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.
* * *
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.