Cardan Grille

Cardan encryption uses another text (understandable or not) of which only a few letters are from the original text.

In practice, Cardan uses a mask, a grille in which some cells are opaque, others empty.

Example: Code DCODE with the mask XXXXXX_XXXXXX_XXX_X_XXXXX_XXXX where X is an opaque cell and _ an empty one.

Superimpose the mask cover, write the text in the empty boxes, remove this cover and fill the rest with random letters.

Example: DCODE is written in empty cells XXXXXXDXXXXXXCXXXOXDXXXXXEXXXX and it is possible to fill with letters like OZPCESDVIDGABCNTEOHDERXJXEMLUE

Cardan decryption requires knowledge of the grille. The decrypted text appears by placing the cover on the text.

Example:

The encrypted message is very long compared to original plain text.

Cardinal Richelieu encrypted his messages in the same way, but the ciphertext took the classical form of a common text. The secret message consisted of only a few of the letters of the message that were revealed with the cache. Any reference to Cardinal Richelieu can be a clue.

The decryption without grille is very difficult, one should try all the grids.

Using a probable word, it is possible to estimate the coverage mask and know the amount of letters to ignore.