**1. Monoalphabetic Cipher :**

A monoalphabetic cipher is any cipher in which the letters of the plain text are mapped to cipher text letters based on a single alphabetic key. Examples of monoalphabetic ciphers would include the Caesar-shift cipher, where each letter is shifted based on a numeric key, and the atbash cipher, where each letter is mapped to the letter symmetric to it about the center of the alphabet.

**2. Polyalphabetic Cipher :**

A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though it is a simplified special case.

**Difference Between Monoalphabetic Cipher and Polyalphabetic Cipher :**

SR.NO | Monoalphabetic Cipher | Polyalphabetic Cipher |
---|---|---|

1 | Monoalphabetic cipher is one where each symbol in plain text is mapped to a fixed symbol in cipher text. | Polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. |

2 | The relationship between a character in the plain text and the characters in the cipher text is one-to-one. | The relationship between a character in the plain text and the characters in the cipher text is one-to-many. |

3 | Each alphabetic character of plain text is mapped onto a unique alphabetic character of a cipher text. | Each alphabetic character of plain text can be mapped onto ‘m’ alphabetic characters of a cipher text. |

4 | A stream cipher is a monoalphabetic cipher if the value of key does not depend on the position of the plain text character in the plain text stream. | A stream cipher is a polyalphabetic cipher if the value of key does depend on the position of the plain text character in the plain text stream. |

5 | It includes additive, multiplicative, affine and monoalphabetic substitution cipher. | It includes autokey, Playfair, Vigenere, Hill, one-time pad, rotor, and Enigma cipher. |

6 | It is a simple substitution cipher. | It is multiple substitutions cipher. |

7 | Monoalphabetic Cipher is described as a substitution cipher in which the same fixed mappings from plain text to cipher letters across the entire text are used. | Polyalphabetic Cipher is described as substitution cipher in which plain text letters in different positions are enciphered using different cryptoalphabets. |

8 | Monoalphabetic ciphers are not that strong as compared to polyalphabetic cipher. | Polyalphabetic ciphers are much stronger. |