Chapter XIII
Le Chiffre Indéchiffrable
During the world war I had been on duty in the intelligence department, and I had taken much interest in the science of cryptography, although not connected personally with the handling of cypher despatches. I could therefore explain to Betty that cypher systems fall under four general heads.
1. The giving to words, or groups of letters, a purely arbitrary significance.
2. The use of mechanical transformers in the shape of a screen or grid.
3. The substitution of numbers or other symbols for the original characters.
4. The transposition of letters according to a constant formula.
"Obviously," I began, "the example before our eyes—long lines of letters without breaks or marks of punctuation—does not come under the first heading. It contains no recognizable words, or phonetic groups, which might correspond in the code book to actual sentences. For example, in the ordinary commercial systems, the word Barbarian may mean: 'The wheat market is advancing.' But if I cable the word Civilisation I really intend to say: 'Australian wool crop is a failure.' The principal value of the elaborate code system is in the saving of cable tolls, a single word conveying the meaning of an entire sentence. It is necessary, of course, that all of the correspondents should possess individual copies of the code, and loss or theft of the book discloses the whole secret. Do you understand?"
Betty thought she did, and seemed so interested that I was emboldened to assume my best lecture manner.
"Under the second head we may consider the mechanical device known as the grid, grille, or screen.