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

1234567
1MEXICAN
2JACKPOT
3BUILDER
4ZIPLOCK
5GYMNAST
6HALFWAY
7QUIVERS

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