Grandpré
History
The Grandpré cipher is a numerical substitution cipher whose origin is unclear though it is believed to have been described first in La Cryptographic Pratique written by A. de Grandpré and published in Paris in 1905. The cipher was first described as using a square table of size 10x10 though 8x8 and 9x9 size squares can be used.
Description
Each row is filled with a single word whose length is the square size with the first column forming another word. The table must include all letters in the alphabet. Each row and column is numbered and these are used as coordinates to replace the letters in the plaintext. Grandpré's original description used the number 0 to indicate the 10th row and column therefore numbering the rows and columns 1 to 0. However this program uses the method described by the American Cryptogram Association of numbering the rows and columns 0 to 9 with the 1st row and column as 0.
1 2 3 4 5 6 7 8
1 A D J A C E N T
2 N A Z A R E N E
3 A G G R I E V E
4 R E Q U I T E D
5 C H A T E A U X
6 H A L F B A C K
7 I M P U N I T Y
8 C R O S S B O W
Encryption is performed by replacing each letter of the plaintext with the coordinates of the letter in the grid forming a two-digit number.
Example
Encipher the following Ingrid Bergman quote "Happiness is good health and a bad memory.” using the above keysquare gives:
h a p p i n e s s i s g o o d h e a l t h a n d a b a d m e m o r y
61 24 73 73 35 75 26 84 85 45 84 33 87 87 48 52 16 11 63 18 52 62 17 48 66 86 22 12 72 38 72 87 25 78
Solving
Solving method: Dictionary search.
The Grandpre solver can be used for 8x8, 9x9 or 10x10 grids and require a tip for the solving routine. On selecting the Solve button a dialog box below will appear to enable the tip and tip location to be set. On selecting OK, the program will search for a solution.
Tip – Enter the tip in the Tip field. Spaces and punctuation will be ignored. A long tip will reduce the number of dictionary words to test so will improve the searching speed.
Click on the Locate tip button to populate the Tip location list with possible locations for the tip.
Only show locations containing repeated pairs – Selecting this option will limit the number of possible tip locations to only include those where the ciphertext pair of numbers is repeated, possibly making it easier to locate the tip.
Tip location – Select the location of the tip from the list then press OK. When a tip location is selected the ciphertext and plaintext positions are shown below the Tip location confirming the location selected.
The tip location within the cipher will also be displayed in the Addition tip location textbox along with any other letters resulting from the tip. In this textbox additional plaintext can be entered to help improve the solution.
After clicking OK and if a word matching the pattern of a row is not found in the dictionary a warning will be given and all words in the dictionary will be tested. The warning may indicate the tip location an incorrect tip location or unusual word in the key grid.
If the Grandpre cipher is re-run the previous entry for the tip will be displayed.
If solving a cipher constructed from a keyword grid of English words, it is recommended to use a small dictionary list of keywords such as the EnglishSml word list as this reduces the number of keywords checked.
If the keywords are known they can be entered into the Enter Key field on the main window separating them by commas. The order of keywords is top to bottom rows.
