substitution cipher is the method of encoding in which unit of plain text are replaced by cipher text. Monoalphabetic cipher uses a fixed message over entire message.A monoalphabetic cipher uses fixed substitution over the entire message.The monoalphabetic substitution takes a letter of an alphabet and substitutes it with another letter, this way a ciphertext is generated. The way of converting is fixed. A character of the plaintext will be replaced by the same ciphertext character, during the entire ciphertext. There is no additional key. The only way of security is to keep the substitution-table secret.

A popular example for the monoalphabetic substitution is the caesar cipher.