1, page 1 screen 2, page 1 screen 3, page 1 screen 4, page 2 screen 1, page 2 screen 2, page 2 screen 3, page 2 screen 4.

Take the keywords and write them below each other, forming a A×B matrix. Write numbers 1 - A along the top side and 1 - B along the left side of this matrix. This will be referred to as the Grandpré Matrix.

Concatenate the numbers on screens A and B in that order. This will be referred to as the Grandpré Key.

For each letter of the encrypted word, do the following:

- Subtract the alphabetic position (A1Z26) of the Nth letter of the encrypted word by X until the number is between 1 - X. X is equal to the number of rows in the Grandpré Matrix.
- Use the resulting number as the row in the Grandpré Matrix.
- Use the Nth number of the Grandpré Key as the column in the Grandpré Matrix.
- The intersection of the row and column in the Grandpré Matrix becomes your Nth decrypted word.

#### Example

Encrypted Word: MYBX

Grandpré Key: 5564

1 | 2 | 3 | 4 | 5 | 6 | 7 | |

1 | M | E | X | I | C | A | N |

2 | J | A | C | K | P | O | T |

3 | B | U | I | L | D | E | R |

4 | Z | I | P | L | O | C | K |

5 | G | Y | M | N | A | S | T |

6 | H | A | L | F | W | A | Y |

7 | Q | U | I | V | E | R | S |

MYBX → 13, 25, 2, 24 → (6, 5), (4, 5), (2, 6), (3, 4) → WOOL