Grille (cryptography) - Cardan Grille and Variations

Cardan Grille and Variations

The Cardan grille was invented as a method of secret writing. The word cryptography became the more familiar term for secret communications from the middle of the 17th century. Earlier, the word steganography was common. The other general term for secret writing was cypher - also spelt cipher. There is a modern distinction between cryptography and steganography

Sir Francis Bacon gave three fundamental conditions for ciphers. Paraphrased, these are:

1. a cipher method should not be difficult to use
2. it should not be possible for others to recover the plaintext (called 'reading the cipher')
3. in some cases, the presence of messages should not be suspected

It is difficult to fulfil all three conditions simultaneously. Condition 3 applies to steganography. Bacon meant that a cipher message should, in some cases, not appear to be a cipher at all. The original Cardan Grille met that aim.

Variations on the Cardano original, however, were not intended to fulfill condition 3 and generally failed to meet condition 2 as well. But, few if any ciphers have ever achieved this second condition, so the point is generally a cryptanalyst’s delight whenever the grille ciphers are used.

The attraction of a grille cipher for users lies in its ease of use (condition 1). In short, it's very simple.