Substitution ciphers may be classified under four major types, each having its subdivisions and variations, and its intercombinations with other types:
1. Simple substitution (also called monoalphabetic substitution) makes use of only one cipher alphabet.
2. Multiple-alphabet substitution (also called double-key substitution, polyalphabetic substitution, etc.) makes use of several different cipher alphabets according to some agreed plan.
The term “multisubstitutional” is sometimes applied to the multiple-alphabet cipher, but more correctly refers to a certain form of the simple substitution cipher, in which the single alphabet is so designed as to provide optional substitutes for all or part of the letters.
3. Polygram substitution provides a scheme by means of which groups of letters are replaced integrally with other groups, which may be of letters or of numbers.
4. Fractional substitution, which requires a certain type of cipher alphabet, breaks up the substitutes for single letters, and subjects these fractions to further encipherment. More often than not, the result is a combination cipher, rather than a purely substitutional one.
CHAPTER IX
Simple Substitution — Fundamentals
Simple substitution is ordinarily defined as a cipher in which each letter of the alphabet has one fixed substitute, and each cryptogram-symbol represents one fixed original. When this cipher is used for puzzle purposes, as we find it in our newspapers and popular magazines, the substitutes (which are invariably letters of the alphabet) may be chosen at random, and the cryptograms must follow certain arbitrary rulings which are designed to make them “fair”: Word-divisions and punctuation must follow religiously those of the original text; a certain minimum of length must be provided; no letter may act as its own substitute; foreign words are not permissible; and so on. Aside from the observance of such rules, however, no holds are barred; the constructor of such a cryptogram, totally unconcerned with the meaning of his plaintext (except that it must have one), sometimes gives his chief attention to distorting the normal language characteristics in an effort to baffle the analyst, and often will carefully search his dictionary for words like yclept, crwth, syzygy, pterodactyl, ichthyomancy, not infrequently producing a plaintext which is almost as incomprehensible as its corresponding cryptogram. Our study here will be confined to the simple substitution cipher as applied to normal English text.