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.
Encrypted Word: MYBX
Grandpré Key: 5564
MYBX → 13, 25, 2, 24 → (6, 5), (4, 5), (2, 6), (3, 4) → WOOL