Substitution Cipher

Why
Substitution ciphers are important because they are examples of simple cryptosystems

What
Substitution ciphers are cryptosystems where the encoding function $$E_k(m)$$ merely substitutes a message for another in a predefined way. The decoding function $$D_k(c)$$ is merely the inverse of the encoding function.

Examples of Substitution Ciphers

 * Simple Substitution Cipher
 * Shift Cipher
 * Vigenere Cipher
 * Vernam Cipher