What is the difference between monoalphabetic and polyalphabetic ciphers

Polygraphic substitution is a cipher in which a uniform substitution is performed on blocks of letters. When the length of the block is specifically known, more precise terms are used: for instance, a cipher in which pairs of letters are substituted is bigraphic. Hill cipher is a polygraphic substitution cipher based on linear algebra. Each letter is represented by a number modulo Which of the following is a type of substitution cipher?

Explanation: In substitution cipher the plain text is replaced by cipher text according to a fixed rule. There are two types of substitution cipher — Mono alphabetic and Poly alphabetic cipher. The Vernam cipher is, in theory, a perfect cipher. Instead of a single key, each plaintext character is encrypted using its own key. The fact that each character of the message is encrypted using a different key prevents any useful information being revealed through a frequency analysis of the ciphertext.

Digitally signed messages may be anything representable as a bitstring: examples include electronic mail, contracts, or a message sent via some other cryptographic protocol. A monoalphabetic substitution is a cipher in which each occurrence of a plaintext symbol is replaced by a corresponding ciphertext symbol to generate ciphertext.

The key for such a cipher is a table of the correspondence or a function from which the correspondence is computed. Monoalphabetic cipher is a substitution cipher in which for a given key, the cipher alphabet for each plain alphabet is fixed throughout the encryption process.

The vigenere cipher is an algorithm of encrypting an alphabetic text that uses a series of interwoven caesar ciphers. It is an example of a polyalphabetic substitution cipher. It uses a Vigenere table or Vigenere square for encryption and decryption of the text. To decrypt, take the first letter of the ciphertext and the first letter of the key, and subtract their value letters have a value equal to their position in the alphabet starting from 0.

This is a simple cipher to decode. All you need to do is create a translation table with the letters of the alphabet written from A to Z across the top and reversed along the bottom.

Find the letter in your cipher text on the bottom row and look above it to see it decrypted. But, it is not. If we look closely at the encrypted word, we can see that the same letter "l" is repeated in the word "Lqgld". Also, the number of alphabets in the code is fixed and the same as the original word. These problems make it vulnerable to many types of attacks.

This type of encryption can be easily broken down using the "Brute Force Algorithm". This algorithm tries to decrypt the message by trying all the possible combinations. Thus, to prevent this type of attack, the words should be long enough, which is impossible for every word in a sentence.

We saw that the disadvantage of Monoalphabetic Cipher is that it keeps the frequency of alphabets similar to the original word. How can we overcome this? It can be done quickly by using a Polyalphabetic cipher.

Polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets. Viva Differences. What Is Monoalphabetic Cipher? In monoalphabetic Cipher, once a key is chosen, each alphabetic character of plaintext is mapped onto a unique alphabetic character of a ciphertext. In monoalphabetic Cipher, the relationship between a character in the plaintext and the characters in the ciphertext is one-to-one,. In Polyalphabetic Cipher, the relationship between a character in the plaintext and the characters in the ciphertext is one-to-many.