Chapter XIII

_Le Chiffre Indechiffrable_

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