Grandpre

The Grandpré Cipher is a classical fractionated substitution cipher that uses a keyed alphabet arranged into a coordinate-based grid. Rather than assigning a single fixed substitution to each plaintext letter, the Grandpré Cipher represents letters through two-digit coordinates derived from their positions in the keyed grid. This gives the cipher characteristics of both a substitution system and a coordinate cipher.

The grid is constructed from a keyword, with repeated letters removed while preserving their first occurrence, followed by the remaining letters of the alphabet. In this implementation, the keyword GRANDPRE is used to establish the keyed alphabet. Each letter is then associated with a two-digit coordinate based on its position in the resulting Grandpré arrangement. The coordinates become the ciphertext, allowing the original letters to be recovered by reversing the same lookup process.

Grandpré Cipher: Encoding

To encode a message using the Grandpré Cipher, enter the plaintext and a keyword. The keyword is used to construct the keyed alphabet and establish the coordinate mapping. Each plaintext character is then replaced by its corresponding two-digit coordinate.

For example, using GRANDPRE as the keyword, the plaintext HELLO produces the following ciphertext:

Plaintext: HELLO
Keyword:   GRANDPRE

H → 21
E → 17
L → 25
L → 25
O → 27

Ciphertext:
21 17 25 25 27

The resulting ciphertext consists of two-digit coordinate values separated by spaces for readability. Each coordinate identifies the corresponding letter in the keyed Grandpré structure.

Grandpré Cipher: Decoding

Decoding reverses the coordinate substitution. The ciphertext is divided into two-digit coordinate values, and each coordinate is looked up in the same keyed structure generated from the original keyword. The corresponding letters are then concatenated to reconstruct the plaintext.

Ciphertext: 21 17 25 25 27
Keyword:    GRANDPRE

21 → H
17 → E
25 → L
25 → L
27 → O

Plaintext:
HELLO

The keyword must remain identical during decoding because it determines the coordinate mapping. Changing the keyword changes the underlying arrangement and causes the same ciphertext coordinates to resolve to different letters.

Grandpré Cipher: Notes

• Type: Coordinate-based substitution cipher
• Keyword: Used to construct the keyed Grandpré alphabet
• Output: Two-digit coordinate values
• Encoding: Each plaintext letter is replaced by its corresponding coordinate
• Decoding: Each coordinate is converted back into its corresponding letter
• Formatting: Coordinate values are separated by spaces for readability
• Security: Primarily useful for historical study, puzzles, and classical cryptography rather than modern secure communication

The Grandpré Cipher demonstrates how a keyed alphabet can be combined with numerical coordinates to conceal plaintext. Its coordinate representation also makes it useful for understanding the relationship between substitution systems and fractionation techniques. Like other classical ciphers, the Grandpré Cipher is not considered secure against modern cryptanalysis, but it remains valuable for studying the development of manual encryption systems.