Grandpré Cipher
Encode and decode homophonic coordinate pairs with an editable letter grid.
The Grandpré cipher replaces each plaintext letter with row and column coordinates from a letter grid. Repeated letters in the grid create homophones, so the same plaintext letter can encrypt to several different coordinate pairs.
How Encryption Works
To encrypt, the tool finds every grid cell that contains the next plaintext letter, then writes that cell's row label followed by its column label. For example, a letter in row 2 and column 5 becomes 25. In a 10-row or 10-column grid, label 0 means the 10th position.
If a letter appears more than once in the grid, every matching coordinate pair is valid. When random coordinate selection is enabled, Encrypt may choose a different valid pair each time, which makes repeated encryptions look different while still decrypting back to the same text with the same grid. Turn the checkbox off for repeatable first-match output.
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