What are the techniques of Monoalphabetic Cipher?

There are various techniques of Monoalphabetic cipher which are as follows −

Additive Cipher − Additive cipher is one method of changing a permutation of the letters of the alphabet. Each letter in the alphabet is cyclically changed by the equivalent amount and the relative order of the letters is kept the similar.

The number of position the letter has been transformed is known as key. For example if it can use a key value of 5, ‘a’ is changed 5 positions right in the alphabet to ‘F’, ‘b’, to ‘G’ and so on.

The letter ‘u’ is changed to ‘Z’ and thus it can wrap around to the starting of the alphabet. The letter ‘v’ is mapped to ‘A’ etc.

In other words, additive cipher can also be completed by using the position numbers of the letters of the alphabet. In this method, the English letters ‘A’ to ‘Z’ are basically mapped to be the position numbers ‘0’ to ‘25’.

Multiplicative Cipher − Multiplicative cipher is another method for creating a permutation of the letters of the alphabet. It can taking a key value and each letter's position number is multiplied by 5 and thus the product is decreased by modulo 26.

For example, because plain text letter (P) is ‘h’ and the key (K) is 5, the cipher text letter is calculated by modular arithmetic multiplication operation including C = PxK (mod26). Thus, C= 7 x 5=9 (mod 26). The position number ‘9’ is mapped to the letter ‘J’. Therefore the cipher text letter is ‘J’.

Affine Cipher − The Affine cipher is a type of monoalphabetic substitution cipher, wherein each letter in an alphabet is mapped to its mathematical equivalent, encrypted using a simple mathematical function, and transformed back to a letter.

The formula used means that each letter encrypts to one other letter, and back again, defining the cipher is basically a standard substitution cipher with a rule governing which letter goes to which.

An Affine cipher is generated by connecting the additive cipher and multiplicative cipher. It is a set of both ciphers with a pair of keys. The first key can be used with the multiplicative cipher, and the second key is used with the additive cipher. The set of keys is shared secret keys for both the sender and the receiver of the message.

Affine cipher is generally two ciphers, and it can be used one after another, such as only one complex operation for the encryption or decryption such as C = ((PxK₁) + K₂)modn and P = ((C − K₂)xK₁ ⁻¹)modn.

T is used as a temporary result and indicates two separate operations such as multiplication and addition for encryption, subtraction and division for decryption.

As a result of a set of ciphers, Affine cipher has reverse transformations in each procedure, encryption or decryption. If addition is the final operation in encryption, thus subtraction should be the first in decryption. If multiplication is the first operation in encryption, thus division should be the final in decryption.