What is Monoalphabetic Cipher in Information Security?

The substitution cipher is the oldest forms of encryption algorithms according to creates each character of a plaintext message and require a substitution process to restore it with a new character in the ciphertext.

This substitution method is deterministic and reversible, enabling the intended message recipients to reverse-substitute ciphertext characters to retrieve the plaintext.

The specific form of substitution cipher is the Monoalphabetic Substitution Cipher, is known as “Simple Substitution Cipher”. Monoalphabetic Substitution Ciphers based on an individual key mapping function K, which consistently replaces a specific character α with a character from the mapping K (α).

A mono-alphabetic substitution cipher is a type of substitution ciphers in which the equivalent letters of the plaintext are restored by the same letters of the ciphertext. Mono, which defines one, it signifies that each letter of the plaintext has a single substitute of the ciphertext.

Caesar cipher is a type of Monoalphabetic cipher. It uses the similar substitution method to receive the cipher text characters for each plain text character. In Caesar cipher, it can see that it is simply for a hacker to crack the key as Caesar cipher supports only 25 keys in all. This pit is covered by utilizing Monoalphabetic cipher.

In Monoalphabetic cipher, the substitute characters symbols supports a random permutation of 26 letters of the alphabet. 26! Permutations of the alphabet go up to 4*10^26. This creates it complex for the hacker to need brute force attack to gain the key.

Mono-alphabetic cipher is a type of substitution where the relationship among a symbol in the plaintext and a symbol in the cipher text is continually one-to-one and it remains fixed throughout the encryption process.

These ciphers are considered largely susceptible to cryptanalysis. For instance, if ‘T’ is encrypted by ‘J’ for any number of appearance in the plain text message, then ‘T’ will continually be encrypted to ‘J’.

If the plaintext is “TREE”, thus the cipher text can be “ADOO” and this showcases that the cipher is possibly mono-alphabetic as both the “O”s in the plaintext are encrypted with “E”s in the cipher text.

Although the hacker will not be capable to need brute force attack, it is applicable for consider the key by using the All- Fearsome Statistical Attack. If the hacker understand the characteristics of plaintext of any substitution cipher, then regardless of the size of the key space, it can simply break the cipher using statistical attack. Statistical attack includes measuring the frequency distribution for characters, comparing those with same statistics for English.

Updated on: 15-Mar-2022

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