You know by now that encryption schemes can either use substitution, transposition, or a combination of both.

<aside> đŸ’¡ Under the substitution category, you have learned about three schemes: the Atbash (and its extension), the Polybius cipher and the Caesar ciphers. In this document, we will discuss substitution ciphers in more details.

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Classification of Substitution Ciphers:

There are several ways to classify substitution ciphers. We will learn about three methods.

Method 1: Uniformity of Substitutions.

  1. Monoalphabetic Substitution: Every character in plaintext always maps to the same character in the ciphertext.
  2. Polyalphabetic Substitution: Every character in plaintext can map to different characters in the ciphertext.

An example of monoalphabetic substitutions is the Caesar cipher and Simple substitution ciphers, and an example of polyalphabetic substitution is the Alberti and Vigenère Ciphers.

Method 2: Substitution Table Construction.

  1. Algebraic Ciphers: use algebraic mathematical equations to generate substitution tables. Examples include Shift, Decimation, Affine and Hill ciphers.
  2. Passphrase Ciphers: use a passphrase to govern how the substitution table is constructed. Examples include Alberti, Passphrase Simple Substitution and Vigenère ciphers.

Method 3: Number of Substituted Characters.

  1. Monograph Substitution: Change one character for one character.
  2. Digraph Substitution: Change every two characters for two characters.
  3. Polygraph Substitution: Change a group of characters with a group of characters.

<aside> đŸ’¡ Most substitution ciphers are monograph. An example of digraph substitution scheme is the Wheatstone Cipher. The BAAAAA Cipher, which will be covered later in the course, is an example of a polygraph substitution.

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Common Features of Substitution Ciphers:

The main operation in substitution ciphers is replacement.