8 Difference Between Monoalphabetic Cipher And Polyalphabetic Cipher With Examples

What Is Monoalphabetic Cipher?

Monoalphabetic cipher is one where each character of a plain text is mapped to a fixed other character of cipher text. 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. The relationship between a character in the plain text and the characters in the cipher text is one-to-one.

Example : If a plain text has a character ‘a’ and any key then if it convert into other character say ‘t’ so wherever there is ‘a’ character in plain text it will be mapped to character ‘t’ ,Therefore it is called as monoalphabetic cipher.

It is a simple type of substitution cipher. Monoalphabetic ciphers are not that stronger as compared to polyalphabetic cipher.

Types of monoalphabetic cipher are

• Additive Cipher
• Caesar Cipher
• Multiplicative Cipher
• Affine Cipher

What You Need To Know About Monoalphabetic Cipher

• Monoalphabetic cipher is one where each symbol in plain text is mapped to a fixed symbol in cipher text.
• The relationship between a character in the plain text and the characters in the cipher text is one-to-one.
• Each alphabetic character of plain text is mapped onto a unique alphabetic character of a cipher text.
• 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.
• It includes additive, multiplicative, affine and monoalphabetic substitution cipher.
• It is a simple substitution cipher.
• 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.
• Monoalphabetic ciphers are not that strong as compared to polyalphabetic cipher.

What Is PolyAlphabetic Cipher?

A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. A polyalphabetic cipher uses a number of substitutions at different positions in the message, where a unit from the plaintext is mapped to one of several possibilities in the ciphertext and vice versa. The Vigenère cipher is probably the best-known example of a polyalphabetic cipher, though it is a simplified special case. The Enigma machine is more complex but is still fundamentally a polyalphabetic substitution cipher.

The Alberti cipher by Leon Battista Alberti around 1467 was an early polyalphabetic cipher. Alberti used a mixed alphabet to encrypt a message, but whenever he wanted to, he would switch to a different alphabet, indicating that he had done so by including an uppercase letter or a number in the cryptogram. For this encipherment Alberti used a decoder device, his cipher disk, which implemented a polyalphabetic substitution with mixed alphabets.

For example, when the disc is set as shown, we see that the plaintext letter “e” (on the outside ring) is encrypted to “Z” (on the inside ring).

What You Need To Know About Polyalphabetic Cipher

• Polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. 
• The relationship between a character in the plain text and the characters in the cipher text is one-to-many.
• Each alphabetic character of plain text can be mapped onto ‘m’ alphabetic characters of a cipher text.
• 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.
• It includes autokey, Playfair, Vigenere, Hill, one-time pad, rotor, and Enigma cipher. 
• It is multiple substitutions cipher.
• Polyalphabetic Cipher is described as substitution cipher in which plain text letters in different positions are enciphered using different cryptoalphabets.
• Polyalphabetic ciphers are much stronger.

Also Read: Difference Between Substitution Cipher And Transposition Cipher

The Difference Between Monoalphabetic Cipher and Polyalphabetic Cipher In Tabular Form

Also Read: Difference Between Block Cipher And Stream Cipher

Examples

Polyalphabetic Cipher

Plain Alphabet: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Cipher Alphabet #1: B D F H J L N P R T V Z A C E G I K M O Q S U W Y.

Cipher Alphabet #2: Z Y X W V U T S R Q P O N M L K J I H G F E D C B A.

Monoalphabetic Cipher

Example: An affine cipher E (x)= (ax+b)MOD26  is an example of a monoalphabetic substitution.

Summary

A monoalphabetic cipher is one where each symbol in the input (known as the ‘’plaintext’’ is mapped to a fixed symbol in the output (referred to ciphertext). Polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets.