CHAPTER VIII

Substitution Types

Substitution cipher presupposes the selection of a set of symbols which can represent the letters or words of messages. As to what these symbols may be, there is practically no limit; we meet substitution in our every-day life: the dots and dashes of the Morse alphabet, the pot-hooks of shorthand, the combinations of Braille, and so on; and we hear of its use in the sign-language of Indians and Gipsies, or in the drum-language of the African jungle. These, of course, are not cipher, yet in each case the plain language has been replaced with symbols. Considering the use of symbols for cipher purposes, there are doubtless many among us who played, as children, with the alphabet of the “Masonic” cipher, based upon a design like the one used for ticktacktoe. Lord Bacon’s alphabet has already been mentioned. The use of printers’ symbols, and similar characters, can be seen in the works of Edgar Allan Poe. Charles I of England is said to have used a cipher alphabet in which letters were represented by a series of dots, placed in certain positions with reference to the line of writing. An endless number of queer symbols is met with in fiction, such as the use of the little dancing men by Sir Arthur Conan Doyle. Cipher alphabets of the nature mentioned do not produce ciphers in any way different from those produced by substitution with letters and numbers; as a matter of fact, the decryptor who must deal with a cryptogram made up of arbitrary signs usually begins the work by making a substitution of his own, replacing each unfamiliar symbol with some one letter (or number). We will confine our discussion, then, to those characters which are transmissible by Morse.

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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.