Grandpré cipher

The Grandpré cipher is a clever homophonic variant of the Polybius square, published in the early 20th century. Instead of a 5×5 grid holding the alphabet, it uses a 10×10 grid containing ten 10-letter keywords.

Because most alphabet letters appear many times across those ten words, each plain letter can be represented by several coordinate pairs — the cipher is homophonic by construction.

Principle

The grid

Pick ten 10-letter words that together cover the full alphabet (or nearly). Example:

         1 2 3 4 5 6 7 8 9 0
     1 : C R Y P T O G R A M
     2 : N E W H A M P S H R
     3 : P O R T L A N D I A
     4 : F O U N T A I N H D
     ...
     0 : C H A R L E S T O N

Each plain letter can sit in many cells. To encrypt C, find any C in the grid: say row 1 column 1 → 11. To encrypt E, pick among several possible positions.

Homophony

Frequent letters (E, A, R) naturally appear in several words → they have many codes. Rare letters (Z, W) appear less often → fewer codes but still representable.

The sender varies the codes per letter, achieving a flatter distribution than plain Polybius.

Example (simplified)

With a fictional grid, CIPHE might encode as:

C → 23    I → 17    P → 42    H → 38    E → 15  (or 45, or 32, or 07…)

Ciphertext: 23 17 42 38 15.

Variants

• Classic Polybius — 5×5 alphabetic ancestor, non-homophonic.
• Nihilist — Polybius + additive key.
• Generic homophonic — same idea without the 10×10 word constraint.

Weaknesses

• The ten keywords are the key. If the attacker knows or guesses them (culturally charged words: surnames, cities, mottoes), everything collapses.
• Homophony is only partial: rare letters have few codes and still stick out in frequency analysis.
• Common plain digrams (TH, HE, RE) still produce statistical patterns on the code pairs.

In CipherChronicle

Grandpré is a cultural cipher: keywords often reference places, mottoes, proper nouns. Its grids can lean into that theme — guessing the keyword becomes part of the puzzle itself.