What is Polyalphabetic Substitution Cipher in Information Security?

A poly-alphabetic cipher is any cipher based on substitution, using several substitution alphabets. In polyalphabetic substitution ciphers, the plaintext letters are enciphered differently based upon their installation in the text. Rather than being a one-to-one correspondence, there is a one-to-many relationship between each letter and its substitutes.

For example, ‘a’ can be enciphered as ‘d’ in the starting of the text, but as ‘n’ at the middle. The polyalphabetic ciphers have the benefit of hiding the letter frequency of the basic language. Therefore attacker cannot use individual letter frequency static to divide the ciphertext.

The first Polyalphabetic cipher was the Alberti Cipher which was introduced by Leon Battista Alberti in the year 1467. It used a random alphabet to encrypt the plaintext, but at different points and it can change to a different mixed alphabet, denoting the change with an uppercase letter in the cipher text.

It can utilize this cipher, Alberti used a cipher disc to display how plaintext letters are associated to cipher text letters. In this cipher, each ciphertext character based on both the corresponding plaintext character and the position of the plaintext character in the message.

As the name polyalphabetic recommend this is achieved by using multiple keys rather than only one key. This implies that the key should be a stream of subkeys, in which each subkey depends somehow on the position of the plaintext character that needs subkey for encipherment.

In other words, it is required to have s key stream k = (K1, K2, K3 ...) in which Ki is used to encipher the ith character in the plaintext to make the ith character in the ciphertext. The best known and simplest of such algorithm is defined as Vigenere cipher.

Vigenere cipher is one of the simplest and popular algorithms in polyalphabetic cipher. In this approach, the alphabetic text is encrypted using a sequence of multiple Caesar ciphers based on the letters of a keyword.

The Caesar cipher restoring each letter in the plaintext with the letters standing constant position to the right in the alphabet. This shift is implemented modulo 26. For instance, in a Caesar cipher of shift 3, A can become D, B can become E and so on.

The Vigenère cipher includes several simple substitution ciphers in sequence with several shift values. In this cipher, the keyword is repeated just before it connects with the duration of the plaintext.

Encryption is implemented by going to the row in the table correlating to the key, and discover the column heading the corresponding letter of the plaintext character; the letter at the intersection of corresponding row and column of the Vigenere Square create the ciphertext character. The rest of the plaintext is encrypted in the similar method.

Updated on: 15-Mar-2022

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