Cryptography - Polyalphabetic Cipher

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Polyalphabetic ciphers use multiple alphabets to substitute letters, determining the encryption based on the letter's position in the text. Unlike basic ciphers, polyalphabetic ciphers create a one-to-many relationship, where each letter has several possible encryptions. The Alberti Cipher, invented in 1467, is the earliest known polyalphabetic cipher. It utilizes a series of random alphabets for encryption, signified by uppercase letters in the ciphertext that indicate switches between alphabets.

It can use this cipher, and Alberti used a cipher disc to show how plaintext letters are connected with ciphertext letters. In this cipher, each ciphertext character is determined by both the plaintext character and its place in the message.

As the name suggests, polyalphabetic means using numerous keys rather than just one. This means that the key should be a stream of subkeys, with each subkey depending on the position of the plaintext character that requires the subkey for encipherment.

How it works?

It is required to have a key stream k = (K1, K2, K3,. . .) in which Ki is used to encipher the ith character in the plaintext and make the ith character in the encrypted text. Vigenere cipher is the most well-known and simple among these algorithms.

The Vigenere cipher is one of the most simple and widely used polyalphabetic ciphers. In this method, the alphabetic text is encrypted with a series of Caesar ciphers depending on the letters of a keyword.

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

The Vigenere cipher consists of a series of simple substitution ciphers with various shift values. This cipher repeats the keyword right before it joins with the plaintext's duration.

Encryption is performed by going to the row in the table corresponding to the key and determining the column heading of the corresponding letter of the plaintext character; the letter at the intersection of the corresponding row and column of the Vigenere Square generates the ciphertext character. The remaining plaintext is encrypted using a similar approach.

Features

These approaches share the following common features −

• A set of related monoalphabetic substitution rules is applied.

• A key determines the rule used for a transformation.

Example

For example, 'a' can be encoded as 'd' in the beginning of the text but as 'n' in the middle. The polyalphabetic ciphers have the advantage of concealing the letter frequency of the fundamental language. As a result, the attacker cannot divide the ciphertext using static individual letter frequencies.

Names of Polyalphabetic Cipher

Polyalphabetic ciphers are a form of encryption process in which different letters are replaced in unique ways, making them harder to crack. Below are some examples of polyalphabetic ciphers −

• Vigenere Cipher

• Beaufort Cipher

• Playfair Cipher

• Autokey Cipher

• Running Key Cipher

These ciphers use different techniques for changing the substitution pattern, making them more secure than basic ciphers like the Caesar cipher.

MonoAlphabetic Cipher vs. Polyalphabetic Cipher

See the difference between MonoAlphabetic Cipher and Polyalphabetic Cipher in detail below −