14.

Learn Encryption Techniques with BASIC and C++
(Publisher: Wordware Publishing, Inc.)
Author(s): Gil Held
ISBN: 1556225989
Publication Date: 10/01/98

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Chapter 2
Monoalphabetic Substitution Concepts

At first glance, the title of this chapter may evoke a vision of coverage of a medical disease. Thus, let’s clarify the scope and content of this chapter by discussing what we intend to cover.

The focus of this chapter is on the use of single letters or characters contained in one alphabet to replace plaintext letters and characters in a message. Because replacement letters and characters are restricted to those included in one alphabet, we use the term “monoalphabet.” In recognition of the fact that characters in the alphabet are substituted for plaintext characters, we refer to the encipherment process as a monoalphabetic substitution process.

In some books you may note the term “uniliteral substitution” or “monoalphabetic uniliteral substitution.” Here, the word “uniliteral” denotes the fact that the replacement process occurs on a character-by-character basis with one character from the replacement alphabet substituted for each character in the plaintext message.

Evolution

Monoalphabetic uniliteral substitution represents one of the earliest methods of cryptology, with the technique employed by Julius Caesar approximately 20 centuries ago to hide the contents of messages he sent by courier. Although Caesar’s method of encipherment was relatively elementary, his name has been associated with a class of displacement enciphering techniques and the term “Caesar cipher” is used to denote encipherment by displacement.

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According to historical records, Julius Caesar wrote to Cicero and other friends using a cipher system in which the plaintext letters in his message were replaced by letters three positions further down the alphabet. Thus, in the English language, INVADE would be enciphered as LQYDGH and ENGLAND would be enciphered as HQJODQG.

Augustus Caesar, the nephew of Julius and first emperor of Rome, modified his uncle’s technique. Augustus substituted for each plaintext character, reducing the displacement from three positions to one. Whether Augustus had difficulty in counting, arthritis which precluded counting by the use of his fingers, or just wanted to make deciphering easier is unknown. What is known is that any enciphering system in which plaintext characters are replaced by characters in an alphabet displaced from the plaintext alphabet is referred to as a Caesar cipher, while the displaced alphabet is called a Caesar alphabet.

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Although a Caesar cipher is a rather elementary enciphering technique, it provided a foundation for the development of more advanced monoalphabetic substitution techniques. Thus, an understanding of that class of ciphers provides you with the ability to recognize the advantages and disadvantages of other techniques whose foundation can be traced to the Caesar cipher.

Prior to examining the use of uniliteral substitution systems, let us define a few terms that will be used throughout this chapter. In doing so, let us restrict our alphabet to the uppercase letters A through Z and ignore lowercase letters, punctuation characters, and numerics for simplicity of illustration (numerics and punctuation characters can be spelled out—ZERO, COMMA, and so on). In fact, in many military systems the English alphabet is restricted to the 26 uppercase characters, so we can be considered as “borrowing” military techniques by initially restricting the alphabet we will use to uppercase letters. However, you can easily expand the examples presented in this chapter to incorporate the use of larger character sets if you so desire.

Alphabets

There are two basic types of alphabets you must consider in developing a monoalphabetic uniliteral substitution system—a plaintext alphabet and a ciphertext alphabet. The plaintext alphabet represents the alphabet from which the characters of a message are constructed, while the ciphertext alphabet represents the sequence of characters used to replace the plaintext characters during the uniliteral substitution process. Because the characters in the ciphertext alphabet are used to replace the characters in the plaintext alphabet, the ciphertext alphabet is also commonly known as a substitution alphabet.

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